Programming in C++ Structure of main() function

Transcription

1 1 Programming in C++ A C++ program is a collection of one or more functions (or procedures) Functions (in C++ program) are very much like functions in mathematics A function is like a black box There must be a function called main( ) in every executable C++ program Execution always begins with the first statement in the function main( ) Any other functions in your program are sub-programs and are not executed until they are called (either from main() or from functions called by main()) 2 Structure of main() function

2 3 Preprocessor and Program Codes std::cout << "Hello World!" << std::endl; preprocessor The actual program Preprocessor: Run before compiling Instruct the compiler on how to compile the program Will not generate machine codes Start with a pound (#) symbol Actual program: Every C++ program must have the main() function It is the beginning point of every C++ program 4 Preprocessor Preprocessor is called automatically each time you run the compiler The preprocessor looks for lines that begin with the # symbol If the line is found then the preprocessor modifies the code The compiler will compile the modified codes In addition to #include, there is also #define, #if etc

3 5 Preprocessor When compiling a file, we need to obtain the definitions of some terms in the program codes These definitions are recorded in some header files These files are shipped with the compiler or other resources #include tells the compiler where to find the header files and insert this file to that location of the program e.g. tells the compiler it should get the file iostream through the default path e.g. #include "myfile" tells the compiler it should get the file myfile in the current folder. 6 Program Codes The basic element of a program is function A function is composed of: Return Type Function name std::cout << "Hello World!" << std::endl; Input parameters Think from the point of view of the compiler Program codes enclosed by by the opening and closing braces Note: The meaning of std::cout is checked in iostream

4 7 Themain() function is the beginning point of a program When executing a program, the operating system will first call the main() function of this program If the above main() function executes successfully, it should return an integer 0 to the operating system Call main() main() Return 0 Means everything fine on executing main()as it is the last statement 8 Program Codes std::cout << "Hello World!" << std::endl; Send the string Hello World! to to std::cout the standard output, defined in in iostream Return an an integer 0 to to the operating system In console mode, the standard output is just the console, i.e. the Command prompt window In C++,character string is represented by a sequence of characters enclosed by " " std::endl means newline (or Enter), also defined in iostream

5 9 Namespaces std::cout and std::endl means that we are referring to the cout and endl of the std namespace Thestd namespace is defined in iostream Namespace A new feature of C++ Folders and files concept in XP Design to help programmers develop new software components without generating naming conflicts Naming conflict A name in a program that may be used for different purpose by different people cout and endl are not a part of C++, people can use these two names for any purpose; not necessarily referring to standard output and newline. 10 namespace myns int cout=0; //Integer variable //No semi-colon This cout refers to to We can have our own cout by putting it it in in a namespace defined by ourselves the standard output std::cout << myns::cout << std::endl; This cout refers to to the number 0 In fact, another definition of cout can be found in iostream The result of this program is a number 0 shown on the standard output.

6 11 namespace myns int cout=0; That s why using cout without the associate namespace is is an error since the system does not know which cout you are referring to to std::cout << cout << std::endl; 12 It may be a bit cumbersome to write the namespace of names every time A short form is to use the using statement cout << "Hello World!" << endl; All names that are not a part of of C++ will belong to to the namespace std, unless otherwise stated No need to to put std in in front of of cout and endl

7 13 We can also print integers, floatingpoint numbers or or even combination of of string and integers in in standard output \n \n--another way to to show newline cout << "Hello there.\n"; escape sequence cout << "Here is 5: "<< 5 << "\n"; cout << "endl writes a new line to the screen."; cout << \t Add a tab character Another line endl; cout << "Here is a very big number:\t" << << endl; cout << "Here is the sum of 8 and 5:\t" << 8+5 << endl; cout << "Here's a fraction:\t\t" << 5.0/8 << endl; cout << "And a very very big number:\t"; cout << *7000 << endl; // (double) 7000 casting integer to double cout << "Replace Frank with your name...\n"; cout << "Frank is a C++ programmer!\n"; \t Add a tab character 14 Result

8 15 Comments A program needs to be well commented to explain the important points of the program Adding comments in the program will not affect the program execution but only improve readability Comments can be added in two ways: /* Text between these two marks are comments */ cout << "Hello World!\n"; // Text after that are also comments 16 Hello.cpp Preprocessor directives Function named main() indicates start of program // Program: Display Hello world // Author(s): ENG2002 // Date: 28/8/2013 Comments Provides simple access otherwise, std::cout cout << "Hello world!" << endl; Ends execution of main() which ends program Insertion statement Program

9 17 Exercise 3.1a a. Build the program in p.13. Note the output on the console. b. Add one statement to the program which will show your name and age in a single sentence. The name should be shown as a character string. The age should be shown as an integer. c. Use the comment symbols /* and */ to comment the statements from line 5 to line 9 (inclusive). Is there any change to the results output? 18 Function main is a function What is a function? A collection of C++ statements to carry out a specific task! You can have more functions in your program! How to declare a function? A proper name must be unique Return type The function body

10 19 Example of a function Function body Return type Function name /* Text between these two marks are comments */ cout << "Hello World!\n"; // Text after that are also comments 20 The Function DEFINITION The mathematical definition of the function f(x) = 5x 3 The C++ programmatic definition would be: int f(int x) int y = 5*x 3; return y; //return 5*x 3;

11 21 Function Definition ReturnDataType FunctionName ( Parameter List ) Statement(s)... int Cube ( int n ) return n * n * n ; header body 22 More on Functions Although a single main() function is enough for any C++ program, it s a bad habit to do everything by a single function C++ allows nesting of functions to facilitate "divide and conquer" of jobs The function main() can call other functions to help it complete a task When a function is called, the program branches off from the normal program flow When the function returns, the program goes back to where it left before.

12 23 They must be be the same //function DemonstrationFunction() // show a useful message void DemonstrationFunction() cout << "In Demonstration Function\n"; cout << "Print one more line\n"; A function is is defined //function main - prints out a message, then //calls DeomonstrationFunction, then shows //the second message. cout << "In main\n"; DemonstrationFunction(); cout << "Back in main\n"; Return nothing A function is is called 24 The execution sequence is like that //function DemonstrationFunction() // show a useful message void DemonstrationFunction() cout << "In Demonstration Function\n"; cout << "Print one more line\n"; //function main - prints out a message, then //calls DeomonstrationFunction, then shows //the second message. cout << "In main\n"; DemonstrationFunction(); cout << "Back in main\n";

13 25 cout << "In main\n"; DemonstrationFunction(); cout << "Back in main\n"; //function DemonstrationFunction() void DemonstrationFunction() cout << "In Demonstration Function\n"; cout << "Print one more line\n"; Can you write your program like this? 26 Function prototype A prototype looks like a header but must end with a semicolon; and its parameter list just needs to contain the type of each parameter. int Cube( int ); // prototype

14 27 Function prototype void DemonstrationFunction(); This the function prototype Put this before main then your function declaration can be written after main void DemonstrationFunction() cout << "In Demonstration Function\n"; cout << "Print one more line\n"; 28 Passing Parameters to Function To let the called function really help the main(), sometimes parameters are passed from main() to the called function After finishing the computation, the function should pass back the results to main() It can be achieved by the return statement. main() function(a,b) return c Branch to function(a,b)

15 29 Function with input parameters Two input parameters of type integer int Add (int x, int y) cout << "In Add(),received "<<x<<" and "<<y<<"\n"; return(x+y); Now inside the function Add, the name x refers to the first input parameter, and y refers to the second parameter 30 int Add (int x, int y) cout << "In Add(),received "<<x<<" and "<<y<<"\n"; return(x+y); cout << "I'm in main()!\n"; int a,b,c; cout << "Enter two numbers: "; cin >> a; cin >> b; cout << "\ncalling Add()\n"; c = Add(a,b); cout << "\nback in main().\n"; cout << "c was set to " << c; cout << "\nexiting...\n\n"; Input parameters need need to to declare type type-the - the same as as those in in the the calling function Add() will will return an an integer x+y x+yback to to main() Add() is is called with with two two parameters c holds the the return value of of Add()

16 31 Exercise 3.1b a. Build the program in the last slide. Note the output on the console. b. Modify main() to calculate the square of c. Add one more function called Square() to achieve this. The Square() function will take the square of the parameter that is passed to it. It will return the result in the form of an integer back to the calling function. 32 Variable Concepts Variables Variable names (identifiers) correspond to locations in the computer's memory Every variable has a name, a type, a size and a value Whenever a new value is placed into a variable, it replaces (and destroys) the previous value. (Destructive write) Reading variables from memory does not change them int i = 45; i 45 int bytes variable value datatype address size

17 33 Variables and Memory A variable is actually a place to store information in a computer It is a location (or series of locations) in the memory The name of a variable can be considered as a label of that piece of memory Variables char a int b short int c bool d Memory Address 10 0A 21 3A One address for one byte, i.e. 8 bits. in hex in bin 34 Size of Variables In memory, all data are groups of 1 and 0, byte by byte Depending on how we interpret the data, different types of variables can be identified in the memory Type Size bool 1 byte unsigned short int 2 bytes short int 2 bytes unsigned long int 4 bytes long int 4 bytes unsigned int 4 bytes int 4 bytes char 1 byte float 4 bytes double 8 bytes Values true or false (1 or 0) 0 to 65,535 (2 16-1) -32,768 to 32,767 0 to 4,294,967,295 (2 32-1) -2,147,483,648 to 2,147,483,647 0 to 4,294,967,295-2,147,483,648 to 2,147,483, character values (+/-)1.40x10-45 to (+/-)3.40x10 38 (+/-)4.94x to (+/-)1.80x unsigned long long 8 bytes 0 to (2 64-1) long long 8 bytes to (2 63-1)

18 35 Declaring Variables Before we use a variable, we need to declare its type so that the compiler can reserve suitable memory space for it Variables are declared in this way: int myvariable; It defines myvariable to be an integer. Hence 4 bytes will be reserved for its storage in memory The name of the variable is case sensitive, hence myvariable is NOT the same as myvariable 36 Variable Declarations type v 1,v 2,v 3,, v n Example: int count; int j; float k; double area; char c; short int x; long int y; unsigned int z; int a1;

19 37 Declaring Variables We can declare one or more variables of the same kind at once int myvar1, myvar2, yourvar1, yourvar2; It defines myvar1, myvar2, yourvar1, yourvar2 are all integers Some names are keywords of the C++ language that cannot be used as variable names e.g. return, while, for, if, etc. We can even initialize the value of the variables when making the declaration Does an uninitialized variable have a value? int myvar1 = 10, myvar2(10), yourvar1, yourvar2 = 5; 38 Variable Initialization If a variable is not initialized, the value of variable may be either 0 or garbage depending on the storage class of the variable. int count = 5; // int count(5); double length = 1.23; char c = A ; // char c; c = A ; int i = 1, j, k = 5; char c1 = B, c2 = 66; float x = 1.23, y = 0.1;

20 39 Maximum and Minimum Each data type has its own maximum and minimum limits When the value goes beyond those limits, unexpected behavior may result unsigned short int smallnumber; smallnumber = 65535; // = cout << "small number:" << smallnumber << endl; smallnumber++; // = 0 // smallnumber = smallnumber + 1; cout << "small number:" << smallnumber << endl; smallnumber++; // = 1 cout << "small number:" << smallnumber << endl; 40 Maximum and Minimum Signed integers do NOT behave in the same way short int smallnumber; smallnumber = 32767; // = cout << "small number:" << smallnumber << endl; smallnumber++; // = cout << "small number:" << smallnumber << endl; smallnumber++; // = cout << "small number:" << smallnumber << endl;

21 41 Characters Besides numbers, C++ also has a data type called char, i.e. character If a variable is declared as char, the compiler will consider the variable as an 8-bit ASCII character // ASCII code of '5' is 53 char c = 53, d = '5'; // They are the same int e = 53; cout << "c = " << c << endl; // c = 5 cout << "d = " << d << endl; // d = 5 cout << "e = " << e << endl; // e = ASCII Table decimal : A 6 6 B 6 7 C 6 8 D 6 9 E : 7 0 F 7 1 G 7 2 H 7 3 I 7 4 J 9 7 a 9 8 b 9 9 c d e Details of the table can be found in many places, for instance, f g h i j

22 43 Special Printing Characters C++ compiler recognizes some special characters for output formatting Start with an escape character \ (e.g. \n means new line) Character \n \t \b \" \' \a \\ What it Means new line tab backspace double quote single quote bell backslash escape sequence //using goto, not recommended int i = 0; char c = i; beg: // label for goto 44 cout << "[ " << c << " ] = " << i << endl; i = i + 1; c = i; // c = ++i; if (i < 256) goto beg; c = 'A'; cout << "c = " << c << endl; c = 65; cout << "c = " << c << endl; i = 'A'; cout << "i = " << i << endl; // A == 65 cin.get();

23 45 Floating-point numbers Floating-point number (real number): zero or any positive or negative number containing a decimal point Examples: No special characters are allowed Three floating-point data types in C++: float (single precision) 7 significant digits 1.2F double (double precision) 15 significant digits 1.2 long double (same as double for Visual C++) 1.2L 46 Exponential Notation Floating point numbers can be written in exponential notation, where e or E stands for exponent 6.25E

24 47 //Comparing int, double and float int main () cout.precision(20); cout << "1/3 = " << 1/3 << endl << endl; cout << "1.0/3 = " << 1.0/3 << "\n"; cout << "(double)1/3 = " << (double)1/3 << "\n\n"; cout << "(float)1/3 = " << (float)1/3 << "\n1.0f/3 = " << 1.0F/3 << "\n\n"; cout << "int size = " << sizeof(int) << " bytes\n"; cout << "double size = " << sizeof(double) << " bytes\n"; cout << "float size = " << sizeof(float) << " bytes\n"; cin.get(); 48 Constants Unlike variables, the values of constants cannot be changed The value of a constant will be fixed until the end of the program There are two types of constants Literal Constants - come with the language e.g. a number 39 (you cannot assign another value to 39) Symbolic Constants - Like variables, users define a special name as a label to it Unlike variables, its value cannot be changed once it is initialized.

25 49 Defining Symbolic Constants A symbolic constant can be defined in two ways 1. Old way - use the preprocessor command #define #define studentperclass 87 No type needs to be defined for studentperclass Preprocessor just replaces the word studentperclass with 87 whenever it is found in the source program 2. A better way - use the keyword const const int studentperclass = 87; Variable studentperclass now has a fixed value 87 It has a type now. This feature helps the compiler to debug the program. 50 Why do we need Constants? Help debugging the program Compiler will automatically check if a constant is modified by the program later (and reports it if modified.) Improve readability Give the value a more meaningful name e.g. rather than writing 360, can use degreeinacircle Easy modification If a constant really needs to be changed, we only need to change a single line in the source program.

26 51 const int totalstudentnumber = 87; // Student no. must < 87 int num; cout << "Enter a student number: "; cin >> num; if (num > totalstudentnumber) cout << "\n Number not valid!!!\n"; Give better readability Modify once and apply to to whole program cout << "\n There are " << totalstudentnumber << " student in this class\n"; 52 Enumerated Constants Enumerated constants allow one to create a new type that contains a number of constants enum COLOR RED, BLUE, GREEN, WHITE, BLACK; Makes COLOR the name of the new type Set RED = 0, BLUE = 1, GREEN = 2, WHITE = 3, BLACK = 4 enum RED, BLUE, GREEN, WHITE, BLACK; Another example enum COLOR2 RED=100, BLUE, GREEN=500, WHITE, BLACK=700; Makes COLOR2 the name of the new type Set RED = 100, BLUE = 101, GREEN = 500, WHITE = 501, BLACK = 700

27 53 const int Sunday = 0; const int Monday = 1; const int Tuesday = 2; const int Wednesday = 3; const int Thursday = 4; const int Friday = 5; const int Saturday = 6; int choice; cout << "Enter a day (0-6): "; cin >> choice; Exercise 3.2 a. Build the project and note the result b. Use enumerated constants method to simplify the program if (choice == Sunday choice == Saturday) cout << "\nhave a nice weekend!\n"; else cout << "\ntoday is not holiday yet.\n"; 54 A Typical Program function function function int abc (...)... ; return... ; int cde (...)... ; return... ;...; statements

28 55 Statements A C++ program comprises a number of functions A function comprises a number of statements A statement can be as simple as x = a + b; It can be as complicated as if (choice == Sunday choice == Saturday) cout << "\nyou're already off on weekends!\n"; A statement must end with a semicolon ; In a statement, space carries nearly no information x = a + b; x = a + b ; 56 Expressions A statement comprises one or many expressions Anything that returns a value is an expression Examples of expressions 3.2 // Returns the value 3.2 studentperclass // Returns a constant, say 87 x = a + b // Here, two expressions // Return a + b and return the value of a variable x Since x = a + b is an expression (i.e. return a value), it can certainly be assigned to another variable For example, y = x = a + b; // assign y to the value of x which // is equal to the sum of a and b

29 57 Operators Any expression can be an operand An expression often comprises one of more operators The assignment operator '=' assigns a value to a variable or constant, e.g. x = 39; Several mathematical operators are provided by C++ Let a be 3, b be 4 and x is declared as an integer, Operators Meaning + Add - Subtract * Multiply / Divide % modulus remainder Examples x = a + b; // x = 7 x = a - b; // x = -1 x = a * b; // x = 12 x = a / b; // x = 0 (why?) x = a % b; // x = 3 (why?) 3 modulo 4 is 3 58 Arithmetic Operators Binary Operator (two operands) + (addition) - (subtraction) * (multiplication) / (division) % (modulus, remainder) (no ** ) Unary Operator (single operands) - (no + ) Example: int i=1, j=2, k=3, x; x=i+2*j-22/k; x=-1+j; x=1+-j; x=+i+j; x=22%k; float f=1.5, g=0.5, y; y=2.5*f+4.0*g; Exercise: Try -5%3-5%-3 5%-3 (x= = -2) (x= 1) (x= -1) (x=3) (x= 1, remainder) (y=5.75) (hint: -5/3=-1-5/-3=1 5/-3=-1 and R=x-y*i) Ans: Mixed data types will be discussed later

30 59 Mixed Type Arithmetic Rule #1 char, short int float double Rule #2 (double long unsigned int) If either operand is double, the other is converted to double, and the result is double Otherwise, if either operand is long, the other is converted to long, and the result is long Otherwise, if either operand is unsigned, the other is converted to unsigned, and the result is unsigned Otherwise, the operand must be int 60 Mixed Type Arithmetic Assign real to int Cut off fractional part of real Assign int value to real Put decimal point at end, converts method of storage to exponential binary form Modify with cast operators Change type of expression Keyword static_cast<type>expression Old form: (type) expression /4 == == (double)3 /4 == static_cast<double>(3) /4 == 1.85

31 61 Integer and Floating Point Divisions x, y of main() are not x, y of intdiv(int, int) void intdiv(int x, int y) Casting int z = x/y; Ask Ask the the compiler to to temporarily cout << "z: " << z << endl; consider x as as a floating point no., no., i.e. i.e in in this this case case void floatdiv(int x, int y) float a = (float)x; // old style float b = static_cast<float>(y); float c = a/b; cout << "c: " << c << endl; int x = 5, y = 3; intdiv(x,y); floatdiv(x,y); Only give integer division result, i.e. i.e. 1 // ANSI style Give floating point division result, i.e. i.e Shorthand of Expressions For some commonly used expressions, there are some shorthands c = c + 1; c += 1; c++; ++c; //increment c = c - 1; c -= 1; c--; --c; //decrement c = c + 2; c += 2; c = c - 3; c -= 3; c = c * 4; c *= 4; // there is no c** or c//, c = c / 5; c /= 5; // since it is meaningless Let c be an integer with value 5. Note that a = c++; a=c; c=c+1; a = ++c; c=c+1; a=c; // a = 5, c = 6 as a = c first and then c++ // a = 6, c = 6 as ++c first and then a = c

32 63 Precedence and Parentheses Mathematical operations are basically performed from left to right It follows the normal mathematical precedence that multiplication / division first and addition / subtraction next Hence x = * 8; // x = 29 since we evaluate 3 * 8 first To change the precedence, we can use parentheses x = (5 + 3) * 8; // x = 64 since we evaluate first Parentheses can be nested as in normal arithmetic x = 5 * ((3 + 2) + 3 * 2); // x = 5 * (5 + 6) = Rules of Operator Precedence Parentheses are applied first. If an expression contains nested parentheses, the innermost pair is evaluated first. If there are several pairs of parentheses on the same level (that is, not nested), they are evaluated left to right. Unary plus (+) and minus (-) are applied next. If an expression contains several unary plus and minus operators, these operators are applied from right to left. Multiplication (*), division (/) and remainder (%) are applied next. If an expression contains several multiplication, division and remainder operations, these operators are applied from left to right. Addition (+) and subtraction (-) are applied next. If an expression contains several addition and subtraction operations, these operators are applied from left to right. Assignment (=) is applied last. This operator has the lowest precedence so far. If an expression contains several assignment operators, these operators are applied from right to left. Figure 3.15 Rules of operator precedence. Precedence is the order in which operators are applied Redundant parentheses make expressions easier to read

33 65 Relational Operators Besides arithmetic operations, we can also perform relational operations with C++ A number of relational operators are defined in C++ Each relational operator returns a bool result (true or false) Operators Meaning == Equals!= Not Equals > Greater than >= Greater than or equals < Less than <= Less than or equals Examples 100 == 50; // return false 100!= 50; // return true 100 > 50; // return true 100 >= 50; // return true 100 < 50; // return false 100 <= 50; // return false 66 Relational Operators Binary Operators ==!= < > <= >= Result is a integer: 1 means true 0 means false Declare logical type variable and constants using bool No space between the operators Example: Meaning equal not equal greater less greater equal less equal C ==!= > < >= <= Expression 5 == 3 5!= 3 5 > 3 5 < 3 5 >= 3 5 <= 3 int i=10, j=10, k=1; i + j <= 3 + k Result

34 67 Applications of Relational Operators Relational operators are often used with if statement The if statement provides branching of the program execution depending on the conditions int bignumber = 10, smallnumber = 5; bool bigger = bignumber > smallnumber; //bigger = true if (bigger) dosomething(); Or Or you can simply write: if if (bignumber > smallnumber) dosomething(); 68 Logical Operators Logical operators do logical operations for bool variables Each logical operation returns a bool result (true or false) Operators && AND OR! NOT Meaning Examples expression1 && expression2 expression1 expression2!expression int bignum = 10, smallnum = 5; if (bignum < 10 && smallnum > 0) dosomething(); Would dosomething() be be called??? NO

35 69 int bignum = 10, smallnum = 5; if ((bignum < 10 && smallnum > 0) bignum > smallnum) dosomething(); Would dosomething() be be called??? YES F T int bignum = 10, smallnum = 5; if (!(bignum > smallnum)) dosomething(); Would dosomething() be be called??? NO 70 Logical (Boolean) Operators Binary Operators && (and) Unary Operator! (not) Operand should be int (OR) Use float, result may be affected by round off error nonzero (true) zero (false) Result is int 1 (true) 0 (false) Example: Expression Result &&3 1 5&&0 0 i&&j (if i=0, j=0) 0 i&&j+1 (if i=5, j=0) 1!5 0!0 1!i (if i=5) 0 Express connected by && or are evaluated from left to right, and evaluation stops as soon as the truth or falsehood of the result is known. i.e. expr1 && expr2 is not equal to expr2 && expr1. This is called short-circuit evaluation. inward == 0 normally be written as!inward Example: 3 < 7 < 5 3 < 7 && 7 < 5 (3 < 7) < 5 1 < && 0 0

36 71 Exercise 3.3 The following program asks the user to enter 3 numbers. If the sum of the first two is equal to the last one, a message is printed to the console a. Build the project and note the result. Does it perform correctly? b. If not, fix the problem and re-build the program int a, b, c; cout << "Enter 3 numbers: \n"; cin >> a; cin >> b; cin >> c; if (c = (a+b)); cout << "c = a+b\n"; 72 Control of Program Flow Normal program execution is performed from top-todown and one-statement by one-statement Often, the program modifies the program flow depending on some conditions set by the programmer or user C++ provides many approaches to control program flow if... else switch... case... break if (cond) // if the cond doa(); // is true, // then do doa() else // else do dob() dob(); switch (expression) case value1: doa(); break; case value2: dob(); break;

37 73 The if / else Selection Structure Compound statement: Set of statements within a pair of braces Example: if ( grade >= 60 ) cout << "Passed.\n"; else cout << "Failed.\n"; cout << "You must take this course again.\n"; Without the braces, if ( grade >= 60 ) cout << "Passed.\n"; else cout << "Failed.\n"; cout << "You must take this course again.\n" ; the statement cout << "You must take this course again.\n" ; would be executed under every condition. 74 #include 3. The <iostream> Nuts and Bolts of C++ int firstnum, secondnum = 10; cout << "Please enter: \n"; cin >> firstnum; cout << "\n\n"; if (firstnum >= secondnum) if ((firstnum% secondnum) == 0) if (firstnum == secondnum) Nested if... else cout << "They are the same!\n"; else cout << "They are evenly divisible!\n"; else cout << "They are not evenly divisible!\n"; else cout << "Hey! The second no. is larger!\n"; can be omitted for single statement

38 75 Using braces with if else What s wrong with this program? How do do you solve the problem? int x; cout << "Enter a number x < 10 or x > 100: "; cin >> x; cout << "\n";? if (x >= 10) if (x > 100)? cout << "More than 100, thanks!\n"; else // not the else intended! cout << "Less than 10, thanks!\n"; 76 Equality (==) vs. Assignment (=) Operators Dangerous error Does not ordinarily cause syntax errors Any expression that produces a value can be used in control structures Nonzero values are true, zero values are false Example: using ==: if ( paycode == 4 ) cout << "You get a bonus!\n"; Checks paycode, if it is 4 then a bonus is awarded Example: replacing == with =: if ( paycode = 4 ) cout << "You get a bonus!\n"; This sets paycode to 4 4 is nonzero, so expression is true, and bonus awarded no matter what the paycode was Logic error, not a syntax error

39 77 Multiple-Selection Structure: switch switch Useful when a variable or expression is tested for all the values it can assume and different actions are taken Format Series of case labels and an optional default case switch ( value ) case 1: Actions; break; case 2: Actions; break; default: actions break; exits from structure case 1 true case 1 action(s) break false case 2 true case 2 break false action(s) true case n case n break action(s) false default action(s) 78 switch if (burger == 1) unit_price = 8.5; else if (burger == 2) unit_price = 12; else if (burger == 3) unit_price = 15.3; else cout<< error \n ; switch(burger) case 1 : unit_price = 8.5; break; case 2 : unit_price = 12; break; case 3: unit_price = 15.3; break; default : cout<< error \n ; // break; optional

40 79 switch case break Only accept number or or expression that that returns a number unsigned short int number; cout << "Enter a number between 1 and 5: "; cin >> number; switch (number) case 0: cout << "Too small, sorry!"; break; case 3: cout << "Excellent!\n"; //falls through case 2: cout << "Masterful!\n"; //falls through case 1: default: cout << "\n\n"; See See the the differences between break and and without break cout << "Incredible!\n"; break; cout << "Too large!\n"; break; Do Do the the default if if all all tests tests fail fail 80 Implement switch with if else switch case statement can be implemented by the if else statement but with more complicated structure However, switch case can only be applied to simple numerical tests if (num == 0) cout << "Too small, sorry!"; else if (num == 3 num == 2 num == 1) cout << "Incredible!\n"; if (num == 3 num == 2) cout << "Masterful!\n"; if (num == 3) cout << "Excellent!\n"; else cout << "Too large!\n";

41 81 Looping Many programming problems are solved by repeatedly acting on the same data The method for achieving repeated execution is by looping C++ provides many approaches to implement looping if goto // old way, strongly NOT recommended while loop // use when the testing parameters are // complicated do-while loop // use when the repeated part is // expected to be executed at least once for loop // use when the testing parameters are // simple 82 while Loop int count ; count = 4; while (count > 0) cout << count << endl ; count -- ; cout << Done << endl ; // initialize loop variable // test expression // repeated action // update loop variable

42 83 while Loops int counter = 0; //initialize counter while (counter<5) counter++; //top of the loop cout << "counter: " << counter << "\n"; cout << "Complete. Counter: " << counter << ".\n"; while loops do the looping until the testing condition fails Test condition (tested variable must change inside the the loop) Repeat this part until counter >= >= More complicated while Loops while loops can be used with a more complicated testing condition unsigned small, large; unsigned short const MAXSMALL = ; cin>>small; cin>>large; while (small < large && large > 0 && small < MAXSMALL) if (small % 5000 == 0) cout << ".\n"; // write a dot every 5000 small++; large -= 2; Testing parameters are updated in in every loop 3 tests for each loop

43 85 Repetition Structure: do/while The do/while repetition structure Similar to the while structure do/while is a post-test condition. The body of the loop is performed at least once. All actions are performed at least once Format: action(s) do statement; while ( condition ); Example: Prints the integers from 1 to 10. (letting counter = 1): do cout << counter <<, ; while (++counter <= 10); condition false true 86 do while while loops do the test first and then the loop Sometimes we would like to have the loop to be done at least once. In this case, do while can be used Hello will be be shown at at least once even if if user enters 0 for counter int counter; cout << "How many hellos? "; cin >> counter; do cout << "Hello\n"; counter--; while (counter > 0); cout << "Counter is: " << counter << endl;

44 87 Repetition Structure: for for loops syntax for ( initialization ; loopcontinuationtest ; increment ) statement Example: Prints the integers from one to ten for ( counter = 1; counter <= 10; counter++ ) cout << counter << endl; For loops can usually be rewritten as while loops: initialization; while ( loopcontinuationtest ) statement; increment; Initialization and increment Can be comma-separated list of statements Example: for ( i = 0, j = 0; j + i <= 10; j++, i++) cout << j + i; No semicolon (;) after last expression 88 for loops In many loops, we initialize a testing parameter, modify the value of the parameter and test the parameter It can be done in a single line by a for loop int counter; Initialize Test Modify for (counter=0; counter<5; counter++) cout << "Looping! "; //counter then increment cout << "\ncounter: " << counter << ".\n"; 5

45 89 Different varieties of for loops Multiple initialization and increments for (int i=0, j=0; i<3; i++, j++) cout << "i: " << i << " j: " << j << endl; Null int counter = 0; initialization and increments for (; counter < 5;) counter++; cout<<"looping! "; cout << "\ncounter: " << counter << ".\n"; 90 Different varieties of for loops Empty for loop int counter = 0; // initialization int max; cout << "How many hellos?"; cin >> max; for (;;) // a for loop that doesn't end if (counter < max) // test cout << "Hello!\n"; counter++; // increment else break; //end for loop

46 91 continue and break Keywords continue and break allow one to change the program flow during looping Should be used with caution since it will make the program hard to understand owing to the sudden change of direction Execution of of continue will will skip skipthe following part part of of the the loop loop but but not not leaving the the loop loop int counter; for (counter=0; counter<5; counter++) cout << "Looping! counter is " << counter <<"\n"; if ((counter%2) == 1) //odd number continue; cout << "Counter is an even number.\n"; 92 The break and continue Statements break Causes immediate exit from a while, for, do/while or switch structure Program execution continues with the first statement after the structure Common uses of the break statement Escape early from a loop Skip the remainder of a switch structure continue Skips the remaining statements in the body of a while, for or do/while structure Proceeds with the next iteration of the loop while and do/while Loop-continuation test is evaluated immediately after the continue statement is executed for Increment expression is executed, then the loop-continuation test is evaluated

47 93 continue Statement while (expr) statement continue; skip statement do statement continue; skip statement while(expr) for (expr1; expr2; expr3) statement continue; statement skip 94 break Statement while (expr) statement; if (expr) break; statements; statement; for (expr1; expr2; expr3) statement if (expr) break; statements; statements; switch (i) case 1: statement_1; case 2: statement_2; case 3: statement_3; break; case 4: statement_4; statements;

48 95 Nested Loop initialize outer loop while ( outer loop condition )... initialize inner loop while ( inner loop condition ) inner loop processing and update // end inner loop... // end outer loop 96 Nested for loops Nested for loop int rows, columns; char thechar; cout << "How many rows? "; cin >> rows; cout << "How many columns? "; cin >> columns; cout << "What characters? "; cin >> thechar; for (int i=0; i<rows; i++) for (int j=0; j<columns; j++) cout << thechar; cout << "\n"; More explanation on next page Will be be executed (rows columns) times

49 97 Assumption: rows = 2 columns = 3 thechar = x Value of of i, i, j for (int i=0; i<rows; i++) for (int j=0; j<columns; j++) cout << thechar; cout << "\n"; i = 0 j =? i = 1 j = 3 i = 0 j = 0 i = 1 j = 0 i = 0 j = 0 i = 1 j = 0 i = 0 j = 1 i = 1 j = 1 x i = 0 j = 1 i = 1 j = 1 i = 0 j = 2 i = 1 j = 2 i = 0 j = 2 i = 1 j = 2 i = 0 j = 3 i = 1 j = 3 i = 0 j = 3 i = 1 j = 3 i = 2 j = 3 Output on on the screen: x x x 98 If a variable is defined in the initialization part of the for loop, the variable will no longer exist on leaving the for loop. It is not an error for old C++ compilers.

50 99 Exercise 3.4 For the program on p. 96 a. Build the project and note the result. b. Try to rewrite the program using nested while loops instead of the nested for loops. Which program is more complicated? 100 Functions - Revisit input A function is, in effect, a subprogram that can act on data and return a value output When the name of the function is encountered in the program, the function is called When the function returns, execution resumes on the next line of the calling function Callingfunc() Statements; funca(); Statements; funcb(); Statements; Return value funca() is void Statements; return; funcb() Statements; return;

51 101 Function Body return type function name type of input parameters unsigned short int // Opening brace Statements; return (return_value); // Closing brace FindArea (int length, int width) name of input parameters return value 102 Why do we need functions? Functions help us shorten our program by re-using the program codes void floatdiv(int x, int y) float a = (float)x; float b = static_cast<float>(y); float c = a/b; cout << "c: " << c << endl; int w = 7, x = 5, y = 3, z = 2; floatdiv(w,x); floatdiv(x,y); floatdiv(y,z); Reason 1 13 lines

52 103 The same program will be much longer without function Can be even longer if the same operation is done in other part of the program int w = 7, x = 5, y = 3, z = 2; float a, b, c; a = (float)w; b = static_cast<float>(x); float c = a/b; cout << "c: " << c << endl; a = (float)x; b = static_cast<float>(y); float c = a/b; cout << "c: " << c << endl; a = (float)y; b = static_cast<float>(z); float c = a/b; cout << "c: " << c << endl; 17 lines 104 Functions make our program much easier to read void floatdiv(int x, int y) float a = (float)x; float b = static_cast<float>(y); float c = a/b; cout << "c: " << c << endl; int w = 7, x = 5, y = 3, z = 2; floatdiv(w,x); floatdiv(x,y); floatdiv(y,z); Reason 2 In this program, it is easily seen that 3 floating-point divisions are to be done Not the case of the previous program without the function.

53 105 Declaring Functions In C++, anything that is not a part of the C++ language needs to be declared Function prototype However, a function need NOT be separately declared if it is placed before the functions that will call it void DemoFunction() cout << "In Demo Function\n"; cout << "In main\n"; DemoFunction(); cout << "Back in main\n"; Since DemoFunction() is is placed before main(), it it need not be be separately declared DemoFunction() can be be used directly 106 However, it is a bad programming practice to require functions to appear in a particular order because It is difficult for code maintenance (too restrictive) Two functions may call each other (typically inside some loop) void FuncA() : FuncB(); : void FuncB() : FuncA(); : Which one should be be placed first?

54 107 Functions are usually declared by either one of the two ways: Write the prototype of the function at the beginning of the file in which the function is used Put the prototype of the function into a header file and include it in the file in which the function is used Function Prototype unsigned short int FindArea(int, int); return type function name type of input parameters 108 using Declaring namespace std; Functions int Area(int length, int width); // function prototype int lengthofyard; Although it it is is not not int widthofyard; necessary, int areaofyard; adding the the name cout << "\nhow wide is your yard? "; of of the the parameters cin >> widthofyard; makes the the prototype cout << "\nhow long is your yard? "; easier to to read read cin >> lengthofyard; areaofyard = Area(lengthOfYard,widthOfYard); cout << "\nyour yard is "; cout << areaofyard; cout << " square feet \n\n"; int Area(int yardlength, int yardwidth) return yardlength * yardwidth; Area() is is placed after after main(). Note Note the the name of of the the parameters are are NOT the the same as as the the prototype

55 109 Assume the file area.h has the following statement and is is at at the same folder as as main() int Area(int length, int width); // function prototype #include "area.h" // function prototype It It is is equivalent to to place the the content : of of area.h to to here here areaofyard = Area(lengthOfYard,widthOfYard); : int Area(int yardlength, int yardwidth) return yardlength * yardwidth; 110 Function Prototypes in Header file Advantage: if a particular set of function prototypes is often used in different programs, we need not declare them every time they are needed E.g. iostream In different.cpp files One line of #include Vs many lines Contains prototypes of many functions that are related to the manipulation of I/O stream Is needed in most programs that need I/O like cout, cin, etc. Should be included at the beginning of most programs; otherwise, we need to type all prototypes in every program.

56 111 Exercise 3.5 1) For the program on p.102, add the function prototype such that we can place the function floatdiv() after main(). 2) Modify the program you've developed in 1) as follows: Remove the function prototype Prepare a file named floatdiv.h that contains just one statement: the function prototype of floatdiv() Store the file floatdiv.h in the same folder as your C++ file Include this header file at the beginning of the program as the example in p.109 Achieve the same result as 1). 112 In C++, there are 3 types of variables that a function may make use of: Where do variables locate in your machine? Passed parameters and return parameter(s) The links between the called function and the calling function Local variable Visible only within a function For temporary local storage Global variable Visible to all functions in the program An old and dangerous way to communicate between functions

57 113 Variable Scope Local variables: variables declared within a function Such variables have local scope; they can be used only within the function in which they are declared Global variables: variables declared outside any function Such variables have global scope; they can be used by all functions that occur after their declaration 114 Variable Scope Global variable The three storage areas created by a C++ Program. Local variable Local variable

58 115 Local Variables float Convert(float); //function prototype float TempFer; float TempCel = 10; TempFer = 100; TempCel = Convert(TempFer); cout << TempCel << endl; float Convert(float Fer) float Cel; Cel = ((Fer - 32) * 5) / 9; return Cel; A function can define local variables for temporary usage Local variables are only visible within the function defining them 116 float Convert(float); same. float TempFer; float Cel = 10; TempFer = 100; Cel = Convert(TempFer); cout << Cel << endl; float Convert(float Fer) float Cel; Cel = ((Fer - 32) * 5) / 9; return Cel; Cel in in main()is different from the Cel in in Convert() although their name is is the same. Actually for each function, a separate piece of of memory is is allocated to to the local variables of of each function disregarding their name.

59 117 float Convert(float); float TempFer; float Cel = 10; TempFer = 100; Cel=Convert(TempFer); cout << Cel << endl; 37.7 main() float Convert(float Fer) float Cel; Cel = ((Fer - 32) * 5) / 9; return Cel; Convert() Variables TempFer Cel For return Fer Cel Memory Parameter Passing Passed by Value It is seen in the previous example that parameters are passed by value Only a copy of the parameter value is passed to the called function What the called function does to the passed parameters have nothing to do with the original one, since they are just two variables (and occupying different memory, even when their names are the same) However, such behavior sometimes is not preferred. In that case, we need the passed by reference parameter, which will be covered in the section of Pointers

60 119 Parameter Passing - by Value Variables a and x use different memory cells. different copies 120 Parameter Passing - by Reference Variables a and x share the same memory cell. same copy

61 121 Global Variables int Convert(float); //function prototype changed float Cel; // Global variable float TempFer; cout << "Please enter the temperature in Fahrenheit: "; cin >> TempFer; Convert(TempFer); //No need to collect the return value cout << "\nhere's the temperature in Celsius: "; cout << Cel << endl; int Convert(float Fer) Cel = ((Fer - 32) * 5) / 9; Global variable are visible to to all all functions Must be be carefully used Make your program difficult to to debug. 122 Scope of Variables It is a rule of thumb that variables defined within a pair of braces are visible only to the statements in that braces after the variable is defined void myfunc() int x = 8; Be careful! x = 8 cout << "\nin myfunc, local x: " << x << endl; x = 8 cout << "\nin block in myfunc, x is: " << x; int x = 9; //This x is not the same as the previous x cout << "\nvery local x: " << x; x = 9 cout << "\nout of block, in myfunc, x: " << x << endl; x = 8 myfunc();

62 123 Default Parameters Calling function should pass parameters of exactly the same types as those defined in the prototype of the called function long myfunction(int); //function prototype It means that any function that calls myfunction() should pass an integer to it The only exception is if the function prototype has a default value long myfunction(int x=50); //default value If the calling function does not provide a parameter, 50 will be automatically used : myfunction( ); myfunction(50); 124 Default Parameters You You may may use use other names int volumecube(int, int width = 25, int height = 1); int length = 100, width = 50, height = 2, volume; volume = volumecube(length, width, height); cout << "First volume equals: " << volume << "\n"; volume = volumecube(length, width); cout << "Second volume equals: " << volume << "\n"; volume = volumecube(length); cout << "Third volume equals: " << volume << "\n"; volume = volume = volume = = = = int volumecube(int length, int width, int height) return (length*width*height); Once Once a default value value is is assigned, the the parameters following must must have have default values.

63 125 Overloading Functions C++ allows overloading of function, i.e. create more than one function with the same name e.g. int myfunction (int, int); 3 different int myfunction (long, long); functions long myfunction (long); Differ by just the return type is NOT allowed When a function calls myfunction(), the compiler checks the number and type of the passed parameters to determine which function should be called Function overloading is also called polymorphism Poly means many, morph means form A polymorphic function is many-formed 126 int intdouble(int); float floatdouble(float); int myint = 6500, doubledint; float myfloat = 0.65, doubledfloat; doubledint = intdouble(myint); doubledfloat = floatdouble(myfloat); cout << "doubledint: " << doubledint << "\n"; cout << " doubledfloat : " << doubledfloat << "\n"; int intdouble(int original) is is to to double the the passed return 2*original; parameter float floatdouble(float original) return 2*original; single function name but but different parameter types The The objective of of both both functions It It looks much better to to have have a different parameter types Overloading allows us us to to do do so so

64 127 int Double(int); float Double(float); int myint = 6500, doubledint; float myfloat = 0.65, doubledfloat; doubledint = Double(myInt); doubledfloat = Double(myFloat); cout << "doubledint: " << doubledint << "\n"; cout << " doubledfloat : " << doubledfloat << "\n"; int Double(int original) return 2*original; float Double(float original) return 2*original; Overloading 128 Parameter Passing Passed by Reference void Double(int& original); void Double(float& original); int myint = 6500; float myfloat = 0.65; Double(myInt); Double(myFloat); cout << "doubledint: " << myint << "\n"; cout << " doubledfloat : " << myfloat << "\n"; void Double(int& original) original = 2*original; void Double(float& original) original = 2*original; Overloading

65 129 Recursive Function Example: factorials: 5! = 5 * 4 * 3 * 2 * 1 5! = 5 * 4! 4! = 4 * 3! 1 n! = n( n 1)! n = 0 n > 0 Can compute factorials recursively Solve base case (1! = 0! = 1) then plug in 2! = 2 * 1! = 2 * 1 = 2; 3! = 3 * 2! = 3 * 2 = 6; long factorial(int n) if (n == 0) return 1; else return n * factorial(n-1); 130 Exercise 3.6a void myfunc() int x = 8; cout << "\nin myfunc, local x: " << x << endl; cout << "\nin block in myfunc, x is: " << x; int x = 9; cout << "\nvery local x: " << x; cout << "\nout of block, in myfunc, x: " << x << endl; The main() on the right is used to call myfunc() above. Build the program. What is the error message when compiling? Why? Correct the error accordingly. void myfunc(); myfunc();