1 EEE1008 C Programming

Transcription

1 1 1 EEE1008 C Programming Dr. A.M. Koelmans

2 2 Administrative details Dr. A.M. Koelmans Location: Merz Court, room E4.14 Telephone: 8155 Web:

3 3 BOOK C How to Program, by P.J. Deitel, H.M. Deitel, published by Pearson International, 6 th edition.

4 4 Web page The web page contains information about the course, and downloads of all the source code used in the lecture slides. Other material may be made available on Blackboard. We will follow Deitel's book closely. There are many other books and web sites dedicated to C.

5 5 Course structure Basics of computer languages First look at C: a basic program Data types and variables Basic I/O, using keyboard and screen The main building blocks: expressions, conditionals, and repetition Advanced data types: arrays Functions

6 6 Why study software? This course intends to show you that software design is very logical and modular. It is a good case study of good engineering practice. A lot of software development is intimately tied to computer architectures: Compilers generate code for certain processors Operating systems control the system hardware Motors are often controlled by software Many embedded systems (from cars, mobile phones and TVs down to microwave ovens and washing machines) need both hardware and software.

7 7 Why study software? It is therefore no exaggeration to say that most Electrical and/or Electronic Engineering related jobs have some kind of software aspect. Many traditional Electrical Engineering functions are now performed by microprocessors. If you end up in a management position you may well have to take important (and potentially expensive) decisions that are affected by your knowledge about both software and hardware. Therefore, understanding and being comfortable with software will greatly enhance your career prospects. Professional organisations (such as IET and IEEE) demand that you develop some software skills.

8 8 Course structure There are practicals every week, where you do exercises and projects. Details are always on the web page. Project deadlines must be strictly adhered to. Some of the lectures will be devoted to exercises, quizzes, and self-assessments.

9 9 Second year In the first year, you also learn the basics of Computer Architecture (as part of the Digital Electronics I course) In the second year, we combine the two topics in EEE2007, where you learn more advanced Machine Architecture, Embedded C, and Assembly language You then build and program your own computer, based around a Motorola microprocessor

10 What is a Computer? Computer Device capable of performing computations and making logical decisions Computers process data under the control of sets of instructions called computer programs Hardware Various devices comprising a computer Keyboard, screen, mouse, disks, memory, CD-ROM, and processing units Software Programs that run on a computer

11 11 The ENIAC

12 12 Computer technology evolution Feature 1945: ENIAC Scale factor 2005: PENTIUM complexity tubes M transistors size 200 m cm 3 speed 150 ops/s M ops/s power consumption 10 kw W cost $ 10M 10-4 $ 1000 reliability 3 hours 10 3 years

13 13 What if cars had improved like that speed 70 m/hour of light fuel 50 mpg mpg cost 10K pounds pound reliability 1 year years weight 1 ton mg

14 Computer Organization Six logical units in every computer: Input unit Obtains information from input devices (keyboard, mouse) Output unit Outputs information (to screen, to printer, to control other devices) Memory unit Rapid access, low capacity, stores input information Arithmetic and logic unit (ALU) Performs arithmetic calculations and logic decisions Central processing unit (CPU) Supervises and coordinates the other sections of the computer Secondary storage unit Cheap, long-term, high-capacity storage Stores inactive programs

15 1.6 Machine Languages, Assembly Languages, and High-level Languages 15 Three types of programming languages Machine languages Strings of numbers giving machine specific instructions Example: Assembly languages English-like abbreviations representing elementary computer operations (translated via assemblers) Example: LOAD BASEPAY ADD OVERPAY STORE GROSSPAY

16 1.6 Machine Languages, Assembly Languages, and High-level Languages 16 Three types of programming languages (continued) 1. High-level languages - Codes similar to everyday English - Use mathematical notations (translated via compilers) - Example: grosspay = basepay + overtimepay

17 History of C C Evolved by Ritchie from two previous programming languages, BCPL and B Used to develop UNIX Used to write modern operating systems Hardware independent (portable) Standardization Many slight variations of C existed, and were incompatible Committee formed to create a "unambiguous, machineindependent" definition Standard created in 1989, updated in 1999

18 18 Portability Tip Because C is a hardware-independent, widely available language, applications written in C can run with little or no modifications on a wide range of different computer systems.

19 C Standard Library C programs consist of pieces/modules called functions A programmer can create her own functions Advantage: the programmer knows exactly how it works Disadvantage: time consuming Programmers will often use the C library functions Use these as building blocks Avoid re-inventing the wheel If a pre-made function exists, generally best to use it rather than write your own Library functions carefully written, efficient, and portable

20 20 Performance Tips Using Standard C library functions instead of writing your own comparable versions can improve program performance, because these functions are carefully written to perform efficiently. This can also improve program portability, because these functions are used in virtually all Standard C implementations.

21 1.14 Typical C Program Development Environment 21 Phases of C Programs: Edit: Using a text editor, you create the program and put it into a file Preprocess: The C Preprocessor includes system definitions and removes comments from the file. The result is stored in another file Compile: The C compiler turns the file into object code, and stores the result in a file Link: The Linker combines the object code with libraries, to create a complete executable file Load: The Loader (part of the Operating System) puts the executable file in memory Execute: The microprocessor executes (runs) the program Fig. 1.1 Typical C development environment.

22 1.18 General Notes About C 22 Program clarity Programs that are convoluted are difficult to read, understand, and modify C is a portable language Programs can run on many different computers However, portability is an elusive goal We do a careful walk-through of C Some details and subtleties are not covered If you need additional technical details Read the C standard document Read other books there are many

23 23 Good Programming Practice Write your C programs in a simple and straightforward manner. This is sometimes referred to as KIS ( keep it simple ). Do not stretch the language by trying bizarre usages.

24 24 Software Engineering Observation Your computer and compiler are good teachers. If you are not sure how a feature of C works, write a sample program with that feature, compile and run the program and see what happens.

25 25 2 Introduction to C Programming

26 26 Line numbers are not part of the program! 1 /* Fig. 2.1: fig02_01.c 2 A first program in C */ 3 #include <stdio.h> comments ignored by compiler load a particular file 4 5 /* function main begins program execution */ 6 int main( void ) beginning of main function 7 { 8 printf( "Welcome to C!\n" ); perform an action 9 10 return 0; /* program ended successfully */ ends the function } /* end function main */ end of main function Welcome to C! 2007 Pearson Education, Inc. All rights reserved.

27 2.2 A Simple C Program: Printing a Line of Text 27 Comments Text surrounded by /* and */ is ignored by computer Used to describe program #include <stdio.h> Preprocessor directive Tells computer to load contents of a certain file <stdio.h> allows standard input/output operations

28 28 Common Programming Errors Forgetting to terminate a comment with */. Starting a comment with the characters */ or ending a comment with the characters /*.

29 2.2 A Simple C Program: Printing a Line of Text 29 int main() C programs contain one or more functions, exactly one of which must be main Parenthesis used to indicate a function int means that main "returns" an integer value Braces ({ and }) indicate a block The bodies of all functions must be contained in braces

30 30 Good Programming Practice Every function should be preceded by a comment describing the purpose of the function.

31 2.2 A Simple C Program: Printing a Line of Text 31 printf( "Welcome to C!\n" ); Instructs computer to perform an action Specifically, prints the characters within quotes (" ") Entire line called a statement All statements must end with a semicolon (;) Escape character (\) Indicates that printf should do something out of the ordinary \n is the newline character

32 Escape Description 32 \n Newline. Put the cursor at the beginning of the next line. \t Horizontal tab. Move the cursor to the next tab stop. \a Alert. Sound the system bell. \\ Backslash. Insert a backslash character in a string. \" Double quote. Insert a double-quote character in a string. Fig. 2.2 Some common escape sequences.

33 33 Common Programming Error Typing the name of the output function printf as print in a program.

34 2.2 A Simple C Program: Printing a Line of Text 34 return 0; A way to exit a function return 0, in this case, means that the program terminated normally Right brace } Indicates end of main has been reached Linker When a function is called, linker locates it in the library Inserts it into object program If function name is misspelled, the linker will produce an error because it will not be able to find function in the library

35 35 Good Programming Practices Add a comment to the line containing the right brace, }, that closes every function, including main. The last character printed by a function that displays output should be a newline (\n). This ensures that the function will leave the screen cursor positioned at the beginning of a new line. Conventions of this nature encourage software reusability a key goal in software development environments.

36 36 Good Programming Practice Indent the entire body of each function one level of indentation (we recommend three spaces) within the braces that define the body of the function. This indentation emphasizes the functional structure of programs and helps make programs easier to read. How would you like this instead: /* Fig. 2.1: fig02_01.c A first program in C*/ #include <stdio.h>/*function main begins program*/ int main(void){printf("welcome to C!\n");return 0; /*program ended successfully*/}/*end function main*/

37 37 Good Programming Practice Set a convention for the size of indent you prefer and then uniformly apply that convention. The tab key may be used to create indents, but tab stops may vary. We recommend using three spaces per level of indent.

38 1 /* Fig. 2.3: fig02_03.c 2 Printing on one line with two printf statements */ 3 #include <stdio.h> 4 5 /* function main begins program execution */ 6 int main( void ) 7 { 8 printf( "Welcome " ); 9 printf( "to C!\n" ); 10 printf statement starts printing from where the last statement ended, so the text is printed on one line. 11 return 0; /* indicate that program ended successfully */ } /* end function main */ 38 Welcome to C! 2007 Pearson Education, Inc. All rights reserved.

39 1 /* Fig. 2.4: fig02_04.c 2 Printing multiple lines with a single printf */ 3 #include <stdio.h> 4 5 /* function main begins program execution */ 6 int main( void ) 7 { 8 printf( "Welcome\nto\nC!\n" ); Newlines move the cursor to the next line 9 10 return 0; /* indicate that program ended successfully */ } /* end function main */ 39 Welcome to C! 2007 Pearson Education, Inc. All rights reserved.

40 1 /* Fig. 2.5: fig02_05.c 2 Addition program */ 3 #include <stdio.h> 4 5 /* function main begins program execution */ 6 int main( void ) 7 { 8 int integer1; /* first number to be input by user */ 9 int integer2; /* second number to be input by user */ 10 int sum; /* variable in which sum will be stored */ printf( "Enter first integer\n" ); /* prompt */ 13 scanf( "%d", &integer1 ); /* read integer */ printf( "Enter second integer\n" ); /* prompt */ 16 scanf( "%d", &integer2 ); /* read integer */ 17 variables 18 sum = integer1 + integer2; /* assign total to sum */ Assigns a value to sum printf( "Sum is %d\n", sum ); /* print sum */ return 0; /* program ended successfully */ } /* end function main */ 40 scanf obtains a value from the user and assigns it to a variable 2007 Pearson Education, Inc. All rights reserved.

41 41 Enter first integer 45 Enter second integer 72 Sum is Pearson Education, Inc. All rights reserved.

42 2.3 Another Simple C Program: Adding Two Integers 42 As before Comments, #include <stdio.h> and main int integer1, integer2, sum; Definition of variables, called a declaration Variables: locations in memory where a value can be stored int (its type) means the variables can hold integers (-1, 3, 0, 47) Variable names (identifiers) integer1, integer2, sum Identifiers: consist of letters, digits (cannot begin with a digit) and underscores( _ ) Case sensitive Definitions appear before executable statements If an executable statement references and undeclared variable it will produce a syntax (compiler) error

43 43 Common Programming Errors Using a capital letter where a lowercase letter should be used (for example, typing Main instead of main). Placing variable definitions among executable statements causes syntax errors.

44 44 Good Programming Practice Choosing meaningful variable names helps make a program self-documenting, i.e., fewer comments are needed. The first letter of an identifier used as a simple variable name should be a lowercase letter. Later in the text we will assign special significance to identifiers that begin with a capital letter and to identifiers that use all capital letters.

45 45 Good Programming Practice Multiple-word variable names can help make a program more readable. Avoid running the separate words together as in totalcommissions. Rather, separate the words with underscores as in total_commissions, or, if you do wish to run the words together, begin each word after the first with a capital letter as in totalcommissions. The latter style is preferred.

46 46 Good Programming Practice Separate the definitions and executable statements in a function with one blank line to emphasize where the definitions end and the executable statements begin.

47 2.3 Another Simple C Program: Adding Two Integers 47 scanf( "%d", &integer1 ); Obtains a value from the user scanf uses standard input (usually keyboard) This scanf statement has two arguments %d - indicates data should be a decimal integer conversion specifier &integer1 - location in memory to store variable address operator & is confusing in beginning for now, just remember to include it with the variable name in scanf statements When executing the program the user responds to the scanf statement by typing in a number, then pressing the enter (return) key

48 2.3 Another Simple C Program: Adding Two Integers 48 = (assignment operator) Assigns a value to a variable (assignment statement) Is a binary operator (has two operands) sum = variable1 + variable2; sum gets the value (variable1 + variable2); Variable receiving value on left The right hand side is an expression printf( "Sum is %d\n", sum ); Similar to scanf %d means decimal integer will be printed sum specifies what integer will be printed Calculations can be performed inside printf statements printf( "Sum is %d\n", integer1 + integer2 );

49 49 Good Programming Practice Place spaces on either side of a binary operator. This makes the operator stand out and makes the program more readable. Place a space after each comma (,) to make programs more readable.

50 50 Common Programming Errors A calculation in an assignment statement must be on the right side of the = operator. It is a syntax error to place a calculation on the left side of an assignment operator. Forgetting one or both of the double quotes surrounding the format control string in a printf or scanf.

51 51 Common Programming Errors Forgetting the % in a conversion specification in the format control string of a printf or scanf. Placing an escape sequence such as \n outside the format control string of a printf or scanf.

52 52 Common Programming Errors Forgetting to include the expressions whose values are to be printed in a printf containing conversion specifiers. Not providing a conversion specifier when one is needed in a printf format control string to print the value of an expression.

53 53 Common Programming Errors Placing inside the format control string the comma that is supposed to separate the format control string from the expressions to be printed. Forgetting to precede a variable in a scanf statement with an ampersand when that variable should, in fact, be preceded by an ampersand.

54 54 Common Programming Error Preceding a variable included in a printf statement with an ampersand when, in fact, that variable should not be preceded by an ampersand.

55 Memory Concepts Variables Variable names correspond to locations in the computer's memory Every variable has a name, a type, a size and a value Types are: int (integer), char (character), float, double (both floating point they have decimal points in them) Whenever a new value is placed into a variable (through scanf, for example), it replaces (and destroys) the previous value (destructive operation) Reading variables from memory does not change them, so in: sum = variable1 + variable2; variable1 and variable2 are unchanged (nondestructive operation), but sum is likely to change

56 56 integer1 integer2 sum Fig. 2.8 Memory locations after a calculation.

57 Arithmetic Arithmetic calculations Use * for multiplication and / for division Integer division truncates remainder 7 / 5 evaluates to 1 Modulus operator(%) returns the remainder 7 % 5 evaluates to 2, because 7 = 5*1 + 2 Operator precedence Some arithmetic operators act before others (i.e. multiplication before addition) Use parenthesis when needed Example: Find the average of three variables a, b and c Do not use: a + b + c / 3 Use: (a + b + c ) / 3

58 C operation operator Algebraic C 58 Addition + f + 7 f + 7 Subtraction p c p - c Multiplication * bm b * m Division / x / y x / y Remainder % r mod s r % s Fig. 2.9 Arithmetic operators.

59 59 Common Programming Error An attempt to divide by zero is normally undefined on computer systems and generally results in a fatal error, i.e., an error that causes the program to terminate immediately without having successfully performed its job. Nonfatal errors allow programs to run to completion, often producing incorrect results.

60 60 Operator(s) Operation(s) Order of evaluation (precedence) ( ) Parentheses Evaluated first. If the parentheses are nested, the expression in the innermost pair is evaluated first. If there are several pairs of parentheses on the same level (i.e., not nested), they are evaluated left to right. * / % Multiplication Division Remainder Evaluated second. If there are several, they are evaluated left to right. + - Addition Subtraction Evaluated last. If there are several, they are evaluated left to right. Fig Precedence of arithmetic operators.

61 Step 1. y = 2 * 5 * * 5 + 7; /* leftmost multiplication */ 2 * 5 is Step 2. y = 10 * * 5 + 7; /* leftmost multiplication */ 10 * 5 is 50 Step 3. y = * 5 + 7; /* multiplication first */ 3 * 5 is 15 Step 4. y = ; /* leftmost addition */ is 65 Step 5. y = ; /* last addition */ Step 6. y = 72; Fig Order in which a second-degree polynomial is evaluated.

62 62 Good Programming Practice Using redundant parentheses in complex arithmetic expressions can make the expressions clearer.

63 2.6 Decision Making: Equality and Relational Operators 63 Statements: Perform actions (calculations, input/output of data) Perform decisions May want to print "pass" or "fail" given the value of a test grade if control statement Simple version in this section, more detail later If a condition is true, then the body of the if statement executed 0 is false and non-zero is true Control always resumes after the if structure Keywords Special words reserved for C Cannot be used as identifiers or variable names

64 64 Keywords auto double int struct break else long switch case enum register typedef char extern return union const float short unsigned continue for signed void default goto sizeof volatile do if static while C s keywords.

65 65 Standard algebraic equality operator or relational operator C equality or relational operator Example of C condition Meaning of C condition Equality operators = == x == y x is equal to y!= x!= y x is not equal to y Relational operators > > x > y x is greater than y < < x < y x is less than y >= x >= y x is greater than or equal to y <= x <= y x is less than or equal to y Fig Equality and relational operators.

66 66 Common Programming Errors A syntax error will occur if the two symbols in any of the operators ==,!=, >= and <= are separated by spaces. A syntax error will occur if the two symbols in any of the operators!=, >= and <= are reversed as in =!, => and =<, respectively.

67 67 Common Programming Errors Confusing the equality operator == with the assignment operator =. Placing a semicolon immediately to the right of the right parenthesis after the condition in an if statement.

68 68 Good Programming Practices Indent the statement(s) in the body of an if statement. Place a blank line before and after every if statement in a program for readability. Although it is allowed, there should be no more than one statement per line in a program.

69 69 Good Programming Practice Consistently applying responsible indentation conventions greatly improves program readability. In this course, we use three blanks per indent.

70 1 /* Fig. 2.13: fig02_13.c 2 Using if statements, relational 3 operators, and equality operators */ 4 #include <stdio.h> 5 6 /* function main begins program execution */ 7 int main( void ) 8 { 9 int num1; /* first number to be read from user */ 10 int num2; /* second number to be read from user */ printf( "Enter two integers, and I will tell you\n" ); 13 printf( "the relationships they satisfy: " ); scanf( "%d%d", &num1, &num2 ); /* read two integers */ if ( num1 == num2 ) { Checks if num1 is equal to num2 18 printf( "%d is equal to %d\n", num1, num2 ); 19 } /* end if */ if ( num1!= num2 ) { Checks if num1 is not equal to num2 22 printf( "%d is not equal to %d\n", num1, num2 ); 23 } /* end if */ Continued on next slide Pearson Education, Inc. All rights reserved.

71 25 if ( num1 < num2 ) { Checks if num1 is less than num2 26 printf( "%d is less than %d\n", num1, num2 ); 27 } /* end if */ if ( num1 > num2 ) { 30 printf( "%d is greater than %d\n", num1, num2 ); 31 } /* end if */ if ( num1 <= num2 ) { Checks if num1 is greater than num2 Checks if num1 is less than or equal to num2 34 printf( "%d is less than or equal to %d\n", num1, num2 ); 35 } /* end if */ if ( num1 >= num2 ) { Checks if num1 is greater than equal to num2 38 printf( "%d is greater than or equal to %d\n", num1, num2 ); 39 } /* end if */ return 0; /* indicate that program ended successfully */ 42 } Pearson Education, Inc. All rights reserved.

72 Enter two integers, and I will tell you the relationships they satisfy: is not equal to 7 3 is less than 7 3 is less than or equal to 7 72 Enter two integers, and I will tell you the relationships they satisfy: is not equal to is greater than is greater than or equal to 12 Enter two integers, and I will tell you the relationships they satisfy: is equal to 7 7 is less than or equal to 7 7 is greater than or equal to Pearson Education, Inc. All rights reserved.

73 73 Good Programming Practice A lengthy statement may be spread over several lines. If a statement must be split across lines, choose breaking points that make sense (such as after a comma in a comma-separated list). If a statement is split across two or more lines, indent all subsequent lines.

74 74 Good Programming Practice Refer to the operator precedence chart when writing expressions containing many operators. Confirm that the operators in the expression are applied in the proper order. If you are uncertain about the order of evaluation in a complex expression, use parentheses to group expressions or break the statement into several simpler statements.

75 75 Operators Associativity ( ) left to right * / % left to right + - left to right < <= > >= left to right ==!= left to right = right to left Fig Precedence and associativity of the operators discussed so far.

76 76 3 Structured Program Development in C

77 Introduction Before writing a program: Have a thorough understanding of the problem Carefully plan an approach for solving it While writing a program: Know what building blocks are available Use good programming principles

78 Algorithms Computing problems All can be solved by executing a series of actions in a specific order Algorithm: procedure in terms of Actions to be executed The order in which these actions are to be executed Program control Specify order in which statements are to be executed

79 Pseudocode Pseudocode Artificial, informal language that helps us develop algorithms Similar to everyday English Not actually executed on computers Helps us think out a program before writing it Easy to convert into a corresponding program Consists only of executable statements

80 Control Structures Sequential execution Statements executed one after the other in the order written Transfer of control When the next statement executed is not the next one in sequence Building blocks All programs written in terms of 3 control structures Sequence structures: Built into C. Programs executed sequentially by default Selection structures: C has three types: if, if else, and switch Repetition structures: C has three types: while, do while and for

81 81 Add grade to total total = total + grade; Add 1 to counter counter = counter + 1; Fig. 3.1 Flowcharting C s sequence structure.

82 Control Structures Flowchart Graphical representation of an algorithm Drawn using certain special-purpose symbols connected by arrows called flowlines Rectangle symbol (action symbol): Indicates any type of action Oval symbol: Indicates the beginning or end of a program or a section of code Single-entry/single-exit control structures Connect exit point of one control structure to entry point of the next (control-structure stacking) Makes programs easy to build

83 The if selection statement Selection structure: Used to choose among alternative courses of action Pseudocode: If student s grade is greater than or equal to 60 Print Passed If condition true Print statement executed and program goes on to next statement If false, print statement is ignored and the program goes onto the next statement Indenting makes programs easier to read C ignores whitespace characters

84 84 Good Programming Practice Pseudocode is often used to think out a program during the program design process. Then the pseudocode program is converted to C.

85 The if selection statement Pseudocode statement in C: if ( grade >= 60 ) printf( "Passed\n" ); C code corresponds closely to the pseudocode Diamond symbol (decision symbol) Indicates decision is to be made Contains an expression that can be true or false Test the condition, follow appropriate path

86 86 grade >= 60 true print passed false Fig. 3.2 Flowcharting the single-selection if statement.

87 3.6 The if else selection statement 87 if Only performs an action if the condition is true if else Specifies an action to be performed both when the condition is true and when it is false Pseudocode: If student s grade is greater than or equal to 60 Print Passed else Print Failed Note spacing/indentation conventions

88 88 Good Programming Practices Indent both body statements of an if...else statement. If there are several levels of indentation, each level should be indented the same additional amount of space.

89 3.6 The if else selection statement C code: if ( grade >= 60 ) printf( "Passed\n"); else printf( "Failed\n"); Ternary conditional operator (? :) Takes three arguments (condition, value if true, value if false) Our pseudocode could be written: printf( "%s\n", grade >= 60? "Passed" : "Failed" ); Or it could have been written: grade >= 60? printf( Passed\n ) : printf( Failed\n ); 89

90 90 print failed false grade >= 60 true print passed Fig. 3.3 Flowcharting the double-selection if...else statement.

91 3.6 The if else selection statement 91 Nested if else statements Test for multiple cases by placing if else selection statements inside if else selection statement Once condition is met, rest of statements skipped Deep indentation usually not used in practice

92 3.6 The if else selection statement 92 Pseudocode for a nested if else statement If student s grade is greater than or equal to 90 Print A else If student s grade is greater than or equal to 80 Print B else If student s grade is greater than or equal to 70 Print C else If student s grade is greater than or equal to 60 Print D else Print F

93 3.6 The if else selection statement Compound statement: Set of statements within a pair of braces Example: if ( grade >= 60 ) printf( "Passed.\n" ); else { printf( "Failed.\n" ); printf( "You must take this course again.\n" ); } Without the braces, the statement printf( "You must take this course again.\n" ); would be executed automatically 93

94 94 Software Engineering Observation A compound statement can be placed anywhere in a program that a single statement can be placed. It is also possible to have no statement at all, i.e., the empty statement. The empty statement is represented by placing a semicolon (;) where a statement would normally be.

95 95 Common Programming Error Forgetting one or both of the braces that delimit a compound statement. This usually leads the compiler to issue error messages that make little sense initially they may occur much later in the program.

96 96 Common Programming Error Placing a semicolon after the condition in an if statement as in if ( grade >= 60 ); leads to a logic error in single-selection if statements and a syntax error in doubleselection if statements.

97 97 Error-Prevention Tip Typing the beginning and ending braces of compound statements before typing the individual statements within the braces helps avoid omitting one or both of the braces, preventing syntax errors and logic errors (where both braces are indeed required).

98 The while repetition statement Repetition structure Programmer specifies an action to be repeated while some condition remains true Pseudocode: While there are more items on my shopping list Purchase next item and cross it off my list while loop repeated until condition becomes false Example: int product = 2; while ( product <= 1000 ) product = 2 * product;

99 99 Common Programming Errors Not providing the body of a while statement with an action that eventually causes the condition in the while to become false. Normally, such a repetition structure will never terminate an error called an infinite loop. Spelling the keyword while with an uppercase W as in While (remember that C is a casesensitive language). All of C s reserved keywords such as while, if and else contain only lowercase letters.

100 100 product <= 1000 true product = 2 * product false Fig. 3.4 Flowcharting the while repetition statement.

101 Counter-Controlled Repetition Counter-controlled repetition Loop repeated until counter reaches a certain value Definite repetition: number of repetitions is known Example: A class of ten students took a quiz. The grades (integers in the range 0 to 100) for this quiz are available to you. Determine the class average on the quiz.

102 102 1 Set total to zero 2 Set grade counter to one 3 4 While grade counter is less than or equal to ten 5 Input the next grade 6 Add the grade into the total 7 Add one to the grade counter 8 9 Set the class average to the total divided by ten 10 Print the class average Fig. 3.5 Pseudocode algorithm that uses counter-controlled repetition to solve the class average problem.

103 1 /* Fig. 3.6: fig03_06.c 2 Class average program with counter-controlled repetition */ 3 #include <stdio.h> 4 5 /* function main begins program execution */ 6 int main( void ) 7 { 8 int counter; /* number of grade */ 9 int grade; /* grade value */ 10 int total; /* sum of grades input by user */ 11 int average; /* average of grades */ /* initialization phase */ 14 total = 0; /* initialize total */ 15 counter = 1; /* initialize loop counter */ /* processing phase */ 18 while ( counter <= 10 ) { /* loop 10 times */ 19 printf( "Enter grade: " ); /* ask for input */ 20 scanf( "%d", &grade ); /* read grade */ 21 total = total + grade; /* add grade to total */ 22 counter = counter + 1; /* increment */ 23 } /* end while */ Counter to control while loop Initialize counter to 1 while loop iterates as long as counter <= 10 Increment the counter Continued on next slide Pearson Education, Inc. All rights reserved.

104 25 /* termination phase */ 26 average = total / 10; /* integer division */ Calculate the average printf( "Class average is %d\n", average ); /* result */ return 0; } /* end function main */ 104 Enter grade: 98 Enter grade: 76 Enter grade: 71 Enter grade: 87 Enter grade: 83 Enter grade: 90 Enter grade: 57 Enter grade: 79 Enter grade: 82 Enter grade: 94 Class average is Pearson Education, Inc. All rights reserved.

105 105 Common Programming Error If a counter or total is not initialized, the results of your program will probably be incorrect. This is an example of a logic error. You must initialize all counters and totals.

106 3.9 Formulating Algorithms with Top- Down, Stepwise Refinement 106 Problem becomes: Develop a class-averaging program that will process an arbitrary number of grades each time the program is run. Unknown number of students How will the program know to end? Use sentinel value Also called signal value, dummy value, or flag value Indicates end of data entry. Loop ends when user inputs the sentinel value Sentinel value chosen so it cannot be confused with a regular input (such as -1 in this case)

107 107 Common Programming Error Choosing a sentinel value that is also a legitimate data value.

108 3.9 Formulating Algorithms with Top- Down, Stepwise Refinement 108 Top-down, stepwise refinement Begin with a pseudocode representation of the top: Determine the class average for the quiz Divide top into smaller tasks and list them in order: Initialize variables Input, sum and count the quiz grades Calculate and print the class average Many programs have three phases: Initialization: initializes the program variables Processing: inputs data values and adjusts program variables accordingly Termination: calculates and prints the final results

109 3.9 Formulating Algorithms with Top- Down, Stepwise Refinement 109 Refine the initialization phase from Initialize variables to: Initialize total to zero Initialize counter to zero Refine Input, sum and count the quiz grades to Input the first grade (possibly the sentinel) While the user has not as yet entered the sentinel Add this grade into the running total Add one to the grade counter Input the next grade (possibly the sentinel)

110 3.9 Formulating Algorithms with Top- Down, Stepwise Refinement 110 Refine Calculate and print the class average to If the counter is not equal to zero Set the average to the total divided by the counter Print the average else Print No grades were entered

111 111 1 Initialize total to zero 2 Initialize counter to zero 3 4 Input the first grade 5 While the user has not as yet entered the sentinel 6 Add this grade into the running total 7 Add one to the grade counter 8 Input the next grade (possibly the sentinel) 9 10 If the counter is not equal to zero 11 Set the average to the total divided by the counter 12 Print the average 13 else 14 Print No grades were entered Fig. 3.7 Pseudocode algorithm that uses sentinel-controlled repetition to solve the class average problem.

112 112 Good Programming Practice When performing division by an expression whose value could be zero, explicitly test for this case and handle it appropriately in your program (such as printing an error message) rather than allowing the fatal error to occur.

113 113 Software Engineering Observation Many programs can be divided logically into three phases: an initialization phase that initializes the program variables; a processing phase that inputs data values and adjusts program variables accordingly; and a termination phase that calculates and prints the final results.

114 114 Software Engineering Observation You terminate the top-down, stepwise refinement process when the pseudocode algorithm is specified in sufficient detail for you to be able to convert the pseudocode to C. Implementing the C program is then normally straightforward.

115 115 Floating point numbers Integer numbers have obvious limitations No fractions needed in most scientific and engineering applications Integers are relatively small Numbers with fractions (i.e. decimal point) are called floating point numbers, and are declared as float or double Double allows larger numbers Printed with %f Examples: etc.

116 1 /* Fig. 3.8: fig03_08.c 2 Class average program with sentinel-controlled repetition */ 3 #include <stdio.h> 4 5 /* function main begins program execution */ 6 int main( void ) 7 { 8 int counter; /* number of grades entered */ 9 int grade; /* grade value */ 10 int total; /* sum of grades */ float average; /* number with decimal point*/ /* initialization phase */ 15 total = 0; /* initialize total */ 16 counter = 0; /* initialize loop counter */ /* processing phase */ 19 /* get first grade from user */ float type indicates variable can be a noninteger 20 printf( "Enter grade, -1 to end: " ); /* prompt for input */ 21 scanf( "%d", &grade ); /* read grade from user */ Continued on next slide Pearson Education, Inc. All rights reserved.

117 23 /* loop while sentinel value not yet read from user */ 24 while ( grade!= -1 ) { while loop repeats until user enters a value of total = total + grade; /* add grade to total */ 26 counter = counter + 1; /* increment counter */ /* get next grade from user */ 29 printf( "Enter grade, -1 to end: " ); /* prompt for input */ 30 scanf("%d", &grade); /* read next grade */ 31 } /* end while */ /* termination phase */ 34 /* if user entered at least one grade */ 35 if ( counter!= 0 ) { 36 Ensures the user entered at least one grade 37 /* calculate average of all grades entered */ 38 average = ( float ) total / counter; /* avoid truncation */ Converts total to float type /* display average with two digits of precision */ 41 printf( "Class average is %.2f\n", average ); 42 } /* end if */ Prints result with 2 digits after decimal point... Continued on next slide Pearson Education, Inc. All rights reserved.

118 43 else { /* if no grades were entered, output message */ 44 printf( "No grades were entered\n" ); 45 } /* end else */ return 0; } /* end function main */ 118 Enter grade, -1 to end: 75 Enter grade, -1 to end: 94 Enter grade, -1 to end: 97 Enter grade, -1 to end: 88 Enter grade, -1 to end: 70 Enter grade, -1 to end: 64 Enter grade, -1 to end: 83 Enter grade, -1 to end: 89 Enter grade, -1 to end: -1 Class average is Enter grade, -1 to end: -1 No grades were entered 2007 Pearson Education, Inc. All rights reserved.

119 119 Good Programming Practice In a sentinel-controlled loop, the prompts requesting data entry should explicitly remind the user what the sentinel value is.

120 120 Print width specification Integers are printed with %d Prints from current position Not helpful if you want align numbers in vertical direction A print width specification (also called padding ) can be inserted between the % and the d For example: %6d says print at least 6 characters, and print spaces first if necessary Example on next slide

121 1 /* Example of print width specification */ 2 #include <stdio.h> 3 4 int main( void ) 5 { 6 int i = 3; 7 int j = 850; 8 int k = ; 9 10 printf( "%d\n%d\n\n", i, j ); 11 printf( "%6d\n%6d\n\n", i, j ); 12 printf( "%6d\n", k ); 13 return 0; /* program ended successfully */ } /* end function main */ Pearson Education, Inc. All rights reserved.

122 122 Print width specification Examples of printing of floating point numbers: %f print from current position %20f print at least 20 characters %.2f print two digits after decimal point %20.2f print at least 20 characters, with two after the decimal point If you use %f, you often get many digits after the decimal point, which is not necessarily what you want

123 1 /* Example of float print width specification */ 2 #include <stdio.h> 3 4 int main( void ) 5 { 6 7 float f = 10.0 / 7.0; 8 9 printf( "%f\n", f ); 10 printf( "%20f\n", f ); 11 printf( "%.2f\n", f ); 12 printf( "%20.2f\n", f ); return 0; /* program ended successfully */ } /* end function main */ Pearson Education, Inc. All rights reserved.

124 124 Common Programming Errors Using precision in a conversion specification in the format control string of a scanf statement is wrong. Precisions are used only in printf conversion specifications. Using floating-point numbers in a manner that assumes they are represented precisely can lead to incorrect results. Floating-point numbers are represented only approximately by most computers. Do not compare floating-point values for equality.

125 Nested Control Structures Problem A college has a list of test results (1 = pass, 2 = fail) for 10 students Write a program that analyzes the results If more than 8 students pass, print "Raise Tuition" Notice that The program must process 10 test results Counter-controlled loop will be used Two counters can be used One for number of passes, one for number of fails Each test result is a number either a 1 or a 2 If the number is not a 1, we assume that it is a 2

126 Nested Control Structures Top level outline Analyze exam results and decide if tuition should be raised First Refinement Initialize variables Input the ten quiz grades and count passes and failures Print a summary of the exam results and decide if tuition should be raised Refine Initialize variables to Initialize passes to zero Initialize failures to zero Initialize student counter to one

127 Nested Control Structures Refine Input the ten quiz grades and count passes and failures to While student counter is less than or equal to ten Input the next exam result If the student passed Add one to passes else Add one to failures Add one to student counter Refine Print a summary of the exam results and decide if tuition should be raised to Print the number of passes Print the number of failures If more than eight students passed Print Raise tuition

128 1 Initialize passes to zero 2 Initialize failures to zero 3 Initialize student to one 4 5 While student counter is less than or equal to ten 6 Input the next exam result 7 8 If the student passed 9 Add one to passes 10 else 11 Add one to failures Add one to student counter Print the number of passes 16 Print the number of failures 17 If more than eight students passed 18 Print Raise tuition 128 Fig. 3.9 Pseudocode for examination results problem.

129 129 1 /* Fig. 3.10: fig03_10.c 2 Analysis of examination results */ 3 #include <stdio.h> 4 6 int main( void ) 7 { 8 /* initialize variables in definitions */ 9 int passes = 0; /* number of passes */ 10 int failures = 0; /* number of failures */ 11 int student = 1; /* student counter */ 12 int result; /* one exam result */ /* process 10 students using counter-controlled loop */ 15 while ( student <= 10 ) { 16 while loop continues until 10 students processed 17 /* prompt user for input and obtain value from user */ 18 printf( "Enter result ( 1=pass,2=fail ): " ); 19 scanf( "%d", &result ); 2007 Pearson Education, Inc. All rights reserved.

130 21 /* if result 1, increment passes */ 22 if ( result == 1 ) { 23 passes = passes + 1; 24 } /* end if */ 25 else { /* otherwise, increment failures */ 26 failures = failures + 1; 27 } /* end else */ 28 if and else statements are nested inside while loop 29 student = student + 1; /* increment student counter */ 30 } /* end while */ /* termination phase; display number of passes and failures */ 33 printf( "Passed %d\n", passes ); 34 printf( "Failed %d\n", failures ); /* if more than eight students passed, print "raise tuition" */ 37 if ( passes > 8 ) { 38 printf( "Raise tuition\n" ); 39 } /* end if */ return 0; } Pearson Education, Inc. All rights reserved.

131 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 2 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 1 Passed 9 Failed 1 Raise tuition Pearson Education, Inc. All rights reserved.

132 132 Performance Tip Initializing variables when they are defined can help reduce a program s execution time. Many of the performance tips we mention result in nominal improvements, so the reader may be tempted to ignore them. Note that the cumulative effect of all these performance enhancements can make a program perform significantly faster. Also, significant improvement is realized when a supposedly nominal improvement is placed in a loop that may repeat a large number of times.

133 Assignment Operators Assignment operators abbreviate assignment expressions c = c + 3; can be abbreviated as c += 3; using the addition assignment operator Statements of the form variable = variable operator expression; can be rewritten as variable operator= expression; Examples of other assignment operators: d -= 4 (d = d - 4) e *= 5 (e = e * 5) f /= 3 (f = f / 3) g %= 9 (g = g % 9)

134 134 Assignment operator Sample expression Explanation Assigns Assume: int c = 3, d = 5, e = 4, f = 6, g = 12; += c += 7 c = c to c -= d -= 4 d = d to d *= e *= 5 e = e * 5 20 to e /= f /= 3 f = f / 3 2 to f %= g %= 9 g = g % 9 3 to g Fig Arithmetic assignment operators.

135 3.12 Increment and Decrement Operators 135 Increment operator (++) Can be used instead of c+=1 Decrement operator (--) Can be used instead of c-=1 Preincrement Operator is used before the variable (++c or --c) Variable is changed before the expression it is in is evaluated Postincrement Operator is used after the variable (c++ or c--) Expression executes before the variable is changed

136 3.12 Increment and Decrement Operators 136 If c equals 5, then printf( "%d", ++c ); Prints 6 printf( "%d", c++ ); Prints 5 In either case, c now has the value of 6 When variable not in an expression Preincrementing and postincrementing have the same effect ++c; printf( %d, c ); Has the same effect as c++; printf( %d, c );

137 137 Operator Sample expression Explanation ++ ++a Increment a by 1, then use the new value of a in the expression in which a resides. ++ a++ Use the current value of a in the expression in which a resides, then increment a by b Decrement b by 1, then use the new value of b in the expression in which b resides. -- b-- Use the current value of b in the expression in which b resides, then decrement b by 1. Fig Increment and decrement operators.

138 1 /* Fig. 3.13: fig03_13.c 2 Preincrementing and postincrementing */ 3 #include <stdio.h> 4 5 /* function main begins program execution */ 6 int main( void ) 7 { 8 int c; /* define variable */ 9 10 /* demonstrate postincrement */ 11 c = 5; /* assign 5 to c */ 12 printf( "%d\n", c ); /* print 5 */ 13 printf( "%d\n", c++ ); /* print 5 then postincrement */ 14 printf( "%d\n\n", c ); /* print 6 */ /* demonstrate preincrement */ 17 c = 5; /* assign 5 to c */ 18 printf( "%d\n", c ); /* print 5 */ 19 printf( "%d\n", ++c ); /* preincrement then print 6 */ 20 printf( "%d\n", c ); /* print 6 */ return 0; } /* end function main */ c is printed, then incremented c is incremented, then printed Pearson Education, Inc. All rights reserved.

139 Pearson Education, Inc. All rights reserved.

140 140 Good Programming Practice Unary operators should be placed directly next to their operands with no intervening spaces.

141 141 Common Programming Error Attempting to use the increment or decrement operator on an expression other than a simple variable name is a syntax error, e.g., writing ++(x + 1).

142 142 Operators Associativity Type ++(post) -- (post) right to left postfix + - (type) ++ (pre) -- (pre) right to left unary * / % left to right multiplicative + - left to right additive < <= > >= left to right relational ==!= left to right equality?: right to left conditional = += -= *= /= %= right to left assignment Fig Precedence of the operators encountered so far in the text.

143 143 Debugging Your program compiles, but it does not run correctly! What to do now? Ask your more knowledgeable colleagues? Read the lecture notes Read a C textbook Read the code statement by statement Walk-through Check the effect of each statement on the state of the program

144 144 Debugging Read the code statement by statement Walk-through: Check the effect of each statement on the state of the program State = Values of attributes of each object Value of each simple variable Value of individual attributes of other program elements Make a table