Functions. 6.1 Modular Programming. 6.2 Defining and Calling Functions. Gaddis: 6.1-5,7-10,13,15-16 and 7.7

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1 Functions Unit 6 Gaddis: 6.1-5,7-10,13,15-16 and 7.7 CS 1428 Spring 2018 Ji Seaman 6.1 Moduar Programming Moduar programming: breaking a program up into smaer, manageabe components (modues) Function: a coection of statements that perform a task, grouped into a singe named unit. Why is moduar programming important? Improves maintainabiity/readabiity of programs by giving structure and organization to the code Simpifies the process of writing programs: programmer can write one sma function at a time Defining and Caing Functions Function definition: statements that make up a function, aong with its name, parameters and return type. return-type function-name (parameters) statements Function ca: statement (or expression) that causes a function to execute function-name (arguments) 3 4

2 Function Definition A Function definition incudes: return type: data type of the vaue that the function returns to the part of the program that caed it. function-name: name of the function. Function names foow same rues as variabes. parameters: optiona ist of variabe definitions. These wi be assigned vaues each time the function is caed. body: statements that perform the function s task, encosed in. 5 Function header (in the box) Function Definition return-type function-name (parameters) statements 6 Function Return Type Caing a Function If a function computes and returns a vaue, the type of the vaue it returns must be indicated as the return type: int getrate() return 8; If a function does not return a vaue, its return type is void: void printheading() cout << "Monthy Saes\n"; 7 To execute the statements in a function, you must ca it from within another function (ike main). To ca a function, use the function name foowed by a ist of expressions (arguments) in parens: printheading(); Whenever caed, the program executes the body of the caed function (it runs the statements). After the function terminates, execution resumes in the caing function after the function ca. 8

3 Exampe: Functions in a program #incude <iostream> void dispaymessage() cout << Heo from the function dispaymessage.\n ; Functions in a program Heo from main. Heo from the function dispaymessage. Back in function main again. Fow of Contro (order of statements): int main() cout << Heo from Main.\n ; dispaymessage(); cout << Back in function Main again.\n ; 9 Contro aways starts at main 10 Caing Functions: rues 6.3 Function Prototypes A program is a coection of functions, one of which must be caed main. Function definitions can contain cas to other functions. A function must be defined before it can be caed In the program text, the function definition must occur before a cas to the function Uness you use a prototype 11 Compier must know the foowing about a function before it can process a function ca: name, return type and data type (and order) of each parameter Not necessary to have the body of the function before the ca. Sufficient to put just the function header before a functions containing cas to that function The compete function definition must occur ater in the program. The header aone is caed a function prototype 12

4 #incude <iostream> // function prototypes void first(); void second(); Prototypes in a program cout << "I am starting in function main.\n"; first(); // function ca second(); // function ca cout << "Back in function main again.\n"; Prototype Stye Notes Pace prototypes near the top of the program (before any other function definitions)--good stye. Using prototypes, you can pace function definitions in any order in the source fie // function definitions void first() cout << "I am now inside the function first.\n"; void second() cout << "I am now inside the function second.\n"; 13 Common stye: a function prototypes at beginning, foowed by definition of main, foowed by other function definitions Sending Data into a Function You can pass (or send) vaues to a function in the function ca statement. This aows the function to work over different vaues each time it is caed. Arguments: Expressions (or vaues) passed to a function in the function ca. Parameters: Variabes defined in the function definition header that are assigned the vaues passed as arguments. A Function with a Parameter void dispayvaue(int num) cout << "The vaue is " << num << end; num is the parameter. Cas to this function must provide an argument (expression) that has an integer vaue: dispayvaue(5); 5 is the argument

5 Function with parameter in program #incude <iostream> // Function Prototype void dispayvaue(int); cout << "I am passing 5 to dispayvaue.\n"; dispayvaue(5); cout << "Back in function main again.\n ; dispayvaue(8); //ca again with diff. argument // Function definition void dispayvaue(int num) cout << "The vaue is " << num << end; I am passing 5 to dispayvaue. The vaue is 5 Back in function main again. The vaue is 8 17 Parameter Passing Semantics Given this function ca, with the argument of 5: dispayvaue(5); Before the function body executes, the parameter (num) is initiaized to the argument (5), ike this: int num = 5; //this stmt is executed impicity Then the body of the function is executed, using num as a reguar variabe: cout << "The vaue is " << num << end; 18 Parameters in Prototypes and Function Definitions The prototype must incude the data type of each parameter inside its parentheses: void evenorodd(int); //prototype The definition must incude a definition for each parameter in its parens void evenorodd(int num) //header if (num%2==0) cout << even ; ese cout << odd ; The ca must incude an argument (expression) for each parameter, inside its parentheses evenorodd(x+10); //ca 19 Passing Mutipe Arguments When caing a function that has mutipe parameters: the foowing must a match: the number and order of data types in the prototype the number and order of parameters in the function definition the number and order of arguments in the function ca the first argument wi be used to initiaize the first parameter, the second argument to initiaize the second parameter, etc. they are assigned in order. void power(int, int); //prototype 20

6 Exampe: function cas function Exampe: ca function more than once void deeper() cout << I am now in function deeper.\n ; void deep() cout << Heo from the function deep.\n ; deeper(); cout << Back in function deep.\n ; cout << Heo from Main.\n ; deep(); cout << Back in function Main again.\n ; Heo from Main. Heo from the function deep. I am now in function deeper. Back in function deep. Back in function Main again. 21 #incude <iostream> #incude <cmath> void puses(int count) for (int i = 0; i < count; i++) cout << "+"; cout << end; int x = 2; puses(4); puses(x); puses(x+5); puses(pow(x,3.0)); Exampe: mutipe parameters 6.7 The return statement #incude <iostream> #incude <cmath> void puses(char ch, int count) for (int i=0; i < count; i++) cout << ch; cout << end; int x = 2; char cc = '!'; puses('#',4); puses('*',x); puses(cc,x+5); puses('x',pow(x,3.0)); #### **!!!!!!! xxxxxxxx return; Used to stop the execution of a void function Can be paced anywhere in the function body the function immediatey transfers contro back to the statement that caed it. Statements that foow the return statement wi not be executed In a void function with no return statement, the compier adds a return statement before the ast 23 24

7 The return statement: exampe void somefunc (int x) if (x < 0) cout << x must not be negative. << end; ese // Continue with ots of statements, indented //... // so many it s hard to keep track of matching void somefunc (int x) This is equivaent, easier to read if (x < 0) cout << x must not be negative. << end; return; // Continue with ots of statements, ess indentation, // no brackets to try to match Returning a vaue from a function You can use the return statement in a non-void function to send a vaue back to the function ca: return expr; The vaue of the expr wi be sent back. The data type of expr must be paced in the function header: Return type: int doubeit(int x) return x*2; Vaue being returned 26 Caing a function that returns a vaue If the function returns void, the function ca is a statement: puses(4); If the function returns a vaue, the function ca is an expression: int y = doubeit(4); The vaue of the function ca (underined) is the vaue of the expr returned from the function, and you shoud do something with it. 27 Returning the sum of two ints #incude <iostream> int sum(int,int); int vaue1; int vaue2; int tota; cout << "Enter 2 numbers: " << end; cin >> vaue1 >> vaue2; tota = sum(vaue1, vaue2); cout << "The sum is " << tota << end; int sum(int x, int y) return x + y; Enter 2 numbers: The sum is 60 28

8 Data transfer The function ca from main: sum(vaue1, vaue2) passes the vaues stored in vaue1 and vaue2 (20 and 40) to the function, assigning them to x and y. The resut, x+y (60), is returned to the ca and stored in tota. x x y y 29 Function ca expression When a function ca cas a function that returns a vaue, it is an expression. The function ca can occur in any context where an expression is aowed: assign to variabe (or array eement) output via cout use in a more compicated expression pass as an argument to another function tota = sum(x,y); cout << sum(x,y); cout << sum(x,y)*.1; z = pow(sum(x,y),2); The vaue of the function ca is determined by the vaue of the expression returned from the 30 function. 6.9 Returning a booean vaue boo isvaid(int number) boo status; if (number >=1 && number <= 100) status = true; ese status = fase; return status; the above function is equivaent to this one: boo isvaid (int number) return (number >=1 && number <= 100); 31 Returning a booean vaue You can ca the function in an if or whie: boo isvaid(int); int va; cout << Enter a vaue between 1 and 100: cin >> va; whie (!isvaid(va)) cout << That vaue was not in range.\n ; cout << Enter a vaue between 1 and 100: cin >> va; //... 32

9 6.5 Passing Data by Vaue (review) Pass by vaue: when an argument is passed to a function, its vaue is copied into the parameter. Parameter passing is impemented using variabe initiaization (behind the scenes): int param = argument; Changes to the parameter in the function definition cannot affect the vaue of the argument in the ca 33 #incude <iostream> void changeme(int); Exampe: Pass by Vaue int number = 12; cout << "number is " << number << end; changeme(number); cout << "Back in main, number is " << number << end; int myvaue = number; void changeme(int myvaue) myvaue = 200; cout << "myvaue is " << myvaue << end; changeme faied! number is 12 myvaue is 200 Back in main, number is Pass by Vaue notes When the argument is a variabe (as in f(x)): The parameter is initiaized to a copy of the argument s vaue. Even if the body of the function changes the parameter, the argument in the function ca is unchanged. The parameter and the argument are stored in separate variabes, separate ocations in memory Passing Data by Reference Pass by reference: when an argument is passed to a function, the function has direct access to the origina argument. Pass by reference in C++ is impemented using a reference parameter, which has an ampersand (&) in front of it: void changeme (int &myvaue); A reference parameter acts as an aias to its argument. Changes to the parameter in the function DO affect the vaue of the argument 36

10 Exampe: Pass by Reference #incude <iostream> void changeme(int &); number is 12 myvaue is 200 Back in main, number is 200 int number = 12; cout << "number is " << number << end; changeme(number); cout << "Back in main, number is " << number << end; myvaue is an aias for number, ony one shared variabe void changeme(int &myvaue) this statement changes number myvaue = 200; cout << "myvaue is " << myvaue << end; 37 Using Pass by Reference for input doube square(doube number) return number * number; void getradius(doube &rad) cout << "Enter the radius of the circe: "; cin >> rad; const doube PI = ; doube radius; doube area; cout << fixed << setprecision(2); getradius(radius); area = PI * square(radius); cout << "The area is " << area << end; During the function execution, rad is an aias to radius in the main program. 38 Pass by Reference notes Changes made to a reference parameter are actuay made to its argument The & must be in the function header AND the function prototype. The argument passed to a reference parameter must be a variabe it cannot be a constant or contain an operator (ike +) Use when appropriate don t use when: the argument shoud not be changed by function (!) the function returns ony 1 vaue: use return stmt! Loca and Goba Variabes Variabes defined inside a function are oca to that function. They are hidden from the statements in other functions, which cannot access them. Because the variabes defined in a function are hidden, other functions may have separate, distinct variabes with the same name. This is not bad stye. These are easy to keep straight Parameters are aso oca to the function in which they are defined. 40

11 Loca variabes are hidden from other functions #incude <iostream> void anotherfunction(); In main, num is 1 In anotherfunction, num is 20 Back in main, num is 1 This num variabe is visibe int num = 1; ony in main cout << "In main, num is " << num << end; anotherfunction(); cout << "Back in main, num is " << num << end; void anotherfunction() This num variabe is visibe ony in anotherfunction int num = 20; cout << "In anotherfunction, num is " << num << end; 41 Loca Variabe Lifetime A function s oca variabes and parameters exist ony whie the function is executing. When the function begins, its parameters and oca variabes (as their definitions are encountered) are created in memory, and when the function ends, the parameters and oca variabes are destroyed. This means that any vaue stored in a oca variabe is ost between cas to the function in which the variabe is decared. 42 Goba Variabes A goba variabe is any variabe defined outside a the functions in a program. The scope of a goba variabe is the portion of the program starting from the variabe definition to the end of the fie This means that a goba variabe can be accessed by a functions that are defined after the goba variabe is defined A oca variabe may have the same name as a goba variabe. The goba variabe is hidden in that variabe s bock. 43 Goba Variabes: exampe #incude <iostream> void anotherfunction(); int num = 2; cout << "In main, num is " << num << end; anotherfunction(); cout << "Back in main, num is " << num << end; In main, num is 2 In anotherfunction, num is 2 But now it is changed to 50 Back in main, num is 50 void anotherfunction() cout << "In anotherfunction, num is " << num << end; num = 50; cout << "But now it is changed to " << num << end; 44

12 Goba Variabes/Constants Do not use goba variabes!!! Because: They make programs difficut to debug. If the wrong vaue is stored in a goba var, you must scan the entire program to see where the variabe is changed Functions that access gobas are not sefcontained cannot easiy reuse the function in another program. cannot understand the function without understanding how the goba is used everywhere It is ok (and good) to use goba constants because their vaues do not change. 45 const doube PI = ; doube getarea(doube number) return PI * number * number; doube getperimeter(doube number) return PI * 2 * number; doube radius; cout << fixed << setprecision(2); cout << "Enter the radius of the circe: "; cin >> radius; Goba Constants: exampe Enter the radius of the circe: 2.2 The area is The perimeter is cout << "The area is " << getarea(radius) << end; cout << "The perimeter is " << getperimeter(radius) << end; 46 Functions and Array Eements Functions and Array Eements An array eement can be passed to any parameter of the same (or compatibe) type: doube square (doube); doube numbers[5] = 2.2, 3.3, 5.11, 7.0, 3.2; for (int i=0; i<5; i++) cout << square(numbers[i]) << " "; cout << end; doube square (doube x) return x * x; An array eement can be passed by reference. What is output by this program? void changeme(int &myvaue) myvaue = 200; int numbers[5] = 2, 3, 5, 7, 3; for (int i=0; i<5; i++) changeme(numbers[i]); for (int i=0; i<5; i++) cout << numbers[i] << " "; cout << end; 48

13 7.7 Arrays as Function Arguments An entire array can(!) be passed to a function that has an array parameter void showarray(int[], int); int numbers[5] = 2, 3, 5, 7, 3; showarray(numbers,5); void showarray(int vaues[], int size) for (int i=0; i<size; i++) cout << vaues[i] << " "; cout << end; Passing arrays to functions In the function definition, the parameter type is a variabe name with an empty set of brackets: [ ] Do NOT give a size for the parameter void showarray(int vaues[], int size) In the prototype, empty brackets go after the eement datatype. void showarray(int[], int); In the function ca, use the variabe name for the array (no brackets!). showarray(numbers, 5); 50 Passing arrays to functions An array is aways passed by reference. The parameter name is an aias to the array being passed in, even though it has no &. Changes made to the array (eements) inside the function DO affect the array in the function ca. 51 Passing arrays to functions Changing an array inside a function: void incrarray(int[], int); void showarray(int[], int); int numbers[5] = 2, 3, 5, 7, 3; incrarray(numbers,5); showarray(numbers,5); void incrarray(int vaues[], int size) for (int i=0; i<size; i++) (vaues[i])++; //vaues[i]=vaues[i]+1; 52

14 Passing arrays to functions Usuay functions that have an array parameter aso have an int parameter for the count of the number of eements in the array. so the function knows how many eements to process. The count parameter is just a reguar int parameter and must be incuded in the parameter ist and a corresponding argument vaue must appear in the function ca. 53