C++ Functions CS242 COMPUTER PROGRAMMING T.Banan Al-Hadlaq.

C++ Functions

CS242COMPUTER PROGRAMMING

T.Banan Al-Hadlaq

Content2

C++ Modules Function syntax Function parameters and return types Defining functions within programs Using and invoking function Function prototypes Function, file, and block scope Function signature Argument coercion Function Overloading Reference parameters Passing arguments (pass-by-value, pass-by-reference) Common errors

Modules

Blocks of codes with an independent functionality.

Written and tested separately.

Combined to form a complete program.

Modules either control or general:

Control

Called “Main”

each program include only one control module.

General

Perform manipulations on data e.g. calculation, print, read and

validation.

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C++ Modules

All Modules in C++ are called “functions”

called methods or procedures in other languages.

Functions can be

Programmer-defined.

Built-in: pow, sqrt, .. etc.

Advantage

make programs easier to write, test, debug and maintain.

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Functions Syntax5

ReturnType FctName(Parameter List)

{

action1;

action2;

..

actionN;

return value ;

}

Function Header

Function Body

321

1

2

Function Name and Return Type

Function name follows rules of naming identifiers as variables and

constants.

Function return ONLY one value.

Function return types: Built-in data types. User-defined data types. Nothing is returned by using keyword void.

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int f1(Parameter List){

return value ;

}

void f2(Parameter List){

return;

}

void f2(Parameter List){

}

Function Parameters

Provide the communication links between the main program and its

functions.

Parameters declared in the function header within parenthesis.

Either have a single, multiple parameters or have no parameters.

List of a parameters are separated by commas.

Each parameter must be defined as follows:

DataType parameterName

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char f1(int x, int y){

return value ;

}

Problem

Write a program that asks the user for the exam grade and prints a

message indicating if the user has passed the exam or not.

Can you solve the problem using functions ?

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#include <iostream>using namespace std; int main(){ int grade;

cout<<"Enter your grade:\n"; cin>>grade;

if (grade>60) cout<<"You passed"; else cout<<“You failed";

return 0;}

1 -Selecting Functions

Divide the program into tasks:

Read student grade.

Print appropriate message.

Naming functions:

Read_Grade

Display_Msg

main

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Read_Grade Display_Msg

main Fct

2-Analyzing Flow of Control10


main Fct Start here

Invo

ke to

read

inpu

t fro

m u

ser

1

2

Ret

urn

the

inpu

t rea

d fro

m th

e us

er

3

Invoke to display message A

ND

send

the grade returned by Read_G

rade

4

Display m

essage on screen

AN

D return control to M

ain

5

return 0 ;6

3-Defining Functions

For each function decide on: Possible Parameter List Possible Return Value

int Read_Grade(){

}

void Display_Message(int g){

}

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Read_Grade

Main FctInvoke to read

input from

user

Retu

rn the in

put read fro

m the user

Display_Msg

Main Fct

Invoke to display messag

e AN

D send

the grade return

ed by Read_

Grade

Disp

lay message on screen

AN

D return

control to Main

3-Defining Functions (cont.)12


Main Fct

#include <iostream>using namespace std; int main(){ int grade;

cout<<"Enter your grade:\n"; cin>>grade;

if (grade>60) cout<<"You passed"; else cout<<“You failed";

return 0;}

Read_Grade

Display_Msg

3-Defining Functions (cont.)13

int Read_Grade(){ int grade; //grade is a local variable cout<<"Enter your grade:\n"; cin>>grade;

return grade;}

void Display_Msg(int g){ if (g>60) cout<<"You passed"; else cout<<“You failed";}

grade scope

g scope

4-Using Functions14

#include <iostream>using namespace std;// ====== You Must Define your Modules before Main ======int Read_Grade(){ int grade; //grade is a local variable cout<<"Enter your grade:\n"; cin>>grade;

return grade;}void Display_Msg(int g){ if (g>60) cout<<"You passed"; else cout<<“You failed";} // ====================== Main Fct ======================int main(){ // Invoke read module and store returned input // Invoke display module and pass the grade return 0;}

4-Using Functions (cont.)

In our example, all invokations occur in main function.

Invoking C++ functions syntax:

Write function name followed by ().

Within ( ) passing values (arguments) to parameters.

Functions could have No parameters FunctionName() One or more parameters FunctionName(v1,v2)

Capturing returned values

Functions return type could be void FunctionName() OR FunctionName(v1,v2) Non-void must use returned value in expression.

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Capturing returned values Non-void must use returned value in expression

Function Read_grade() returns an int

int n = Read_Grade();

if ( Read_Grade() == 3 ) …… … …

cout << Read_Grade();

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Each value passed in a function call is referred to as an Argument.

Arguments (Passed in a Function call) and parameters (defined in a

function header) MUST match in:

Number

Order

Data types

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void Display_Msg(int g){ if (g > 60) cout<<"You passed"; else cout<<“You failed";}

int main(){

Display_Msg(87);

return 0;}

Argument

Parameter

Final Solution18

#include <iostream>using namespace std;// ====== You Must Define your Modules before Main ======int Read_Grade(){ int grade; //grade is a local variable cout<<"Enter your grade:\n"; cin>>grade;

return grade;}void Display_Msg(int g){ if (g>60) cout<<"You passed"; else cout<<“You failed";} // ====================== Main Fct ======================int main(){ int n = Read_Grade(); // Invoke read function Display_Msg(n); // Invoke display function return 0;}

Using Library Functions

Need to know the points below to write invokation of built-in functions

correctly:

Name of the function.

Number , type and order of parameters.

Type of return value if it has.

Header file.

Examples

Header File Function call Example Value

<cmath> pow(x,y) pow(2,3) 8 <iomanip> setprecision(n) setprecision(2)<< 1.2345 1.23

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Function Prototype

Consists of the function's return type, name and parameter list

(number, types and order of parameters).

Also called function declaration.

Usually placed at the top of the program to tell the compiler that the

function exists.

Same as corresponding function header except that:

Parameter names are optional

Must end with a semicolon ;

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Examples21

int square( int x )

{

return x * x;

}

void useLocal( void )

{

int x = 25;

cout << "\nlocal x is " << x << endl;

x++;

cout << "local x is " << x << endl;

}

void useLocal();

int square( int );

Compilation Error22

#include <iostream>using std::cin;using std::cout;using std::endl;

int main(){ int a = 10; cout << a << " squared: " << square( a ) << endl; return 0; }

int square( int x ){ return x * x; }

Error: function undeclared before use'square': identifier not found

Solution -123



int main(){ int a = 10; cout << a << " squared: " << square( a ) << endl; return 0; }

Solution -224

#include <iostream>using std::cin;using std::cout;

using std::endl;

int square( int ); // prototype for function square

int main(){ int a = 10;

cout << a << " squared: " << square( a ) << endl; return 0; }


Empty Parameter Lists

Specified by writing either void or nothing at all in parentheses.

int sum(); OR int sum(void);

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Scope

Identifiers Variable or object. Function.

Scope of Identifier Portion of the program where an identifier can be used.

Types of scopes for an identifier: Function scope. File scope. Block scope.

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File Scope

For an identifier declared outside any function or class

Identifier is “known” and can be used in all functions from the point at

which it is declared until the end of the file.

Examples

Global variables, function definitions and function prototypes placed

outside a function all have file scope.

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Function Scope

Can be used anywhere in the function in which they appear.

Cannot be referenced outside the function body.

Examples

Local variables and function parameters.

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Block Scope

Blocks are defined by the beginning left brace ( { ) and the

terminating right brace ( } ).

Identifiers declared inside a block have block scope

Block scope begins at the identifier’s declaration.

Block scope ends at the terminating right brace (}) of the block in which

the identifier is declared.

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#include <iostream>using std::cout;using std::endl;

int x = 1; // global variable

int main(){ int x = 5; // local variable to main

cout << "local x in main's outer scope is " << x << endl; { // start new scope int x = 7; // hides x in outer scope cout << "local x in main's inner scope is " << x << endl; } // end new scope cout << "local x in main's outer scope is " << x << endl;

return 0;}

File Scope (defined outside any class or function)

Function / Outer Block Scope (can be used only inside the outer block defined in Main)

Function / Inner Block Scope (can be used only inside the inner block defined in Main)

Function Signature

The portion of a function prototype that includes the name of the

function and the types of parameters.

Does not specify the function’s return type.

Functions in the same scope must have unique signatures.

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int square( int ); Function Signature

Function Prototype

Compilation Error -132



void square( int x) { cout<< x * x; }

int main(){ return 0; } // end main

• Two functions in the global(file) scope• They have the same signature:

square(int)• This results in a compilation error:

'square' : redefinition



class Num {


void square( int x) { cout<< x * x; } };

int main(){ return 0; // indicate successful termination} // end main

• Two functions in the class scope• They have the same signature:

square(int)• This results in a compilation error

'Num::square' : redefinition


Arguments (in function call) and parameters (in function header)

must match in Number, Order, and Data Types.

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#include <iostream> using namespace std;

int boxVolume( int length, int width, int height ) { return length * width * height; }

int main() { cout<< boxVolume(); // Error#1: 'boxVolume' : function does not take 0 arguments cout<< boxVolume(10); // Error#2: 'boxVolume' : function does not take 1 argument cout<< boxVolume(10,5); // Error#3: 'boxVolume' : function does not take 2 arguments cout<< boxVolume(10,5,2); return 0;}

Argument Coercion

Arguments (in function call) and parameters (in function header)

must match in Number, Order, and Data Types.

Promotion/Demotion may occur when the type of a function

argument does not match the specified parameter type.

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1 -Argument Demotion36


int boxVolume( int length, int width , int height ) ;

int main() { cout<< boxVolume(10.5,5.7,2.4); return 0;}

int boxVolume( int length , int width , int height ) { return length * width * height; }

Warning: 'argument' : conversion from 'double' to 'int', possible loss of data

2 -Argument Promotion37


void boxVolume( double length, double width , double height ) ;

int main() { boxVolume(10,5,2); return 0;}

void boxVolume(double length , double width , double height ) { cout<< length * width * height; }

No Warning is issued when promotion of arguments occurs

3 -Return Type Demotion38


double boxVolume( double length, double width , double height ) ;

int main() { int r = boxVolume(10.5,5.7,2.3); return 0; }

double boxVolume(double length , double width , double height ) { return length * width * height; }

warning : conversion from 'double' to 'int', possible loss of data

4 -Return Type Promotion39


int boxVolume( int length, int width , int height ) ;

int main() { double r = boxVolume(10,5,2); return 0; }

int boxVolume(int length , int width , int height ) { return length * width * height; }

No Warning is issued when promotion of return values occurs

Function Overloading

Creating several functions of the same name that perform similar

tasks, but on different data types.

Overloaded functions have:

Same name.

Different signature.

Compiler selects proper function to execute based on number,

types and order of arguments in the function call.

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// function square for int values int square( int x ) { cout << "square of integer " << x << " is "; return x * x; }

// function square for double values double square( double y ) { cout << "square of double " << y << " is "; return y * y; }

int main(){ cout << square( 7 ); // calls int version cout << endl; cout << square( 7.5 ); // calls double version cout << endl; return 0; }

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// function multiply for two parameters int multi( int x, int y ) { return x * y; }

// function multiply for one parameter int multi( int y ) { return y * 1; }

int main(){ cout << multi( 7,5 ); // calls 2

parameter version cout << endl; cout << multi( 7 ); // calls 1 parameter

version cout << endl; return 0; }

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// function multiply for two parameters int multi( int x, double y )

{ return x * y; }

// function multiply for two parameter but with different order

double multi( double y , int x) { return y * x; }

int main(){ cout << multi( 7,5.5 ); // calls

int,double version cout << endl; cout << multi( 7.5,3 ); // calls

double,int version cout << endl; return 0; }

Reference Variables44

// create an integer

int a ;

// Now create another name for the // same integer

int& b = a ;//ORInt &b = a ;

grab memory foran integer and labels it as "a"

"b" is just another label for the same memory

a

b

b is a "reference".

int& means "make a label to reference an integer"

a and b are synonyms for the same thing

Reference Variables (cont.) Can be used as aliases for other variables

An alias is simply another name for the original variable

All operations supposedly performed on the alias (i.e., the reference) are actually performed on the original variable

Must be initialized in their declarations Cannot be reassigned afterward

Example

int count = 1;

int &cRef = count;

cRef++;

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1

count

1

count

cRef

2

count

cRef

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int main(){ int x = 3; int &y = x; // y refers to (is an alias for) x

cout << "x = " << x << endl << "y = " << y << endl; y = 7; // actually modifies x cout << "x = " << x << endl << "y = " << y << endl; return 0; // indicates successful termination} // end main

x = 3y = 3x = 7y = 7


int main(){ int x = 3; int &y; // Error: y must be initialized

cout << "x = " << x << endl << "y = " << y << endl; y = 7; cout << "x = " << x << endl << "y = " << y << endl; return 0; }

Example #1

Example #2

Passing Arguments

Two ways to pass arguments to functions

Pass-by-value

A copy of the argument’s value is passed to the called function

Changes to the copy do not affect the original variable’s value in the

caller

Prevents accidental side effects of functions

Pass-by-reference

Gives called function the ability to access and modify the caller’s

argument data directly

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Reference Parameter

An alias for its corresponding argument in a function call

& placed after the parameter type in the function prototype and

function header

Example

void squareByReference( int &numberRef ) ;

Parameter name in the body of the called function actually refers to

the original variable in the calling function

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Pass By Value49


int squareByValue( int ); // function prototype (value pass)

int main(){ int x = 2;

// demonstrate squareByValue cout << "x = " << x << " before squareByValue\n"; cout << "Value returned by squareByValue: " << squareByValue( x ) << endl; cout << "x = " << x << " after squareByValue\n" << endl;

return 0; }

int squareByValue( int number ) { return number *= number; // caller's argument not modified

}

2

x

2

number

Pass By Reference50


int squareByReference( int& ); // function prototype (value pass)

int main(){ int z = 4;

// demonstrate squareByReference cout << "z = " << z << " before squareByReference" << endl;

squareByReference( z ); cout << "z = " << z << " after squareByReference" << endl;

return 0; }

void squareByReference( int &numberRef ) { numberRef *= numberRef; // caller's argument modified }

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z

numberRef

Common Errors

Compilation errors

Declaring function has parameters of the same type as char a,b instead of

char a, char b.

Invoking function before it is defined, e.g. when f1 invokes f2, f2 must be

defined before f1.

Attempting to use a function that has no return value in an expression.

Two functions in the same scope have the same signature.

Mismatching function header with function call OR function header with

function prototype in the number, type and order of parameters and

arguments.

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C++ Functions CS242 COMPUTER PROGRAMMING T.Banan Al-Hadlaq.

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