Chapter 3

Functions

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Overview 3.2 Using C++ functions

Passing arguments Header files & libraries

3.4-5 Writing C++ functions Prototype Definition Parameters

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Functions

Every C++ program must have a main function execution starts by looking for a “main”

All other functions are called directly from main, or indirectly through a chain of function called from main

Function Calls One function calls another by using the name of the called

function next to ( ) enclosing an argument list

ex. strlen(FirstName) A function call temporarily transfers control from the

calling function to the called function

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FunctionName(Argument List)

The argument list is a way for functions to communicate with each other by passing information

The argument list can contain 0 or more actual arguments, separated by commas

Function call syntax

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What does the function do?

The function uses it actual arguments (inputs) to return a calculation To produce a side-effect

The function's return value is substituted for the function call in the expression

If the function does things other than a calculation (such as print a message to the screen), it is said to produce side-effects

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Example of calling a function

#include <iostream>#include <cmath>using namespace std;int main(){ float x, root; cout << "Enter a number: "; cin >> x; root = sqrt(x); cout << endl << "The square root of " << x << " is " << root << "." << endl; return 0;}

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Using library functions

We've already seen how to use library-provided objects like cin and cout to manage I/O Libraries also contain the implementation of functions

A library has 2 parts Interface (stored in a header file) tells what items are in the

library and how to use them Implementation (stored in another file) contains the definitions of

the items in the library

#include <iostream> Refers to the header file for the iostream library needed for use of

cout and endl

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Some Mathematical Library Functions

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Some Mathematical Library Functions

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User Defined Functions

In addition to the functions in libraries, programmers can use functions that they write on their own

Using functions for common code segments can improve your program in several ways: more readable is easier to update modular code

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int Cube ( int n ){ return n * n * n;

}

The heading declares the function’s name, specifying its return type and the name and type of each of its parameters

The body is a compound statement (block) which defines the behavior of the function

Two parts of a function definition

Heading

Body

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What is in a heading?

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What is in a prototype? A prototype looks like the heading, followed by a

semicolon parameter list must contain the type of each parameter,

but names are optional Prototypes are used to declare functions before they

are definedfloat volume(float, float);

//C++ does require parameter names in prototypes

float volume(float height, float radius);

//it is good style to include them

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#include <iostream>

int GCD (int n1, int n2); // prototype

using namespace std;

int main()

{

int x, y;

cout << "enter 2 positive integers: ";

cin >> x >> y;

cout << "The GCD of " << x << " and " << y

<< " is " << GCD( x, y) << endl;

cout << "The GCD of " << x << " and 100 "

<< " is " << GCD( x, 100) << endl;

cout << "The GCD of " << y << " and " << x*x - 1

<< " is " << GCD( y, x*x -1) << endl;

return 0;

}

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A program with several functions

Square function

Cube function

function prototypes

main function

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A complete program: prototypes

#include <iostream> int Square (int) ; // prototypesint Cube (int) ;using namespace std; int main(){ cout << "The square of 27 is " << Square (27) << endl; cout << "The cube of 27 is " << Cube (27) << endl; return 0;}

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A complete program: definitions

int Square (int n){ return n * n;}

int Cube (int n){ return n * n * n;}

18void function has no return value

void DisplayMessage(int n){ cout << "I have liked math for " << n << " years" << endl; return ;}

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Classified by location

Always appear in a function call, using variables whose scope is within the calling statement.

Always appear in the function heading, or function prototype. Their scope is only within the function definition.

Arguments Parameters

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Equivalent terms The term "argument", as used in this slideshow, is

equivalent to Actual argument used in our text Actual parameter (confusing, but used by others)

The term "parameter", as used in this slideshow, is equivalent to Formal parameter used in our text

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Scope

A variable or constant can be declared outside of any function Such variables or constants are known as globals Globals can be accessed from anywhere in a program

Variables declared in a function are known as local variables Local variables can only be referenced from within the

function in which they are declared You may use global constants, but no CS201

program should use global variables

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Drivers and Stubs

When building a large project, it is sometimes helpful to test each function in isolation This allows the programmer to test a single function before the

entire program is written

The function to be tested must be called A simple main function, called a stub, can call the function with

appropriate arguments

The function to be tested might call other functions which have not yet been implemented Use functions which match the signature of these, but which do

no real work, are called stubs

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Strings

C++ allows programmers to create new data types, called classes

Many classes have been implemented in the standard libraries The <string> library implements the string class,

which is used to store a sequence of characters

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Declaring a string

If you include the string library in your program, you can declare string variables

#include <string>string first_name, last_name;

You can assign string literals to a string variable

last_name = "Doe"; first_name = "John";

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String operations

Mathematical operations, such as multiplication and division, have no meaning for strings But there are operations that do make sense

String concatenation full_name = first_name + last_name;

Nth position

char first_initial = first_name[0];

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String member functions

There are several functions designed for strings Called member functions, they are invoked differently

int size = first_name.length();

int location = first_name.find("o");

There is also a function that allows a programmer to read an entire line of input into a string cin >> my_string; //reads one word

getline(cin, my_string); //reads whole line