Chapter 4

Chapter Four
Functions in C++
1

• Function - a subprogram that can act on data and return
a value
• Every C++ program has at least one function, main(),
where program execution begins
• A C++ program might contain more than one function.
• Functions may interact using function call
• Functions in C++ come in two varieties:
– user-defined
– built-in
2

3
Predefined Functions (continued)
• Some of the predefined mathematical functions are:
sqrt(x)
pow(x,y)
floor(x)
• Predefined functions are organized into separate
libraries
• I/O functions are in iostream header
• Math functions are in cmath header

4
The Power Function (pow)
• pow(x,y) calculates xy, pow(2,3) = 8.0
• pow returns a value of type double
• x and y are called the parameters (or
arguments) of the function pow
• Function pow has two parameters

5
The sqrt and floor Functions
• The square root function sqrt(x)
– Calculates the non-negative square root of x, for
x >= 0.0
– sqrt(2.25) is 1.5
– Type double
– Has only one parameter

6
The sqrt and floor Functions
(continued)
• The floor function floor(x)
– Calculates largest whole number not greater than
x
– floor(48.79) is 48.0
– Type double
– Has only one parameter

Declaration of Functions
• Functions must be declared before use
• The declaration tells the compiler
– The name,
– Return type,
– Parameters of the function
• Three ways
– Write your prototype into a file, and then use the #include
directive to include it in your program.
– Write the prototype into the file in which your function is
used.
– Define the function before it is called by any other function.
9

Function Prototypes
• The declaration of a function is called its prototype
• Is a statement - it ends with a semicolon
• It consists of the function's
– return type,
– name,
– parameter list
• Syntax
– return_type function_name (type
[parameterName1], type [ParameterName2] ... );
• E.g. long Area(int, int);
Or
long Area(int length, int width);
10

Function Prototypes
• All functions have a return type
• If the function doesn’t have a return type void will be used
– void is a reserved word
• The function prototype usually placed at the beginning of the
program
• The definition of the prototype must be given
• Many of the built-in functions have their function prototypes already
written in the header files you include in your program by using
#include
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Defining a Function
• The definition tells the compiler how the function works.
• Consists of :
– the function header :
• like the function prototype except that the parameters must be
named
• there is no terminating semicolon
– its body
• the task of the function
• Syntax
return_type function_name(parameter declarations)
{
declarations;
statements;
}
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Defining a Function
• E.g.
long Area(long l, long w)
{
return l * w;
}
• The return statement without any value is typically used
to exit the function early
• C++ does not allow nested functions
– The definition of one function cannot be included in
the body of another function
• A function definition must agree in return type and
parameter list with its prototype
13

// Creating and using a programmer-defined function.
#include <iostream.h>
int square( int ); // function prototype
int main()
{
// loop 10 times and calculate and output
// square of x each time
for ( int x = 1; x <= 10; x++ )
cout << square( x ) << " "; // function call
cout << endl;
return 0; // indicates successful termination
} // end main
// square function definition returns square of an integer
int square( int y ) // y is a copy of argument to function
{
return y * y; // returns square of y as an int
} // end function square
Definition of square. y is a
copy of the argument passed.
Returns y * y, or y squared.
Function prototype: specifies data types
of arguments and return values.
square expects an int, and returns
an int.
Parentheses () cause function to be called.
When done, it returns the result.
1 4 9 16 25 36 49 64 81 100
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Program Using a Function
double Celsius_to_Fahr(double); //Function Prototype
int main()
{
double temp,result;
cout<<“enter the temperature”<<endl;
cin>>temp;
result= Celsius_to_Fahr(temp);
cout<<“the corresponding Fahrenheit is”<<result<<
endl;
return 0;
}
double Celsius_to_Fahr(double Celsius)
{
double temp; // Declare variables
temp = (9.0/5.0)*Celsius + 32; // Convert
return temp;
}
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Scope of identifier
• Refers to where in the program an identifier is accessible
• Determines how long it is available to your program and
where it can be accessed
• Two kind
– Local identifier - identifiers declared within a function
(or block)
– Global identifier – identifiers declared outside of every
function definition
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Local scope
• You can declare variables within the body of the function
– local variables
– When the function returns, the local variables are no
longer available
• Variables declared within a block are scoped to that
block – Local to that block
– they can be accessed only within that block and "go
out of existence" when that block ends
– E.g.
for (int i = 0;i<5; i++)
cout<<i;
i=+10; // compilation error i is inaccessible
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Global Scope
• Variables defined outside of any function have global
scope
• Available for any function in the program, including main()
• A local variable with the same name as a global
variable hides the global variable - when used within
the function
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void myFunction(); //
prototype
int x = 5, y = 7; //
global variables
int main()
{
cout << "x from main: "
<< x << "n";
cout << "y from main: "
<< y << "nn";
myFunction();
cout << "Back from
myFunction!nn";
cout << "x from main: "
<< x << "n";
cout << "y from main: "
<< y << "n";
return 0;
}
void myFunction()
{
int y = 10;
cout << "x from myFunction:
" << x << "n";
cout << "y from myFunction:
" << y << "nn";
}
Output:
x from main: 5
y from main: 7
x from myFunction: 5
y from myFunction: 10
Back from myFunction!
x from main: 5
y from main: 7
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Unary Scope Resolution Operator ::
• Using ::, one can access an global variable even
if it is over-shadowed by a local variable of the
same name.
• E.g.
float num=10.8;
int main()
{
float num= 9.66;
cout<<“the value of num is:”<<::num<<endl;
return 0;
}
20

inline Functions
• Function calls
– Cause execution-time overhead
• Qualifier inline before function return type
"advises" a function to be inlined
• Puts copy of function's code in place of function call
– Speeds up performance but increases file size
– Compiler can ignore the inline qualifier
• Ignores all but the smallest functions
– E.g.
inline double cube( const double s )
{
return s * s * s;
} 21

inline functions
• Advantage: function call overhead is eliminated,
thus faster and less memory consuming
• Disadvantage: the code is expanded during
compilation so that executable file is large
22

Functions with Default Parameters
• When a function is called
– The number of actual and formal parameters must be
the same
• C++ relaxes this condition for functions with default
parameters
• You specify the value of a default parameter when the
function name appears for the first time, such as in the
prototype
23

# include<iostream>
Using namespace std;
int product (int x, int y=3)
{
int multiply;
multiply = x* y;
return multiply;
}
void main()
{
// First call to function ‘product’
cout<<product(10)<<endl;
// Second call to function ‘product’
cout<<product(20,12);
}
24

Empty Parameter Lists
• functions can take no arguments
• To declare that a function takes no parameters:
– Write void or nothing in parentheses
• E.g
– void print1( void );
– void print2();
25

Parameter Passing
• Call by Value
– Value of the function argument passed to the
formal parameter of the function
– Copy of data passed to function
– Changes to copy do not change original
• Call by Reference
– Address of the function argument passed to the
formal parameter of the function
26

Call by Reference
• If a formal parameter is a reference parameter
– It receives the address of the corresponding actual parameter
– A reference parameter stores the address of the corresponding
actual parameter
• During program execution to manipulate the data
– The address stored in the reference parameter directs it to the
memory space of the corresponding actual parameter
– A reference parameter receives the address of the actual
parameter
• Reference parameters can:
– Pass one or more values from a function
– Change the value of the actual parameter
27

Call by Reference
• Reference parameters are useful in three
situations:
– Returning more than one value
– Changing the actual parameter
– When passing the address would save
memory space and time
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29
Reference Variable Example
int count;
int &x = count;
// Create x as an alias for count
count = 1;
cout << “x is “ << x << endl;
x++;
cout << “count is “ << count << endl;

// Initializing and using a reference.
#include <iostream>
using namespace std;
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;
} // end main
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Call by Value Example
/* Incorrect function to switch two values */
void swap(int a, int b)
{
int hold;
hold = a;
a = b;
b = hold;
return;
}
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int a = 3, b = 5;
Swap( a,b);
Cout<<a<<b;

Call by Reference Example
/* Correct function to switch two values */
void swap2(int& a, int& b)
{
int hold;
hold = a;
a = b;
b = hold;
return;
}
32

33
Recursion
• Functions can call themselves! This is
called recursion.
• Recursion is very useful – it’s often very
simple to express a complicated
computation recursively.

Finding Factorial Recursively
5!
5*4!
4*3!
3*2!
2*1!
1
5!
5*4!
4*3!
3*2!
2*1!
1
Final value=120
1
2!=2*1=2 returned
3!=3*2=6 returned
4!=4*6=24 returned
5!=5*24=120 returned
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Finding Factorial iteratively
#include<iostream.h>
unsigned long factorial(unsigned long);//prototype
int main()
{
int num;
cout<<"enter a positive integer:";
cin>>num;
cout<<"The factorial of "<<num<<" is:
"<<factorial(num)<<endl;
return 0;
}
unsigned long factorial(unsigned long n)
{
unsigned long fact = 1;
for( int i=1; i<=n; i++)
fact*=i;
return fact;
}
35

//Recursive factorial Function
#include<iostream.h>
unsigned long factorial(unsigned long);//prototype
int main()
{
int num;
cout<<“enter a positive integer:”;
cin>>num;
cout<<“factorial=“<<factorial(num);
return 0;
}
unsigned long factorial(unsigned long n)
{
if ( n <= 1) //the base case
return 1;
else
return n * factorial (n - 1);
}
Finding Factorial Recursively
36

37
Designing Recursive Functions
• Define “Base Case”:
– The situation in which the function does not
call itself.
• Define “recursive step”:
– Compute the return value the help of the
function itself.

38
Recursion Base Case
• The base case corresponds to a case in which you know
the answer (the function returns the value immediately),
or can easily compute the answer.
• If you don’t have a base case you can’t use recursion!
(and you probably don’t understand the problem).

39
Recursive Step
• Use the recursive call to solve a sub-
problem.
– The parameters must be different (or the
recursive call will get us no closer to the
solution).
– You generally need to do something besides
just making the recursive call.

40
Recursion is a favorite test topic
• Write a recursive C++ function that
computes the area of an nxn square.
n
n
Base case:
n=1 area=1
Recursive Step:
area = n+n-1+area(n-1)

41
Recursive area function
int area( int n) {
if (n == 1)
return(1);
else
return( n + n - 1 + area(n-1) );
}

42
Recursion Exercise
• Write a function that prints a triangle:
triangle(4); triangle(5);
* *
*** ***
***** *****
******* *******
*********

• Function overloading
– Functions with same name and different
parameters
– Should perform similar tasks
• I.e., function to square ints and function to square floats
int square( int x) {return x * x;}
float square(float x) { return x * x; }
• A call-time c++ complier selects the proper function by
examining the number, type and order of the parameters
Function Overloading
43

// overloaded function
#include <iostream>
using namespace std;
int operate (int a, int b)
{
return (a*b);
}
float operate (float a, float b)
{
return (a/b);
}
int main ()
{
int x=5,y=2;
float n=5.0,m=2.0;
cout << operate (x,y);
cout << "n";
cout << operate (n,m);
cout << "n";
return 0;
}
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