FM-AA-CIA-15 Rev. 0 10-July-2020
Study Guide in CC 103 Intermediate Programming
Module No. 1
STUDY GUIDE FOR MODULE NO. 1
USER-DEFINED FUNCTIONS
MODULE OVERVIEW
Functions are an integral part of C++ programming such that you can’t even create C++ programs without
using functions. In this module, we will be concentrating on the creation of your own functions.
MODULE LEARNING OBJECTIVES
By the end of this module you should be able to:



Identify the structure and appropriate use of user-defined functions
Differentiate and implement the proper use of void and non-void functions
Implement function prototypes
LEARNING CONTENTS (USER-DEFINED FUNCTION DEFINITION)
Introduction
A user-defined function is a group of codes performing a specific task and given with a specific name (function
name or identifier). When the function name is called in any part of the program, all codes inside it are
executed. Hence, we could use functions to segment our code to perform individual tasks.
1.1 Function Declaration
In order to declare a function, the following structure shall be used:
returnDataType functionName(parameter1, parameter2, ...)
{
//function body
statements;
return expression;
}
The returnDataType is the data type of the value in which the function will return. Some functions do not
return a value; which the keyword void should be used.
The functionName is the name of the function. The series of parameters after the function name is called the
parameter list. Parameters are variables that hold the value passed to the function when it is called.
Parameters are optional, that is why functions may contain no parameters.
The function name and the parameter list forms the function signature.
The function body contains the statements that are executed when the function is called.
Let’s start with examining the following codes:
C++ Code [function1.cpp]
#include<iostream>
using namespace std;
void printHello()
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Study Guide in CC 103 Intermediate Programming
Module No. 1
{
cout<<"Hello from User-defined Function!\n";
}
int main()
{
printHello();
cout<<"Hi from Main";
}
[Sample Output]
Hello from User-defined Function!
Hi from Main
Extracted from the code above, we have the following user-defined function:
In the example, the return data type for the function is void, marking the function as a void function. Void functions do not return a
value to the function caller. It is important to note that the function void printHello() is defined before the main function, as we have not
yet created a function prototype.
1.2 Function Call
The main function acts as our starting point for our program. Inside the function, we called the printHello()
function. When the function is called, the function body of that function is executed. After executing the
function body, the execution returns to the function caller, which is the main function.
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Study Guide in CC 103 Intermediate Programming
Module No. 1
You can also call the function multiple times, executing the function body multiple times based on the number
of function calls.
1.3 Return values
We can create functions that return a value to the function caller. These functions are what we call non-void
functions. To declare a non-void function, we need to indicate a data type in the returnDataType part of our
function definition. Also inside the function body, a return statement shall be used to indicate the value to be
returned to the function caller.
Let’s examine the following code:
C++ Code [functio21.cpp]
#include<iostream>
using namespace std;
int square()
{
return 2*2;
}
int main()
{
int num=square();
cout<<num;
return 0;
}
[Sample Output]
4
We have a square() function which returns an int value, given that the returnDataType is int. It has also a
return statement which returns the value 4 from the given expression.
In the main function, after declaring the num variable, it is initialized from the returned value of the square()
function. Hence, the value of the num variable is 4.
Non-void functions can return any value depending on the specified return type.
We have provided before regarding void functions. These functions are not required to return a value which
explains the absence of the return statement inside the function body. Also, void functions declared with void
on the returnDataType.
1.4 Function Parameters
A parameter is a variable that holds the value that is passed when the function is called. In the previous
examples, our parameter list is empty. Let’s examine the following codes with a parameter.
C++ Code [function3.cpp]
#include<iostream>
using namespace std;
int square(int sq)
{
return sq*sq;
}
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Study Guide in CC 103 Intermediate Programming
Module No. 1
int main()
{
int num=0;
cout<<"Enter a number:";
cin>>num;
cout<<"The square of "<<num<<" is "<<square(num);
return 0;
}
[Sample Output]
Enter a number:5
The square of 5 is 25
The square() function has a parameter int sq. In the main function, when the square() function is called, the
value of num is passed to the sq variable. That is used in the sq*sq expression and returned to the function
caller.
You can declare multiple parameters with different data types. Let’s provide another example:
C++ Code [function4.cpp]
#include<iostream>
using namespace std;
float sum(int num1, float num2)
{
float ans=num1+num2;
return ans;
}
int main()
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Study Guide in CC 103 Intermediate Programming
Module No. 1
{
int n1{0};
float n2{0};
cout<<"Enter an integer number: ";
cin>>n1;
cout<<"Enter a float number: ";
cin>>n2;
cout<<"Sum: "<<sum(n1, n2);
return 0;
}
[Sample Output]
Enter an integer number: 3
Enter a float number: 4.7
Sum: 7.7
The sum() function returns a float value but accepts two parameters, int num1 and float num2. The result of
the operation from the function is displayed inside the main function.
1.5 Arguments
Looking back with the previous examples we have, we are passing values to the function through the use of
function parameters. The parameters act as placeholders for the values being passed. However, the value
that is passed to the function is what we call an argument.
Code Snippet
cout<<"Sum: "<<sum(n1, n2);
In the code snippet above, the function call sum will pass the values of n1 and n2 to the parameters of the
function. Hence, n1 and n2 are the arguments, while the num1 and num2 are the parameters.
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1.6 Function Prototype
We have already defined functions in the previous examples. However, you may notice that the function
definition comes before the function call. If we want to define a function after a function call, we need to use a
function prototype.
A function prototype consists of a function’s return type, parameters but with no function body.
returnDataType functionName(parameter1, parameter2, ...);
or
returnDataType functionName(dataType, dataType, ...);
Given the following example, check the line that serves as the function prototype.
C++ Code [function5.cpp]
#include<iostream>
using namespace std;
float sum(int num1, float num2);
int main()
{
int n1{0};
float n2{0};
cout<<"Enter an integer number: ";
cin>>n1;
cout<<"Enter a float number: ";
cin>>n2;
cout<<"Sum: "<<sum(n1, n2);
return 0;
}
float sum(int num1, float num2)
{
float ans=num1+num2;
return ans;
}
[Sample Output]
Enter an integer number: 1
Enter a float number: 1.2
Sum: 2.2
The line float sum(int num1, float num2); is the function prototype for our example. Also, notice that
the function definition for the sum() can now be placed under the main() function. Another way of declaring a
function prototype is:
C++ Code [function5.cpp]
#include<iostream>
using namespace std;
float sum(int, float);
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Study Guide in CC 103 Intermediate Programming
Module No. 1
int main()
{
int n1{0};
float n2{0};
cout<<"Enter an integer number: ";
cin>>n1;
cout<<"Enter a float number: ";
cin>>n2;
cout<<"Sum: "<<sum(n1, n2);
return 0;
}
float sum(int num1, float num2)
{
float ans=num1+num2;
return ans;
}
[Sample Output]
Enter an integer number: 1
Enter a float number: 1.2
Sum: 2.2
1.7 Function Overloading
In C++, different functions can have the same name. We call these as overloaded functions. For example,
int max (int x, int y);
double max (double x, double y);
In the example, two different functions share the same name which is max. The function max() was
overloaded for int and double return types. However, remember that functions can have the same name if
the number and/or data type of the arguments passed is different. Based on the type of parameter passed
during the function call, the appropriate function is called.
C++ Code [function5.cpp]
#include<iostream>
using namespace std;
int max(int, int);
double max(double, double);
int main(){
int n1 = 0, n2 = 0;
double n3 = 0, n4 = 0;
cout << "Enter two numbers(int):" << endl;
cin >> n1 >> n2;
cout << "Max: " << max(n1, n2) << endl;
cout << "Enter two numbers(double):" << endl;
cin >> n3 >> n4;
cout << "Max: " << max(n3, n4);
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Study Guide in CC 103 Intermediate Programming
Module No. 1
return 0;
}
int max (int x, int y){
if(x>y) return x;
else return y;
}
double max(double x, double y){
if(x>y) return x;
else return y;
}
[Sample Output]
Enter two numbers(int):
5
2
Max: 5
Enter two numbers(double):
3.4
6.2
Max: 6.2
LEARNING ACTIVITY 1
Create a non-void function in your program that will ask the user to enter an int number. The entered number
will be returned to the main function and will be displayed.
Create a void function named celsiusToFahrenheit that will convert a Celcius temperature passed to it and
displays the converted Fahrenheit temperature. The input should be done in the main function.
SUMMARY
User-defined functions make your code reusable as you can declare them once and use them multiple times.
Further, your codes divided into functions can be managed more easily and increases code readability.
Hence, the use of user-defined functions is encouraged.
REFERENCES
Zak, D. (2016). An Introduction to Programming with C++ (8E), pages 291-341
Online Reading Materials:
 http://cplusplus.com/doc/tutorial/functions/
 https://www.programiz.com/cpp-programming/function
 https://www.programiz.com/cpp-programming/user-defined-function-types
 https://www.programiz.com/cpp-programming/function-overloading
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