Control structures and statements in C and C++

Control structures form the basic entities of a “structured programming language“. We all know languages like C/C++ or Java are all structured programming languages. Control structures are used to alter the flow of execution of the program.  Why do we need to alter the program flow ? The reason is “decision making“! In life, we may be given with a set of option like doing “Electronics” or “Computer science”. We do make a decision by analyzing certain conditions (like our personal interest, scope of job opportunities etc). With the decision we make, we alter the flow of our life’s direction. This is exactly what happens in a C/C++ program. We use control structures to make decisions and alter the direction of program flow in one or the other path(s) available.

There are three types of control structures available in C and C++

1) Sequence structure (straight line paths)

2) Selection structure (one or many branches)

3)Loop structure (repetition of a set of activities)

All the 3 control structures and its flow of execution is represented in the flow charts given below.

Control statements in C/C++ to implement control structures

We have to keep in mind one important fact:- all program processes can be implemented with these 3 control structures only. That’s why I wrote “control structures are the basic entities of a structured programming language“. To  implements these “control structures” in a C/C++ program, the language provides ‘control statements’.  So to implement a particular control structure in a programming language, we need to learn how to use the relevant control statements in that particular language.

The control statements are:-

• Switch
• If
• If Else
• While
• Do While
• For

As shown in the flow charts:-

• Selection structures are implemented using If , If Else and Switch statements.
• Looping structures are implemented using While, Do While and For statements.  

Selection structures 

The simple If statement

Example program to demo “If” statement

Problem:- 

A simple example program to demo the use of If, If Else and Switch is shown here. An integer value is collected from user.
If the integer entered by user is 1 – output on screen “UNITED STATES”. If the integer is 2 – output “SPAIN”, If the integer is 3 output “INDIA”. If the user enters some other value – output “WRONG ENTRY”.

Note:- The same problem is used to develop example programs for “if else” and “switch” statements

#include
void main()
{
int num;
printf("Hello user, Enter a number");
scanf("%d",&num); // Collects the number from user
if(num==1)
{
printf("UNITED STATES");
}
if(num==2)
{
printf("SPAIN");
}
if(num==3)
{
printf("INDIA");
}
}

The If Else statement.

Example program to demo “If Else”


#include
void main()
{
int num;
printf("Hello user, Enter a number");
scanf("%d",&num); // Collects the number from user
if(num==1)
{
printf("UNITED STATES");
}
else if(num==2)
{
printf("SPAIN");
}
else if(num==3)
{
printf("INDIA");
}
else
{
printf("WRONG ENTRY"); // See how else is used to output "WRONG ENTRY"
}
}

Note:- Notice how the use of If Else statements made program writing easier. Compare this with above program using simple If statement only.

Switch statement

Example program to demo working of “switch” 

#include
void main()
{
int num;
printf("Hello user, Enter a number");
scanf("%d",&num); // Collects the number from user
switch(num)
{
case 1:
printf("UNITED STATES");
break;
case 2:
printf("SPAIN");
break;
case 3:
printf("INDIA");
default:
printf("WRONG ENTRY");
}
}

Loop structures

The while statement

Syntax for while loop is shown below:

while(condition)// This condition is tested for TRUE or FALSE. Statements inside curly braces are executed as long as condition is TRUE
{
statement 1;
statement 2;
statement 3;
}

The condition is checked for TRUE first. If it is TRUE then all statements inside curly braces are executed.Then program control comes back to check the condition has changed or to check if it is still TRUE. The statements inside braces are executed repeatedly, as long as  the condition is TRUE. When the condition turns FALSE, program control exits from while loop.

Note:- while is an entry controlled loop. Statement inside braces are allowed to execute only if condition inside while is TRUE.

Example program to demo working of “while loop”

An example program to collect a number from user and then print all numbers from zero to that particular collected number is shown below. That is, if user enters 10 as input, then numbers from 0 to 10 will be printed on screen.

Note:- The same problem is used to develop programs for do while and for loops

#include
void main()
{
int num;
int count=0; // count is initialized as zero to start printing from zero.
printf("Hello user, Enter a number");
scanf("%d",&num); // Collects the number from user
while(count<=num) // Checks the condition - if value of count has reached value of num or not. { printf("%d",count); count=count+1; // value of count is incremented by 1 to print next number. } }

The do while statement

Syntax for do while loop is shown below:

do
{
statement 1;
statement 2;
statement 3;
}
while(condition);

Unlike while, do while is an exit controlled loop. Here the set of statements inside braces are executed first. The condition inside while is checked only after finishing the first time execution of statements inside braces. If the condition is TRUE, then statements are executed again. This process continues as long as condition is TRUE. Program control exits the loop once the condition turns FALSE.

Example program to demo working of "do while"


#include
void main()
{
int num;
int count=0; // count is initialized as zero to start printing from zero.
printf("Hello user, Enter a number");
scanf("%d",&num); // Collects the number from user
do
{
printf("%d",count); // Here value of count is printed for one time intially and then only condition is checked.
count=count+1; // value of count is incremented by 1 to print next number.
}while(count<=num); }

The for statement

Syntax of for statement is shown below:


for(initialization statements;test condition;iteration statements)
{
statement 1;
statement 2;
statement 3;
}


The for statement is an entry controlled loop. The difference between while and for is in the number of repetitions. The for loop is used when an action is to be executed for a predefined number of times. The while loop is used when the number of repetitions is not predefined.

Working of for loop:

The program control enters the for loop. At first it execute the statements given as initialization statements. Then the condition statement is evaluated. If conditions are TRUE, then the block of statements inside curly braces is executed. After executing curly brace statements fully, the control moves to the "iteration" statements. After executing iteration statements, control comes back to condition statements. Condition statements are evaluated again for TRUE or FALSE. If TRUE the curly brace statements are executed. This process continues until the condition turns FALSE.

Note 1:- The statements given as "initialization statements" are executed only once, at the beginning of a for loop.
Note 2: There are 3 statements given to a for loop as shown. One for initialization purpose, other for condition testing and last one for iterating the loop. Each of these 3 statements are separated by semicolons.

Example program to demo working of  "for loop"

#include
void main()
{
int num,count;
printf("Hello user, Enter a number");
scanf("%d",&num); // Collects the number from user
for(count=0;count<=num;count++)// count is initialized to zero inside for statement. The condition is checked and statements are executed. { printf("%d",count); // Values from 0 are printed. } }

So that is all about "control statements" in C/C++ language. If you have any doubts ask in comments section.