control structure in c

Control structures form the essential entities of a “structured programming language“. We all know languages like C/C++ or Java are all structured programming languages. Control structures are wont to alter the flow of execution of the program. Why can we got to alter the program flow ? the rationale is “decision making“! In life, we could also be given with a group of option like doing “Electronics” or “Computer science”. We do make a choice by analyzing certain conditions (like our personal interest, scope of job opportunities etc). With the choice we make, we alter the flow of our life’s direction. this is often exactly what happens during a C/C++ program. We use control structures to form decisions and alter the direction of program flow in one or the opposite path(s) available.

There are three sorts 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 group of activities)

All the three control structures and its flow of execution is represented within the flow charts given below.

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

We have to stay in mind one important fact:- all program processes are often implemented with these 3 control structures only. That’s why I wrote “control structures are the essential entities of a structured programming language“. To implements these “control structures” during a C/C++ program, the language provides ‘control statements’. So to implement a specific control structure during a programing language , we’d like to find out the way to use the relevant control statements therein particular language.
The control statements are:-

Switch
If
If Else
While
Do While
For

As shown within 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

Selection structures are wont to perform ‘decision making‘ then branch the program flow supported the result of deciding . Selection structures are implemented in C/C++ with If, If Else and Switch statements. If and If Else statements are 2 way branching statements where as Switch may be a multi branching statement.
The simple If statement
The syntax format of an easy if statement is as shown below.

if (expression) // This expression is evaluated. If expression is TRUE statements inside the braces are going to be executed
{
statement 1;
statement 2;
}
statement 1;// Program control is transfered on to this line, if the expression is fake
statement 2;
The expression given inside the brackets after if is evaluated first. If the expression is true, then statements inside the curly braces that follow if(expression) are going to be executed. If the expression is fake , the statements inside curly braces won’t be executed and program control goes on to statements after curly braces.
Example program to demo “If” statement

Problem:-

A simple example program to demo the utilization 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 another value — output “WRONG ENTRY”.

Note:- an equivalent problem is employed 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 amount from user
if(num==1)
{
printf(“UNITED STATES”);
}
if(num==2)
{
printf(“SPAIN”);
}
if(num==3)
{
printf(“INDIA”);
}
}
The If Else statement.
Syntax format for If Else statement is shown below.

if(expression 1)// Expression 1 is evaluated. If TRUE, statements inside the curly braces are executed.
{ //If FALSE program control is transferred to immedate else if statement.

statement 1;
statement 2;
}
else if(expression 2)// If expression 1 is fake , expression 2 is evaluated.
{
statement 1;
statement 2;
}
else if(expression 3) // If expression 2 is fake , expression 3 is evaluated
{
statement 1;
statement 2;
}
else // If all expressions (1, 2 and 3) are FALSE, the statements that follow this else (inside curly braces) is executed.
{
statement 1;
statement 2;
}
other statements;
The execution begins by evaluation expression 1. If it’s TRUE, then statements inside the immediate curly braces is evaluated. If it’s FALSE, program control is transferred on to immediate else if statement. Here expression 2 is evaluated for TRUE or FALSE. the method continues. If all expressions inside the various if and else if statements are FALSE, then the last else statement (without any expression) is executed along side the statements 1 and a couple of inside the curly braces of last else statement.
Example program to demo “If Else”

#include
void main()
{
int num;
printf(“Hello user, Enter a number”);
scanf(“%d”,&num); // Collects the amount 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 employed to output “WRONG ENTRY”
}
}

Note:- Notice how the utilization of If Else statements made program writing easier. Compare this with above program using simple If statement only.
Switch statement
Switch may be a multi branching control statement. Syntax for switch statement is shown below.

switch(expression)// Expression is evaluated. the result of the expression should be an integer or a personality constant
{
case value1: // case is that the keyword wont to match the integer/character constant from expression.
//value1, value2 … are different possible values which will are available expression
statement 1;
statement 2;
break; // break may be a keyword wont to break the program control from switch block.
case value2:
statement 1;
statement 2;
break;
default: // default may be a keyword wont to execute a group of statements inside switch, if no case values match the expression value.
statement 1;
statement 2;
break;
}
Execution of switch statement begins by evaluating the expression inside the switch keyword brackets. The expression should be an integer (1, 2, 100, 57 etc ) or a personality constant like ‘a’, ‘b’ etc. This expression’s value is then matched with each case values. There are often any number of case values inside a switch statements block. If first case value isn’t matched with the expression value, program control moves to next case value then on. When a case value matches with expression value, the statements that belong to a specific case value are executed.
Notice that last set of lines that begins with default. The word default may be a keyword in C/C++. When used inside switch block, it’s intended to execute a group of statements, if no case values matches with expression value. So if no case values are matched with expression value, the set of statements that follow default: will get executed.
Note: Notice the break statement used at the top of every case values set of statements. The word break may be a keyword in C/C++ wont to break from a block of curly braces. The switch block has two curly braces { }. The keyword break causes program control to exit from switch block.
Example program to demo working of “switch”

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

Note:- Switch statement is employed for multiple branching. an equivalent are often implemented using nested “If Else” statements. But use of nested if else statements make program writing tedious and sophisticated . Switch makes it much easier. Compare this program with above one.

Loop structures

A loop structure is employed to execute a specific set of actions for a predefined number of times or until a particular condition is satisfied. There are 3 control statements available in C/C++ to implement loop structures. While, Do while and For statements.
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’s TRUE then all statements inside curly braces are executed.Then program control comes back to see the condition has changed or to see if it’s 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 as long as condition inside while is TRUE.
Example program to demo working of “while loop”

An example program to gather variety from user then print all numbers from zero thereto particular collected number is shown below. That is, if user enters 10 as input, then numbers from 0 to 10 are going to be printed on screen.

Note:- an equivalent problem is employed to develop programs for do while and for loops

#include
void main()
{
int num;
int count=0; // count is initialized as zero to start out printing from zero.
printf(“Hello user, Enter a number”);
scanf(“%d”,&num); // Collects the amount 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 primary 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 out printing from zero.
printf(“Hello user, Enter a number”);
scanf(“%d”,&num); // Collects the amount from user
do
{
printf(“%d”,count); // Here value of count is printed for just one occasion intially 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;
}

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The for statement is an entry controlled loop. The difference between while and for is within the number of repetitions. The for loop is employed when an action is to be executed for a predefined number of times. The while loop is employed when the amount of repetitions isn’t predefined.

Working of for loop:

The program control enters the for loop. initially 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 just one occasion , at the start 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 those 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 amount 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. } }

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