Decision making in C

Decision making is about deciding the order of execution of statements based on certain conditions or repeat a group of statements until certain specified conditions are met. C language handles decision-making by supporting the following statements,

  • if statement
  • switch statement
  • conditional operator statement (? : operator)
  • goto statement

Decision making with if statement

The if statement may be implemented in different forms depending on the complexity of conditions to be tested. The different forms are,

  1. Simple if statement
  2. if....else statement
  3. Nested if....else statement
  4. Using else if statement

Simple if statement

The general form of a simple if statement is,

if(expression)
{
    statement inside;
}
    statement outside;

If the expression returns true, then the statement-inside will be executed, otherwise statement-inside is skipped and only the statement-outside is executed.

Example:

#include <stdio.h>

void main( )
{
    int x, y;
    x = 15;
    y = 13;
    if (x > y )
    {
        printf("x is greater than y");
    }
}

x is greater than y


if...else statement

The general form of a simple if...else statement is,

if(expression)
{
    statement block1;
}
else
{
    statement block2;
}

If the expression is true, the statement-block1 is executed, else statement-block1 is skipped and statement-block2 is executed.

Example:

#include <stdio.h>

void main( )
{
    int x, y;
    x = 15;
    y = 18;
    if (x > y )
    {
        printf("x is greater than y");
    }
    else
    {
        printf("y is greater than x");
    }
}

y is greater than x


Nested if....else statement

The general form of a nested if...else statement is,

if( expression )
{
    if( expression1 )
    {
        statement block1;
    }
    else 
    {
        statement block2;
    }
}
else
{
    statement block3;
}

if expression is false then statement-block3 will be executed, otherwise the execution continues and enters inside the first if to perform the check for the next if block, where if expression 1 is true the statement-block1 is executed otherwise statement-block2 is executed.

Example:

#include <stdio.h>

void main( )
{
    int a, b, c;
    printf("Enter 3 numbers...");
    scanf("%d%d%d",&a, &b, &c);
    if(a > b)
    { 
        if(a > c)
        {
            printf("a is the greatest");
        }
        else 
        {
            printf("c is the greatest");
        }
    }
    else
    {
        if(b > c)
        {
            printf("b is the greatest");
        }
        else
        {
            printf("c is the greatest");
        }
    }
} 

else if ladder

The general form of else-if ladder is,

if(expression1)
{
    statement block1;
}
else if(expression2) 
{
    statement block2;
}
else if(expression3 ) 
{
    statement block3;
}
else 
    default statement;

The expression is tested from the top(of the ladder) downwards. As soon as a true condition is found, the statement associated with it is executed.

Example :


#include <stdio.h>

void main( )
{
    int a;
    printf("Enter a number...");
    scanf("%d", &a);
    if(a%5 == 0 && a%8 == 0)
    {
        printf("Divisible by both 5 and 8");
    }  
    else if(a%8 == 0)
    {
        printf("Divisible by 8");
    }
    else if(a%5 == 0)
    {
        printf("Divisible by 5");
    }
    else 
    {
        printf("Divisible by none");
    }
}

Points to Remember

  1. In if statement, a single statement can be included without enclosing it into curly braces { ... }
    int a = 5;
    if(a > 4)
        printf("success");

    No curly braces are required in the above case, but if we have more than one statement inside if condition, then we must enclose them inside curly braces.

  2. == must be used for comparison in the expression of if condition, if you use = the expression will always return true, because it performs assignment not comparison.
  3. Other than 0(zero), all other values are considered as true.
    if(27)
        printf("hello");

    In above example, hello will be printed.