C - Decision Making

Decision making in C

---

It is about deciding the order of the statement based on certain conditions or a group of statements are repeated until certain specified condition 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

Depending on the complexity of conditions to be tested, the ‘if’ statement may be implemented in different forms. Following are the different forms: -

1.   Simple if statement

2.   If-else statement

3.   Nested if-else statement

4.   Using else-if statement

1.  Simple ‘if’ statement

The general form of a simple ‘if’ statement is,

if(expression)

{

    statement inside;

}

    statement outside;

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

Ex:

#include <stdio.h>

void main( )

{

    int x, y;

    x = 15;

    y = 13;

    if (x > y )

    {

        printf("x is greater than y");

    }

}

Output:

x is greater than y

2. if...else statement

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

if(expression)

{

    statement block1;

}

else

{

    statement block2;

}

Statement-block1 is executed if the expression is true, else statement-blocck2 is executed and Statement-block1 is skipped.

Ex:

#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");

    }

}

Output:

y is greater than x

3. 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;

}

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

Ex:

#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");

        }

    }

} 

4. 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;

After expression is tested from top (of the ladder) downwards and is true condition is found, the statement associated with it is executed.

Ex:

#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");

    }

}

Important Points:

1.   In ‘if’ statement, if there is only one statement, it can be included without enclosing it into curly braces { ... }

2.    int a = 5;

3.    if(a > 4)

    printf("success");

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

4.   ‘==’ must be used for comparison in the expression of ‘if’ condition, if we use ‘=’ then it performs assignment not comparison therefore expression will always return true.

5.   Other than 0(zero), all other values are considered as true.

6.    if(27)

    printf("hello");

hello will be printed in above example.