Conditional Instructions in C

Sometimes we want to watch comedy videos on youtube if the day is Sunday. Sometimes we order junk food if it is our friend’s birthday in the hostel. You might want to buy an umbrella if it’s raining and you have the money. You order the meal if dal or your favorite bhindi is listed on the menu.

All these are decisions that depend on conditions being met.

In ‘C’ language, too, we must be able to execute instructions on a condition(s) being met.

Decision-making instructions in C

• If-else statement
• Switch statement

If-else statement

The syntax of an if-else statement in c looks like this:

if ( condition to be checked) {

   Statements-if-condition-true ;

}

else{

statements-if-condition-false ;

}

Code Example

int a=23;
if (a>18){
printf(“you can drive\n”);
}

Note that else block is not necessary but optional.

Relational Operators in C

Relational operators are used to evaluate conditions (true or false) inside the if statements. Some examples of relational operators are:

Important Note: ‘=’ is used for an assignment, whereas ‘==’ is used for an equality check.

The condition can be any valid expression. In C, a non-zero value is considered to be true.

Logical Operators 

&&, ||, and ! are the three logical operators in C. These are read as “and,””or,” and “not.” They are used to provide logic to our c programs.

Use of logical operators:

1. && (AND) is true when both the conditions are true

    “1 and 0” is evaluated as false

    “0 and 0” is evaluated as false

    “1 and 1” is evaluated as true

2. || (OR) is true when at least one of the conditions is true. (1 or 0 = 1)(1 or 1 = 1)

3. ! returns true if given false and false if given true.

    !(3==3) evaluates to false

    !(3>30) evaluates to true

As the number of conditions increases, the level of indentation increases. This reduces readability. Logical operators come to the rescue in such cases.

Else if clause

Instead of using multiple if statements, we can also use else if along with if, thus forming an if-else if-else ladder.

if {

// statements ;

}

else if { //statements;

           }

else {  //statements;

        }

Using if-else if-else reduces indents. The last “else” is optional. Also, there can be any number of “else if.”

Last else is executed only if all conditions fail.

Operator Precedence

Conditional operators

A shorthand “if-else” can be written using conditional or ternary operators.

Condition ?  expression-if-true ;  expression-if-false

Here, ‘?’ and ‘:’ are Ternary operators.

Switch case-control instruction

Switch-case is used when we have to make a choice between the number of alternatives for a given variable.

Syntax,

Switch(integer-expression)

{

Case c1:

       Code;

Case c2:                                                         //c1,c2,c3 are constants

       Code;                                                       //Code is any valid C code

Case c3:

      Code;

default:

      Code;

}

The value of integer-expression is matched against c1,c2,c3……if it matched any of these cases, that case along with all subsequent “case” and “default” statements are executed.

Quick Quiz: Write a program to find the grade of a student given his marks based on below:

Important notes

• We can use switch case statements even by writing in any order of our choice
• Char values are allowed as they can be easily evaluated to an integer
• A switch can occur within another, but in practice, this is rarely done