What are Pre-increment and Post-increment in C?

In the C/C++ programming language, there exists a operator that is used to increase the value of a variable by 1. The operator is denoted by the ++ symbol. When we increase the value of a variable before assigning it to another variable then it is known as Pre-Increment. When the value of a variable is incremented after assigning it to a variable, it is known as Post-Increment.

So, we can increment the value of a variable in the following two ways -

1. Pre-Increment
2. Post-Increment

Properties of the Increment Operator

Following are the main properties of the preincrement and postincrement in C -

1. It increases the value of a variable by 1.
2. It can only be used with variables.
3. It is represented by a double plus ++ symbol.

Pre-Increment Operator in C

As the name suggests, the pre-increment operator alters the value of the variable before using it in any expression. Therefore, we can say that the pre-increment operator increases the value of the variable first and then use it in the expression.

Syntax:

For example, if the initial value of a were 5, then the value 6 would be assigned to b.

Code:

Output:

Post-Increment Operator in C

The post-increment operator is used when it is required to increment the value of the variable after evaluating the expression. Therefore, in post-increment value is first used in the expression, and then it is incremented.

Syntax:

For example, assume the initial value of a to be 5. Then after executing the above statement the final value of b will be 5 as the value of a will be incremented after performing the expression.

Code:

Output:

Special Case of Post Increment Operator

What will happen if we use the post-increment operator on a variable and assign it to the same variable? In this case, the value will not be incremented $i.e.$ it will be exactly equal to its initial value.

For example, let's say we execute the following statement -

In this case, the value of x will not get changed no matter what was the initial value of x.

Code

Output:

Evaluating Post and Pre-Increment Operators in C

In Operators Precedence & Associativity Table we have seen that the precedence of the post-increment is higher than that of pre-increment. Also, their associativity is as follows -

1. The pre-increment is right to left associative.
2. The post-increment is left to right associative.

Examples

Exmaple 1

Find the output of the following code -

Output:

Explanation:

The initial values of x and y were 5 and 6 respectively. In the expression z = x++ + y++, we are performing post-increment with x and pre-increment with y. So, the value of x will be incremented but the initial value of x will be used to evaluate the expression, while the value y will be incremented and the incremented value will be used to evaluate the expression.

Example 2

Find the output of the following code -

Output:

Explanation:

The initial values of x and y were 5 and 6 respectively. In the expression z = x++ + y++, we are performing post-increment with x and pre-increment with y. So, the increased values of x and y will be used to evaluate the answer, and then it is assigned to z.

Example 3

Find the output of the following code -

Output:

Explanation:

The initial values of x and y were 'a' and 'b' respectively.

Firstly, we are using post-incrementing with the variable x which will change its value to 'b'. Then, we are using the pre-increment operator on x and assigning the result to z, due to which we will have x = 'c' and z = 'c'. At last, we are performing a special case of post-increment operator on y that will not change the value of y.

Example 4

Find the output of the following code -

Output:

Explanation:

Inside the for loop we have initialized the values of i and j to 1 and 120 respectively. After which we are iterating till the condition i < j holds true. Also, after each iteration, we are multiplying i by 2, and performing post-increment on j. So in each iteration, we will have the following values of i and j.

So, after the $8^{th}$ iteration, the condition i < j fails and the final values of i and j get printed.