Pre-increment and Post-increment in C/C++

In C/C++, Increment operators are used to increase the value of a variable by 1. This operator is represented by the ++ symbol. The increment operator can either increase the value of the variable by 1 before assigning it to the variable or can increase the value of the variable by 1 after assigning the variable. Thus it can be classified into two types:

• Pre-Increment Operator
• Post-Increment Operator

1) Pre-increment operator: A pre-increment operator is used to increment the value of a variable before using it in an expression. In the Pre-Increment, value is first incremented and then used inside the expression.

Syntax:

 a = ++x;

Here, if the value of ‘x’ is 10 then the value of ‘a’ will be 11 because the value of ‘x’ gets modified before using it in the expression.

CPP

// CPP program to demonstrate pre increment
// operator
#include <iostream>
using namespace std;

// Driver Code
int main()
{
    int x = 10, a;

    a = ++x;
    cout << "Pre Increment Operation";

    // Value of a will change
    cout << "\na = " << a;

    // Value of x change before execution of a=++x;
    cout << "\nx = " << x;

    return 0;
}

Output

Pre Increment Operation
a = 11
x = 11

2) Post-increment operator: A post-increment operator is used to increment the value of the variable after executing the expression completely in which post-increment is used. In the Post-Increment, value is first used in an expression and then incremented. 

Syntax:

 a = x++;

Here, suppose the value of ‘x’ is 10 then the value of variable ‘a’ will be 10 because the old value of ‘x’ is used.

CPP

// CPP program to demonstrate post increment
// operator
#include <iostream>
using namespace std;

// Driver Code
int main()
{
    int x = 10, a;

    a = x++;

    cout << "Post Increment Operation";

    // Value of a will not change
    cout << "\na = " << a;

    // Value of x change after execution of a=x++;
    cout << "\nx = " << x;

    return 0;
}

Output

Post Increment Operation
a = 10
x = 11

Special Case for Post-increment operator: If we assign the post-incremented value to the same variable then the value of that variable will not get incremented i.e. it will remain the same like it was before.

Syntax:

a = a++;

Here, if the value of ‘x’ is 10 then the value of ‘a’ will be 10 because the value of ‘x’ gets assigned to the post-incremented value of ‘x’.

C++

// CPP program to demonstrate special
// case of post increment operator
#include <iostream>
using namespace std;

int main()
{

    int x = 10;

    cout << "Value of x before post-incrementing";

    cout << "\nx = " << x;

    x = x++;

    cout << "\nValue of x after post-incrementing";

    // Value of a will not change
    cout << "\nx = " << x;

    return 0;
}

Output:

Value of x before post-incrementing
x = 10
Value of x after post-incrementing
x = 10

Note: This special case is only with post-increment and post-decrement operators, while the pre-increment and pre-decrement operators works normal in this case.

Evaluating Post and Pre-Increment Together

The precedence of postfix ++ is more than prefix ++ and their associativity is also different. Associativity of prefix ++ is right to left and associativity of postfix ++ is left to right.  

• Associativity of postfix ++ is left to right
• Associativity of prefix ++ is right to left

This article is contributed by Souvik Nandi. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.