Pre-decrement and Post-decrement operator in C++

Decrement operator

The decrement operator(--) is a unary operator which decreases the value of operand by one(1). It is used to abbreviate the assigning of variable's decreased value (by 1) to the same variable i.e. a-- means a=a-1.

Unary operators are those operators which perform an operation upon one operand to produce a new value.

The decrement operator can be utilized in two different ways, depending on whether the operator is written before or after the operand.

Pre-decrement operator

If the decrement operator precedes the operand, then it is called the pre-decrement operator(E.g. --a). The value of operand will be decremented before its value is utilized in the expression.

Example:

y= --a;   means   a=a-1;   and   y=a;   will be executed serially.

First, the value of a will be decremented, then it will be assigned to y.

Post-decrement operator

If the decrement operator follows the operand, then it is called the post-decrement operator(E.g. a--). The value of operand will be decremented after its value is utilized in the expression.

Example:

y=a--;   means   y=a;   and   a=a-1; will be executed serially.

First, the value of a will be assigned to y, then it will be decremented.

Write a program in C++ to give an idea about the pre-decrement and post-decrement operator.

In this program, we perform different operations using pre-decrement and post-decrement operator. We will input a number from the user and the altered values due to pre-decrement and post-decrement operator will be printed.

Program

1    #include<iostream.h>
2    #include<conio.h>
3    void main()
4    {
5    int a,y;
6    clrscr();
7    cout<<"Enter a value in a= ";
8    cin>>a;
9    cout<<endl<<"a= "<<a<<endl;
10  y=a--;
11  cout<<"y= a--= "<<y<<endl;
12  cout<<"a= "<<a<<endl;
13  y=--a;
14  cout<<"y= --a= "<<y<<endl;
15  cout<<"a= "<<a<<endl;
16  cout<<"a--= "<<a--<<endl;
17  cout<<"--a= "<<--a<<endl;
18  a--;
19  cout<<"a--"<<endl<<"a= "<<a<<endl;
20  getch();
21  }

Output

USE OF DECREMENT OPERATOR IN C++

Explanation

In this program, the user enters 5 as the input number(i.e. a=5). The remaining part of the program will be executed as follows:

Line number                        Action                                                               Output
9                                            Value of a(i.e. 5) is printed.                               5
10                                          y=a-- i.e. y=5 and a=5-1=4                                           
11                                          Value of y(i.e. 5) is printed.                               5
12                                          Value of a(i.e. 4) is printed.                               4
13                                          y= --a i.e. a=4-1=3 and y=3                         
14                                          Value of y(i.e. 3) is printed.                               3
15                                          Value of a(i.e. 3) is printed.                               3                                   
16                                          Value of a-- is printed i.e. the value
                                              of a(i.e. 3) is printed and  a=3-1=2.                   3                                    
17                                          Value of --a is printed i.e. a=2-1=1 
                                              and the value of a(i.e. 1) is printed.                   1       
18                                          a-- i.e. a=1-1=0                                         
19                                          Value of a(i.e. 0) is printed.                               0   

In this way, the statements are executed and we obtain our required output on the screen.