CODEWITHSUNDEEP
java
Dudda
Dudda

Reputation: 19

Postincrement in java expressions

In this example:

int i = 1;

while(i < 10)
    if(i++%2 == 0)
        System.out.println(i);

Why is the output 3,5,7,9 and not 2,4,6,8?

Upvotes: 0

Views: 135

Answers (3)

Ale Sequeira
Ale Sequeira

Reputation: 2039

The ++ operator applied after a variable returns the value of the variable and increments the variable after the expression is evaluated. The semantic is the same than this:

int i = 1;
while(i < 10) { 
    boolean cond = i % 2 == 0;
    i = i + 1;
    if(cond) {
        System.out.println(i);
    }
}

Upvotes: 2

m8L
m8L

Reputation: 119

The post-increment operator, uses the current value of its operand in the expression and then increments it.

We can break this down using a literal value of '2' for example. Basically this is what your present code is doing: 

int i = 2;

if (i % 2 == 0) //true, 2 % 2 = 0
    i = i + 1;  //i now becomes 3
    System.out.println (i);

OR to make it simpler, if we remove the loop and put back your code

int i = 1;

if (i++ % 2 == 0) //1 % 2 != 0 

   System.out.println (i);  //Nothing will print for the if statement

   System.out.print i;  //Will print 2, because this print statement is outside
                        //the body of the if-statement

to get the output that you are looking for, you will have to use the prefix-increment operator (++i)

int i = 1;

if ( ++i % 2 == 0)  
    System.out.println (i);

this is equivalent to 

 int i = 1
 if ( (i + i) % 2 == 0) //++i increments i and then uses it in the expression
      System.out.println (i);

Upvotes: 0

Eran
Eran

Reputation: 393811

The condition is performed on the previous value of i, before it is incremented (which is even), but the output is done on the incremented value of i (which is odd).

Upvotes: 4

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