Increment in for loop give different answers

Question

class Test {
    public static void main(String[] args) {
        for (int i= 1; i < 2; i++) {
            System.out.println(i);
            System.out.println(++i);

            int a = 1;
            System.out.println(a++);
            System.out.println(++a);
        }
    }
}

The output here is 1,2 and 1,3. We are doing post-increment and pre-increment in both the cases why I am getting different results?

you need to know about post and pre incremental operator in java — Mr. Roshan, Jul 26 '18 at 06:27
Use a debugger, and see how values are changing with iterations... — ernest_k, Jul 26 '18 at 06:27
`for (int i= 1; i < 2; i++)` <-- because this `i++` gets executed after each iteration. Please have a look at the [docs](https://docs.oracle.com/javase/tutorial/java/nutsandbolts/for.html). — BackSlash, Jul 26 '18 at 06:30
Dupicate suggestion would have to be in context of loop, not only pre/post incrementation — Antoniossss, Jul 26 '18 at 06:36
@AndyTurner It isn't. The OP is asking why `a++ ... ++a` behaves different than `i++ ... ++i`, and the reason is that `i++` doesn't get executed until the end of each iteration, while `a++` gets executed immediately (so `i` and `a` will have different values). — BackSlash, Jul 26 '18 at 06:47
@BackSlash Even if it were executed, the result would be 2,3 so the question doesn't make sense. — shmosel, Jul 27 '18 at 04:22

Aman Chhabra · Answer 1 · 2018-07-26T06:36:16.117

3

Because post increment happens at the end of the iteration.

Hence the below code :

for (int i= 1; i < 2; i++) {
        System.out.println(i);
        System.out.println(++i);

Will work something like this:

int i = 1;
System.out.println(i); // i is still 1
System.out.println(++i); // pre increment making i to 2 and then print
i++;
if(i<2) 
// iterate again

However, in second case:

int a = 1;
  System.out.println(a++); // post increment first print 1 and then change a to 2
  System.out.println(++a); // pre increment a now changed to 3

edited Jul 26 '18 at 06:36

answered Jul 26 '18 at 06:28

Aman Chhabra

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Why down vote ? – Aman Chhabra Jul 26 '18 at 06:30
"post increment does not work for the first iteration in for loop" is incorrect. Are you trying to say that the last term in the for loop, the i++ is evaluated at the end of the loop? The way a post increment works does not change depending on which iteration you're in. – matt Jul 26 '18 at 06:34
@matt yes, that is what I shared in by explaining where in post increment happens in the end – Aman Chhabra Jul 26 '18 at 06:35
@matt Also updated the statement to avoid confusion – Aman Chhabra Jul 26 '18 at 06:36
1

Makes sense now, cheers. – matt Jul 26 '18 at 06:36

score 2 · Answer 2 · answered Jul 26 '18 at 06:29

    System.out.println(i); // 1 (initial value)
    System.out.println(++i); // 2 (pre increment returns the value after increment)

    int a = 1;
    System.out.println(a++); // 1 (initial value prior to increment, since post increment 
                             // returns the value prior to increment)
    System.out.println(++a); // 3 (pre increment returns the value after increment)

The i++ in the loop's definition only takes place at the end of each iteration. You never print the value of i after i++ is executed, since the loop terminates after the first iteration.

BTW, if i++ was executed prior to System.out.println(i);, you'd still get a different output (compared to the last two println statements) - 2 followed by 3.

Andy Turner · Answer 3 · 2018-07-26T06:54:45.650

The general structure of a for loop (more precisely: this type of basic for statement) is:

for ([ForInit]; [Expression]; [ForUpdate]) Statement

The equivalent while loop is:

[ForInit];
while ([Expression]) {
  Statement;
  [ForUpdate];
}

In other words, the update is executed after the loop body.

So your code is equivalent to:

// ForInit
int i = 1;
while (/* Expression */ i < 2) {
  // Statement
  System.out.println(i);
  System.out.println(++i);

  int a = 1;
  System.out.println(a++);
  System.out.println(++a);

  // ForUpdate
  i++;
}

Hopefully it is then easy to see why there is a difference in the output.

score 0 · Answer 4 · answered Jul 26 '18 at 06:31

In the first case, the post-increment is not called. If you understand how a for loop works, you will know that the increment section is not executed the very first time. The initialization section, here int i = 1, and the condition checking - i < 2 is validated. The increment section is invoked only after each iteration.

In the second case, the post-increment is performed and hence the difference in values. You should focus more on the basics of looping.

score 0 · Answer 5 · answered Jul 26 '18 at 06:32

System.out.println(i);

i isn't postincremented here. The answers would be the same if it were:

System.out.println(i++);
System.out.println(++i);

int a = 1;
System.out.println(a++);
System.out.println(++a);



for (int i= 1; i < 2; **i++)**

doesn't do anything until a pass through loop is done.

Increment in for loop give different answers

5 Answers5