Unexpected output from while loop Java

Question

This is the class whose behaviour I am unable to understand.

class loop1 {
    public static void main(String args[]) {
        int i = 10;

        do
            while(i++ < 15) {
                System.out.println(i);
                i = i + 20;
                System.out.println(i);
            } 
        while(i<2);

        System.out.println(i);    
    }

}

I expected it to print

11
31
31

But it prints

11
31
32

I am not able to understand why this "32" has come up in the output.

This is my understanding of the flow

i = 10
In the while loop because of unary incremental, it becomes 11 so this explains the first output
11 gets incremented to 31 by (+20)
Then 31 < 15 should fail (during the next iteration) so it should proceed to the last print statement and print 31, but it instead it is printing 32.

Can someone tell me what I am missing ?

For the same reason the first print statement print 11. 31 is increment before the last print statement is executed. — Rohit Jain
– Rohit Jain, Commented Aug 19, 2013 at 11:36
Thanks Rohit.You could have put this as answer..would have upvoted it :) — Varun
– Varun, Commented Aug 19, 2013 at 11:43
You're welcome :) Well, there are already 3 duplicate answers. — Rohit Jain
– Rohit Jain, Commented Aug 19, 2013 at 11:45
possible duplicate of Why does this go into an infinite loop? — nawfal
– nawfal, Commented Jul 20, 2014 at 9:15

fommil · Accepted Answer · 2013-08-19 11:55:28Z

5

During the final evaluation of the first while loop i++ still increments i even though the loop does not execute because the condition fails.

class loop1 {
    public static void main(String args[]) {
        //1.  i = 10
        int i = 10;

        do 
            // 2. while loop condition = (10 < 15), i = 11
            // 6. while loop condition = (31 < 15), i = 32
            while(i++ < 15) {
                System.out.println(i);  //3. prints 11
                i = i + 20; //4. i = 31
                System.out.println(i);  //5. prints 31
            } 

        while(i<2); //this really has no effect on the codes execution, given i values

        System.out.println(i); //7.  Prints 32
    }

}

edited Aug 19, 2013 at 11:55

fommil

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answered Aug 19, 2013 at 11:37

Kevin Bowersox

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3 Comments

fommil Over a year ago

i++ increments by 1 but returns the original value, so the comparison check in 6 is 31 < 15 but i == 32.

Kevin Bowersox Over a year ago

@fommil the expression evaluates 31 < 15, but after the evaluation i++ still increments i to 32. I think we are on the same page here.

fommil Over a year ago

yes, I was suggesting a minor improvement to your comments ;-)

christopher · Accepted Answer · 2013-08-19 11:36:45Z

3

i++

You're increasing the value by 1. The value, when you increase it after the first iteration is 31. 31 + 1 is, surprisingly, 32. And you print out the value directly after incrementing it.

answered Aug 19, 2013 at 11:36

christopher

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Comments

gifpif · Accepted Answer · 2013-08-19 12:19:58Z

2

In 2nd iteration when condition of while loop is check
while(i++<15)
at that time i is 31 so condition fail but i++ change the value of i 31 -> 32

answered Aug 19, 2013 at 12:19

gifpif

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Comments

user2695297 · Accepted Answer · 2013-08-19 12:26:54Z

2

while(i++ < 15) compare the value of i with and after that increment i by 1

answered Aug 19, 2013 at 12:26

user2695297

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Comments

Eric · Accepted Answer · 2013-08-19 11:47:49Z

1

My guess is:

while(i++ < 15)

The i++ increments the value from 31 to 32 on the second loop.

Note, the ++ will be executed even if the condition fails - which in your case, the 31 is greater than 15 (condition fails), however because of the ++ the value is incremented to 32, which is being printed out by the System.out at the end.

edited Aug 19, 2013 at 11:47

answered Aug 19, 2013 at 11:38

Eric

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Comments

Sushil Deshmukh · Accepted Answer · 2013-08-19 11:55:20Z

0

In the program :

 while(i++<15)

above statement is condition checking in this statement you are post increment i therefor 31+1=32 is comming

answered Aug 19, 2013 at 11:55

Sushil Deshmukh

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