Java for loop with condition not incrementing

Question

This might be a silly question but I'm confused.

I have a for loop that excludes a given iteration (a random number). It works for any random number chosen that's greater than 0. However, if it's zero, it never does a single iteration:

int x;
for (x = 0; ((x < 2) && (x != r)); x++) {
    // do something if (x != r)

}
System.out.println("X : " + x);

For the example that it's not working, r = 0. Shouldn't that mean it should skip the first iteration but does the second?

The above println yields "X : 0".

Any help? Thanks!

It never does a single iteration because you require x != r while your x is first set to 0, like your r, causing your condition to return false and not to enter the loop — SomethingSomething
– SomethingSomething, Commented Nov 8, 2015 at 8:12
Because of the condition(x < 2) && (x != r). As r =0, the condition fails and will not enter the loop. — Shriram
– Shriram, Commented Nov 8, 2015 at 8:14
No, the second expression is used as a way to end the loop. If the expression evaluates to false, exit the loop: docs.oracle.com/javase/tutorial/java/nutsandbolts/for.html — BLaZuRE
– BLaZuRE, Commented Nov 8, 2015 at 8:14
@tj56 When the first condition (second expression) becomes false, it exists the loop and continues execution after the loop. — BLaZuRE
– BLaZuRE, Commented Nov 8, 2015 at 8:16

Eran · Accepted Answer · 2015-11-08 08:14:28Z

5

If you only want to skip the first iteration, you must move the x != r condition inside the loop. Having the x != r condition in the loop's condition means the loop will never be entered if both r and x are initialized to 0, since the loop terminates when the loop's condition becomes false.

for (x = 0; x < 2; x++) {
    if (x != r) {
    // do something if (x != r)
    }
}

edited Nov 8, 2015 at 8:14

answered Nov 8, 2015 at 8:12

Eran

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2 Comments

tj56 Over a year ago

The variable r is chosen randomly so it might not be always the first iteration. But assuming it's always 0, why wouldn't it work the way I posted above?

Eran Over a year ago

@tj56 I added an explanation. If the loop's condition is false, the loop is terminated. If it's false before the first iteration, the loop is never entered.

hqt · Accepted Answer · 2015-11-08 08:16:29Z

0

for (initialization; termination condition; increment) {
    statement(s)
}

Above code will run:

    initialization
    termination condition
    statement
    increment

    termination condition
    statement
    increment

    termination condition
    statement
    increment

    ....

So you should remove condition x != r because before it runs statement for first time, it check that condition and return false.

You should note that. without condition x != r, your loop is run only twice: x = 0 and x = 1. Maybe in your code, those value (0 and 1) doesn't output any value.

answered Nov 8, 2015 at 8:16

hqt

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Comments

gerrDreamer · Accepted Answer · 2015-11-08 08:19:38Z

0

I think it would be great if you placed (r >= 0) the random number generation would then be picked directly from 0.

answered Nov 8, 2015 at 8:19

gerrDreamer

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