Understanding floating points and infinite loops

Question

I have this code:

while (x < 10.0) {
    x += y;
}

If x and y are floating point numbers, apparently if you add them the loop will continue infinitely. I don't understand how it wont pass 10. It seems impossible to not pass ten but it doesn't according to my text book.

Why is that? Is it because of the rounding that happens with floating points? Or am I missing something?

Edit: x and y are strictly postive by the way

try to put `System.out.println(x)` inside loop to track how much increment each loop get — A-SM, Jun 04 '13 at 00:34
the values of x and y are relevant. if y = 0, this is obviously an infinite loop (if x < 10) :) — bengoesboom, Jun 04 '13 at 00:35
guys their strictly positive so both x and y are above 0. Meaning if they loop through it will increment past 10. — user1832483, Jun 04 '13 at 00:39
OK, @OliCharlesworth knows the trick. I'm not seeing it myself, though ... — John, Jun 04 '13 at 00:44

score 8 · Accepted Answer · answered Jun 04 '13 at 00:46

8

You haven't told us the actual initial values of x or y, so we can't say for sure. But here's an example of why this might not work:

class Main {
    public static void main(String[] args) {
        float x = 1.0f;
        float y = 1e-8f;
        float z = x + y;

        System.out.printf("%g\n", x);      // 1.00000
        System.out.printf("%g\n", y);      // 1.00000e-08
        System.out.printf("%g\n", z);      // 1.00000
        System.out.printf("%g\n", z - x);  // 0.00000

    }
}

See http://ideone.com/RMMPlP.

answered Jun 04 '13 at 00:46

Oliver Charlesworth

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And an example where it could theoretically work, you just haven't waited long enough: `y = 1e-30f` :-) – Adrian Pronk Jun 04 '13 at 00:51
Yes see this is what I mean I plugged in the code and it just keeps running but why is that thought what makes floating point numbers just keep going like that. – user1832483 Jun 04 '13 at 00:56
@user1832483: Because in floating point, 1.0 + 1e-8 == 1.0. You need to review how floating-point works ;) – Oliver Charlesworth Jun 04 '13 at 00:57
@OliCharlesworth: I just meant to joke that even if the precision was sufficient so adding 1e-30f would result in a higher value, because the loop would have to iterate about 1e30 times it would effectively be an infinite loop anyway. – Adrian Pronk Jun 04 '13 at 01:41

score 0 · Answer 2 · answered Jun 04 '13 at 00:39

0

Your y value may be 0 or very small and hence making the loop to run for infinte or for a long time:

Assuming if

y = 0;

This loop will go infinite

while(x < 10.0) {
x += y;
}

answered Jun 04 '13 at 00:39

Juned Ahsan

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i understood this but the problem is that x and y are strictly positive so y or x cant be 0 – user1832483 Jun 04 '13 at 00:40

score 0 · Answer 3 · answered Sep 10 '14 at 15:12

0

Are you using Java?Try this-

while(x < 10.0f) {
x += y;
}

or

while(x < (float)10.0) {
x += y;
}

answered Sep 10 '14 at 15:12

Flying Monkey

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Understanding floating points and infinite loops

3 Answers3