How many times will this string be printed?

Question

I saw this code on Exam: the question is how many times this string will be printed.

I thought first it will be 10 times but this is wrong. Can someone tell me why my answer is wrong. This is part of a code in the C language.

for (float x = 100000001.0f; x <= 100000010.0f; x += 1.0f) {
    printf("lbc");
}

Very related question: http://stackoverflow.com/questions/588004/is-floating-point-math-broken — Some programmer dude, Feb 27 '17 at 09:44
still, this is a for, and im adding to x a value, so x will be a greater then 100000010.0f . and the for will be end. — Elior Sastiel, Feb 27 '17 at 09:49
Not taking in account rounding errors, the answer is 10. Then, depending on the floating point implementation, strange things (for us humans) can happen. Some programmer dude has pointed you to an explanation. Floating point is an _approximation_ of real numbers. — linuxfan says Reinstate Monica, Feb 27 '17 at 09:51
Just because the loop variable changes, doesn't mean the loop will end. It heavily depends on the type of the variable. Case and point: `for(unsigned int x = 10; x > 0; --x);` will be an infinite loop. — StoryTeller - Unslander Monica, Feb 27 '17 at 09:51
The **correct** answer is "it is implementation-dependent". The answer they expect is anyone's guess. — n. m. could be an AI, Feb 27 '17 at 09:55

Kami Kaze · Accepted Answer · 2018-04-19T07:31:20.867

7

Assuming x is a 32 bit floating point:

Floating point values have a limited resolution. 100000001 is 1*10^8 so you lose your 1 at the end. If you would add 1, it again gets lost because the next float value is 1.00000008*10^8. You can add as many 1s as you like the result will always be the same.

That is the reason why your code is an endless loop.

edited Apr 19 '18 at 07:31

answered Feb 27 '17 at 09:53

Kami Kaze

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score 5 · Answer 2 · answered Feb 27 '17 at 09:53

5

float x = 100000001.0f;

will initialize x with the nearest representable float, which is 100000000. Adding 1 to this value will lead to the same value.

If you print the value of x in the loop you will see what happen: http://ideone.com/3FJGTz

answered Feb 27 '17 at 09:53

mch

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How many times will this string be printed?

2 Answers2