In JS - why does 77.6*100 not equal 7760

Asked Oct 21 '16 at 03:18

Active Oct 21 '16 at 03:18

Viewed 33 times

Came across this, trying to understand why

Initially I thought this was due to type conversations between strings, ints and floats etc, but it appears not.

In JS doing 77.6 * 100, the outcome is 7759.999999999999.

If you do 77.6 * 1000 then the outcome is not 77599.999999999999, but instead is actually 77600.

(77.6*100) == (77.6*1000)/10 will give you false, but

(66.7*100) == (66.7*1000)/10 will give you true

This appears to only happen when 77.6 and 100 are involved, the same happens when you divide also.

I've tried this across Windows and Linux OS's, chrome js console, nodejs as well as PHP. All exhibit the same behaviour.

It's not exactly an insurmountable problem, but I am curious.

Any ideas?

Cheers

asked Oct 21 '16 at 03:18

Warren.S

Because floats are not precise, and comparing them with `==` is pretty much always a bad idea. – CollinD Oct 21 '16 at 03:20
and it is not only javascript, every languague has this problem. you have to round your float and convert it to int before comparing – mtizziani Oct 21 '16 at 03:29
@mtizziani - Not *every* language. Some languages correctly represent base 10 decimal values. – nnnnnn Oct 21 '16 at 03:43
@nnnnnn ok, not every language, but nearly to every – mtizziani Oct 21 '16 at 03:45
Thanks guys. I wasn't sure if this was just float precision (lack of), or something else. – Warren.S Oct 21 '16 at 03:51

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