comparing float values with double resulting different behaviour

Question

I found many questions.But none helps me

      float x = 0.1;
      x == 0.1

The above code returns false. since i try to compare double precision value with single precision x.

      float x = 0.5
      x == 0.5

This one returns true. I cant find why is it returning true?? Any suggestions ??

EDIT : So how do i identify which value has same representation in both precisions??

`x == 0.1f` is more likely to compare true (but still not guaranteed) — M.M, Sep 13 '14 at 09:50
http://www.exploringbinary.com/why-0-point-1-does-not-exist-in-floating-point/ — Maroun, Sep 13 '14 at 09:50
another duplicate http://stackoverflow.com/questions/20855633/a-float-set-to-0-1-compares-false-against-0-1?rq=1 — phuclv, Sep 13 '14 at 09:58
Try `printf("%.20e\n", x);printf("%.20e\n", 0.1);` to see the difference. — chux - Reinstate Monica, Sep 13 '14 at 12:07

score 5 · Accepted Answer · answered Sep 13 '14 at 09:48

5

x == 0.1

0.1 is not of type float but of type double. float and double don't have the same precision. 0.1f is of type float.

Why it works with 0.5 is because 0.5 has an exact representation in both float and double (in binary IEEE-754) types.

answered Sep 13 '14 at 09:48

ouah

+1 please see my edits. and let me know why it is downvoted?? – Gibbs Sep 13 '14 at 09:52
2

because there are so many duplicates here – phuclv Sep 13 '14 at 09:57

score 0 · Answer 2 · edited May 23 '17 at 12:21

0

It is because of float/double representation in memory, you are seeing this behavior.

edited May 23 '17 at 12:21

Community

answered Sep 13 '14 at 09:50

µtex

2 Answers2