Reading string in to float produces incorrect accuracy

Question

Now I understand floats are less accurate than double, but does this explain when I have the std::string:

"7.6317"

and I do:

float x = atof(myString.c_str());

getting 7.63170004 is expected? Is there any way I can tell the assignment of x to only read the first 4 decimal places? Or is this because of the way the float representation stores the number 7.6317?

No, a `float` cannot store X decimal places. You can tell it to print with a specific precision. And `double` has the same behaviour. And use `std::stof`. — chris, Jul 27 '14 at 18:21

score 0 · Answer 1 · answered Jul 27 '14 at 18:22

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Yes. It is expected. It is so-called floating point error.

answered Jul 27 '14 at 18:22

Midas

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This does answer the question "Is it expected". – Dan Is Fiddling By Firelight Jul 27 '14 at 18:57
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1. He asked if it is to be expected - I answered that, giving out tips where to look at. 2. I cannot comment any post other than mine ATM. 3. I would probably flag this post if I could. – Midas Jul 27 '14 at 21:53

score 0 · Answer 2 · answered Jul 27 '14 at 18:32

Some floating point literals do not have an accurate representation in the computer, even if -- in decimal notation -- the number seems harmless. This is because the computer uses 2 as a base. So even if a number might have a finite representation in base 10, it might not have on in base 2.

score -1 · Answer 3 · edited May 23 '17 at 12:11

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you can do it like:

float x = floorf(val * 10000) / 10000;

i think it should work! see See

edited May 23 '17 at 12:11

Community

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answered Jul 27 '14 at 18:25

Exciter

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What does this accomplish? The number is already rounded to four decimal places. – chris Jul 27 '14 at 18:31
Yeah thats right, i forgot the float error with matisse and the exponent. sry – Exciter Jul 27 '14 at 18:34

Reading string in to float produces incorrect accuracy

3 Answers3