Comparing floating point numbers in C

c comparison floating-point zero
nick_name picture nick_name · Aug 10, 2011 · Viewed 12.5k times · Source

I've got a double that prints as 0.000000 and I'm trying to compare it to 0.0f, unsuccessfully. Why is there a difference here? What's the most reliable way to determine if your double is zero?

Answer

Steve Jessop picture Steve Jessop · Aug 10, 2011

To determine whether it's close enough to zero that it will print as 0.000000 to six decimal places, something like:

fabs(d) < 0.0000005

Dealing with small inaccuracies in floating-point calculations can get quite complicated in general, though.

If you want a better idea what value you've got, try printing with %g instead of %f.

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