CODEWITHSUNDEEP
.netfloating-pointcomparerounding
Trap
Trap

Reputation: 12372

Is it correct to compare two rounded floating point numbers using the == operator?

Or is there a chance that the operation will fail?

Thanks.

I chose the wrong term and what I really meant was rounding to 0, not truncation.

The point is, I need to compare the integer part of two doubles and I'm just casting them to int and then using ==, but, as someone pointed out in one of my earlier questions, this could throw an overflow exception if the double can't fit into the integer.

So the question would be 'Is it correct to use the == operator to compare two doubles that have previously been rounded to 0, or should I stick to the casting to int method and catch a possible exception?

Upvotes: 7

Views: 5069

Answers (6)

nickf
nickf

Reputation: 546055

Here's the updated site which discusses the pros and cons of several methods of comparing floating point numbers. (You can still view the old site here.)

The method I'd go with is the "relative error" method. Find the difference between the two numbers, convert that to a percentage of the numbers, and if that percentage is sufficiently small, then you've got equality.

Upvotes: 12

DenisBakharev
DenisBakharev

Reputation:

http://www.windojitsu.com/code/floatcomparer.html

Upvotes: 1

rlbond
rlbond

Reputation: 67779

Even worse is that sometimes, even for the exact same number, it will fail. This is because some compilers or processors will use more bits of precision in a CPU register than in memory (MSVC has 3 different floating-point behavior options, for example). Thus a recently-computed value may not have these bits truncated and will appear to be unequal. NEVER use == on floats.

Upvotes: 6

Martin Liesén
Martin Liesén

Reputation: 1338

If your absolut value is less than 2^23 for single or 2^52 for double you can use round() and then do the compare. Larger values can not be precisly stored and this opens for situations where N == N+1.

Upvotes: -1

S.Lott
S.Lott

Reputation: 391854

It's never correct to use == with floating-point.

What does "truncate" mean in a float-point context? What specific library function are you calling? What is the result? What makes you believe that "truncated" values are any more comparable than non-truncated values?

Floating point is an approximation of decimal values. Floating point can only represent powers of two precisely. All other values are subject to some error, no matter what floating-point operations you do.

If, however, you convert to integer, you can use ==.

Upvotes: 3

Bill the Lizard
Bill the Lizard

Reputation: 405765

It can still fail due to the normal problems with floating point representation. Rather than truncating them, use a delta that would represent the equivalent precision.

It can fail in cases where you have two floats that you would normally consider the same,

10.19999999

10.20000001

but when you truncate them they give different results.

10.19

10.20

Whereas, if I had used a delta of 0.001 to compare to the difference, I would have seen that these two values are effectively the same.

Upvotes: 4

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