CODEWITHSUNDEEP
c#parsingfloating-point
Daniel Revell
Daniel Revell

Reputation: 8586

Parsing float resulting in strange decimal values

I'm attempting to parse a string with 2 decimal places as a float.

The problem is, the resultant object has an incorrect mantissa. As it's quite a bit off what I'd expect, I struggle to believe it's a rounding issue.

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However double seems to work.

This value does seem to be within the range of a float (-3.4 × 10^38 to +3.4 × 10^38) so I don't see why it doesn't parse it as I'd expect.

I tried a few more test but it doesn't make what's happening any more clear to me.

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Upvotes: 0

Views: 480

Answers (2)

JoriO
JoriO

Reputation: 1048

The mantissa of a float in c# has 23 bits, which means that it can have 6-7 significant digits. In your example 650512.59 you have 8, and it is just that digit which is 'wrong'. Double has a mantissa of 52 bits (15-16 digits), so of course it will show correctly all your 8 or 9 significant digits.

See here for more: Type float in C#

Upvotes: 1

Jon Skeet
Jon Skeet

Reputation: 1500735

From the documentation for System.Single:

All floating-point numbers have a limited number of significant digits, which also determines how accurately a floating-point value approximates a real number. A Single value has up to 7 decimal digits of precision, although a maximum of 9 digits is maintained internally.

It's not a matter of the range of float - it's the precision.

The closest exact value to 650512.56 (for example) is 650512.5625... which is then being shown as 650512.5625 in the watch window.

To be honest, if you're parsing a decimal number, you should probably use decimal to represent it. That way, assuming it's in range and doesn't have more than the required number of decimal digits, you'll have the exact numeric representation of the original string. While you could use double and be fine for 9 significant digits, you still wouldn't be storing the exact value you parsed - for example, "0.1" can't be exactly represented as a double.

Upvotes: 3

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