Question with floating point addition

Question

float totalAmount = 0;
.
.
.//totalAmount assigned value 1.05 correctly
.
totalAmount  += float.Parse(dataRow["Amt"].ToString()); //where dataRow["Amt"] has value 4.93

The answer I get for totalAmount is 5.97999954 instead of 5.98

Why is this happening?

Answer 1

You state that totalAmount is 1.05, before the accumulation. That would give expected results of:

1.05
4.93 +
------
5.98

You are getting 5.97999954, which is basically the answer as best represented by IEEE floating point, which is a binary format that cannot exactly express every decimal number. For instance, the rather common 0.1 ₁₀ has an infinite binary floating point representation .0001100110011... ₂ .

And a Wikipedia link, for good measure: http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems ;)

Answer 2

This is due to representing base-10 numbers in a base-2 system.

Floating point math on computers always does this. If you know the level of precision you'll need you should use the appropriate decimal representation.

Answer 3

jdmichal是正确的，但我要补充一点，如果您实际上希望将此值正确加起来，则可以使用Decimal类型（包括十进制文字）。