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

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 2

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 0.1₁₀ has an infinite binary floating point representation .0001100110011...₂.

And a Wikipedia link, for good measure: http://en.wikipedia.org/wiki/Floating%5Fpoint#Accuracy%5Fproblems ;)

Answer 3

jdmichal is correct, but I'll add that, if you actually want this to add up correctly, you could use the Decimal type (including decimal literals).