Float Error

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C#

convert

errors

(int)(0.04F*100F) = 3 ????? WHY?

Posted 1-Oct-10 1:32am

Shargon_85

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#realJSOP · Accepted Answer · 2010-10-01T01:49:00

It appears that using a float type induces less precision, and when the resulting value is cast to an int, it rounds down to the nearest whole number. Note the following:

float y = 0.04F * 100F; // 4 - without casting DURING the math operation, it's fine.
int x = (int)y); // 4 - and then it casts without issue

int x = (int)(0.04F * 100F); // 3 - if you cast during the operation, it's just plain wrong

int x = Convert.ToInt32(0.04F * 100F); // 4 - using ConvertTo results in the expect answer, even if you put the expression in as the parameter (probably because the expression is evaluated before passing it to the ToInt32() method.

int x = (int)(0.04 * 100); // 4 - implicit doubles

int x = (int)(0.04M * 100M); // 4 - using decimal for the math - much more precise

int x = (int) 0.04D * 100D); // 4 - doubles seem to act fine

Welcome to the wonderful world of floating point math.

Richard MacCutchan · Accepted Answer · 2010-10-01T01:41:00

Because you can lose precision between floating and integers. Floating point numbers are designed to handle calculations involving very large or very small numbers, but there is a cost in that precision will be lost - how much depends on the actual values. If you want precise answers to your calculations then you should always use integral values.

Float Error

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