Objective C Issue With Rounding Float

Question

I have an issue with rounding the result of a calculation to two decimal places.

It is a financial calculation and when the result involves half a penny I would expect the number to be rounded up but it is in fact being rounded down.

To replicate the issue:

float raw = 16.695;
NSLog(@"All DP: %f",raw);
NSLog(@"2 DP: %.2f",raw);

Returns:

All DP: 16.695000
2 DP: 16.69

Whereas I would expect to see:

All DP: 16.695000
2 DP: 16.70

Can anyone advise if this is by design or if (most likely) I am missing something and if there is anything I can do to get around it. It is vital that it rounds up in this scenario.

Thanks in advance, Oli

Answer 1

Don't use floats for financial calculations!

There are values that floats cannot represent exactly. 16.695 is one of them. When you try to store that value in a float, you actually get the closest representable value. When you perform a series of operations on a float, you can lose precision, and then you lose accuracy. This leads to losing money.

Use an actual currency type, use NSDecimalNumber, or do all your calculations with ints that count the smallest unit you care about (i.e., 1050 is $10.50 in whole cents, or $1.050 if you want fractions of pennies).