What's the best way to create a percentage value from two integers in C#?

Question

I have two integers that I want to divide to get a percentage.

This is what I have right now:

int mappedItems = someList.Count(x => x.Value != null); int totalItems = someList.Count(); (int)(((double)mappedItems /(double) totalItems) * 100) 

This gives the right answer. But that is a lot of casting to do something as simple as get a percentage between two numbers.

Is there a better way to do this? Something that does not involve casting?

Answer 1

How about just mappedItems * 100.0 / totalItems and casting this to the appropriate type?

Answer 2

The right integer-only way to get percentage with proper rounding is:

int result = ( mappedItems * 200 + totalItems ) / ( totalItems * 2 ); 

How do we get there? If we do this thing in floating point, it would be Math.Floor( mappedItems * 100.0 / totalItems + 0.5 ). We need to transform this formula to be integer-only by multiplying and dividing 0.5 by totalItems, then moving 0.5 * totalItems into dividend, and then multiplying both dividend and divisor by 2 to make fractions go away:

mappedItems * 100.0 / totalItems + 0.5 => mappedItems * 100.0 / totalItems + totalItems * 0.5 / totalItems => ( mappedItems * 100.0 + 0.5 * totalItems ) / totalItems => ( mappedItems * 200.0 + totalItems ) / ( totalItems * 2 ).

At this point the formula is integer-only. When we do integer division, we get floored result, so the integer-only result is equivalent to the mentioned floating-point one.