Question

I’m trying to convert celsius to fahrenheit with C# ASP.net web forms in visual studio 2010.

I’m using a function to perform the conversion, including this code line:

return (celsius * (9 / 5)) + 32;

It doesn’t calculate right! If I put in celsius = 16, I’m getting 48.

I’ve tried doing the same in PHP, – I’m getting 60, which is what I’m suppose to get.

Any idea what could be wrong here?

Answer 1

You’re seeing 9 / 5 being evaluated in integer arithmetic, giving the value of 1. So basically your expression is just adding 16.

You want something like:

// Force the 9.0 / 5.0 to be evaluated as a double
return (celsius * (9.0 / 5.0)) + 32;

Or you could stay within integer arithmetic just by changing the evaluation grouping:

return ((celsius * 9) / 5) + 32;

If your input and expected output are integers, I’d go with the latter. Obviously there’s now a greater risk of overflow… but only with very high temperatures 🙂

Note that this will always round down – if you want the mathemtically closest value, you’d probably want to still perform the arithmetic using floating point, then apply rounding, e.g.

return (int) (Math.Round((celsius * (9.0 / 5.0)) + 32));

Note that this will use banker’s rounding (round to even) for mid-points between two integers.