Question

Currently I’m rewriting some old C++ code to C#. When running some tests it didn’t work as it should, and I started debugging, and came across the below which I cant seem to figure out.

As I’m doing a lot of math in the class and using Math.Pow function very often, I created a shortcut method for it:

public double pow(double d, double p)
{
   return Math.Pow(d,p);
}

Then I have a code line as follows, which use this method quite some times:

double y = pow((pow(d12, 2) + pow(d13, 2) + pow(23, 2)), 2) - (2.0 * (pow(d12, 4) + pow(d13, 4) + pow(d23, 4)));

This line didn’t give the expected result, so I started splitting it up into smaller pieces as it should be calculated… for example the value before the minus sign should be equal to q5:

double q1 = pow(d12, 2);
double q2 = pow(d13, 2);
double q3 = pow(d23, 2);
double q4 = q1 + q2 + q3;
double q5 = pow(q4, 2);

After these lines q5 is 8775553070736.0

Then I tried splitting the long line into two parts, where the first should be equal to the above 5 lines, just written as one line:

double q12 = pow((pow(d12, 2) + pow(d13, 2) + pow(23, 2)), 2);

Which I would expect to evaluate to the same as q5, but it does not. q12 instead evaluates to 4479508755225.0

What’s wrong with the code?

Answer 1

Why do you expect:

pow(d23, 2)

to be the same as

pow(23, 2)

?

PROTIP: IN VS, always change the default colour of numbers (black), to something else (I like red).