How do I find if two variables are approximately equals?

Question

I am writing unit tests that verify calculations in a database and there is a lot of rounding and truncating and stuff that mean that sometimes figures are slightly off.

When verifying, I'm finding a lot of times when things will pass but say they fail - for instance, the figure will be 1 and I'm getting 0.999999

I mean, I could just round everything into an integer but since I'm using a lot of randomized samples, eventually i'm going to get something like this

10.5 10.4999999999

one is going to round to 10, the other will round to 11.

How should I solve this problem where I need something to be approximately correct?

Answer 1

Define a tolerance value (or an epsilon), for instance, 0.00001, and then use to compare the difference like so:

if (Math.Abs(a - b) < epsilon)
   // Then they are the same

[You could use Double.Epsilon but you would have to use a multiplying factor.]

Better still, write an extension method to do the same. We have something like Assert.AreSimiliar(a,b) in our unit tests.

Answer 2

You could provide a function that includes a parameter for an acceptable difference between two values. For example

// close is good for horseshoes, hand grenades, nuclear weapons, and doubles
static bool CloseEnoughForMe(double value1, double value2, double acceptableDifference)
{
    return Math.Abs(value1 - value2) <= acceptableDifference; 
}

And then call it

double value1 = 24.5;
double value2 = 24.4999;

bool equalValues = CloseEnoughForMe(value1, value2, 0.001);

If you wanted to be slightly professional about it, you could call the function ApproximatelyEquals or something along those lines.

static bool ApproximatelyEquals(this double value1, double value2, double acceptableDifference)