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

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) 

Answer 2

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

if (Math.Abs(a - b) < delta) {    // Values are within specified tolerance of each other.... } 

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.

Microsoft's Assert.AreEqual() method has an overload that takes a delta: public static void AreEqual(double expected, double actual, double delta)

NUnit also provides an overload to their Assert.AreEqual() method that allows for a delta to be provided.