Question

Can anyone explain, why this program is returning the correct value for sqrt_min?

int n = 1000000;

double[] myArr = new double[n];
for(int i = n-1 ; i>= 0; i--){ myArr[i] = (double)i;}

// sqrt_min contains minimal sqrt-value
double sqrt_min = double.MaxValue;

Parallel.ForEach(myArr, num =>
{
double sqrt = Math.Sqrt(num); // some time consuming calculation that should be parallized
if(sqrt < sqrt_min){ sqrt_min = sqrt;}
});
Console.WriteLine("minimum: "+sqrt_min);

Answer 1

It works by sheer luck. Sometimes when you run it you are lucky that the non-atomic reads and writes to the double are not resulting in “torn” values. Sometimes you are lucky that the non-atomic tests and sets just happen to be setting the correct value when that race happens. There is no guarantee that this program produces any particular result.