Question

I’m having an odd issue in which my math inside a for loop will not calculate at all until the very last iteration, at which point it comes out as 100%. The equation I’m using is a simple percentage equation to calculate how much of an array of files has been processed thus-far. My code is below:

    for(int z=0;z<theFiles.size();z++) {
    ...
        System.out.println(z); // Prints out the current iteration.
        System.out.println(theFiles.size()); // Prints out the length of the array (35 in my test sample).
        double test = Math.abs(z / theFiles.size() * 100); // Calculation to find the percentage of 100 the current iteration is (this is where things seem to break). Comes out as 0 if it's set as an int also.
        System.out.println(test); // Prints out the percentage complete for this iteration.
    }

Anyone have an idea why the variable “test” keeps coming out 0.0? I recall having a similar problem to this in JavaScript, but I’m not sure how to fix it in Java and forgot how I had done so in JS.

Answer 1

It is due to integer divison. Cast one of the value to double

Example:

double test = Math.abs((double)z / theFiles.size() * 100);

As per JLS 15.17.2

Integer division rounds toward 0. That is, the quotient produced for operands n and d that are integers after binary numeric promotion (§5.6.2) is an integer value q whose magnitude is as large as possible while satisfying |d · q| ≤ |n|. Moreover, q is positive when |n| ≥ |d| and n and d have the same sign, but q is negative when |n| ≥ |d| and n and d have opposite signs.