Question

So, I am trying to measure the execution time of some sorting methods.

Here is my code:

public static void main(String[] args)
{
    ...

    MeasureExecutionTime(new Runnable() { public void run() { insertionSort(C); } }, "insertionSort()");
}

=====================

private static void MeasureExecutionTime(Runnable r, String s)
{
    startTime = System.nanoTime();
    try
    {
        r.run();
    }
    finally
    {
        endTime = System.nanoTime();
    }
    elapsedTime = endTime - startTime;
    System.out.println(s + " takes " + elapsedTime + " nano-seconds which is " + formatTime(elapsedTime));
}

=====================

public static String formatTime(long nanoSeconds)
{
    long hours, minutes, remainder, totalSecondsNoFraction;
    double totalSeconds, seconds;

    totalSeconds = (double) nanoSeconds / 1000000000.0;
    String s = Double.toString(totalSeconds);
    String [] arr = s.split("\\.");
    totalSecondsNoFraction = Integer.parseInt(arr[0]);
    hours = totalSecondsNoFraction / 3600;
    remainder = totalSecondsNoFraction % 3600;
    minutes = remainder / 60;
    seconds = remainder % 60;
    seconds = Double.parseDouble(Long.toString((long)seconds) + Double.parseDouble("." + arr[1]));

    StringBuilder result = new StringBuilder(".");
    String sep = "", nextSep = " and ";
    if(seconds > 0)
    {
        if(seconds > 1) result.insert(0, " seconds").insert(0, seconds);
        else result.insert(0, " second").insert(0, seconds);
        sep = nextSep;
        nextSep = ", ";
    }
    if(minutes > 0)
    {
        if(minutes > 1) result.insert(0, sep).insert(0, " minutes").insert(0, minutes);
        else result.insert(0, sep).insert(0, " minute").insert(0, minutes);
        sep = nextSep;
        nextSep = ", ";
    }
    if(hours > 0)
    {
        if(hours > 1) result.insert(0, sep).insert(0, " hours").insert(0, hours);
        else result.insert(0, sep).insert(0, " hour").insert(0, hours);
    }
    return result.toString();
}

My problem is:

After run this program, and I enter int[1000000] as an input, it executes insertionSort() in about 12-13 minutes and then returns:

insertionSort() takes 767186856920 nano-seconds which is 12 minutes and 470.18685692 seconds.

why it gives 470 seconds? what is wrong in my code?

=========================

EDIT:

After replacing seconds = Double.parseDouble(Long.toString((long)seconds) + Double.parseDouble("." + arr[1])); with seconds = seconds + Double.parseDouble("." + arr[1]);, the previous issue is gone, but another issue showed up:

insertionSort() takes 22864 nano-seconds which is 2.000002864 seconds.

It should be 0.000022864 seconds.

=========================

EDIT2:

I might discover the error. when nanoSeconds is large, arr[1] will be ok but when nanoSeconds is small, arr[1] will convert to exponential form, i.e 14931 nano-seconds => 4.931E-6 seconds.. How can I solve this issue?

==========================

EDIT3:

Ok, I found the solution:

if(arr[1].contains("E")) seconds = Double.parseDouble("." + arr[1]);
else seconds += Double.parseDouble("." + arr[1]);

Answer 1

The problem is here:

seconds = Double.parseDouble(Long.toString((long)seconds) + 
                             Double.parseDouble("0." + arr[1]));

Say seconds is 12 entering this line and arr[1] is "456". Then

seconds = Double.parseDouble("12" + Double.parseDouble("0.456"));
seconds = Double.parseDouble("12" + 0.456);
seconds = Double.parseDouble("12" + "0.456");
seconds = Double.parseDouble("120.456");
seconds = 120.456.

Why not just do:

seconds = seconds + Double.parseDouble("0." + arr[1]);