Which loop is faster, while or for?

Question

You can get the same output with for and while loops:

While:

$i = 0;
while ($i <= 10){
  print $i.\"\\n\";
  $i++;
};


        

Answer 1

I also tried to benchmark the different kinds of loop in C#. I used the same code as Shane, but I also tried with a do-while and found it to be the fastest. This is the code:

using System;
using System.Diagnostics;


public class Program
{
    public static void Main()
    {
        int max = 9999999;
        Stopwatch stopWatch = new Stopwatch();

        Console.WriteLine("Do While Loop: ");
        stopWatch.Start();
        DoWhileLoop(max);
        stopWatch.Stop();
        DisplayElapsedTime(stopWatch.Elapsed);
        Console.WriteLine("");
        Console.WriteLine("");

        Console.WriteLine("While Loop: ");
        stopWatch.Start();
        WhileLoop(max);
        stopWatch.Stop();
        DisplayElapsedTime(stopWatch.Elapsed);
        Console.WriteLine("");
        Console.WriteLine("");

        Console.WriteLine("For Loop: ");
        stopWatch.Start();
        ForLoop(max);
        stopWatch.Stop();
        DisplayElapsedTime(stopWatch.Elapsed);
    }

    private static void DoWhileLoop(int max)
    {
        int i = 0;
        do
        {
            //Performe Some Operation. By removing Speed increases
            var j = 10 + 10;
            j += 25;
            i++;
        } while (i <= max);
    }

    private static void WhileLoop(int max)
    {
        int i = 0;
        while (i <= max)
        {
            //Performe Some Operation. By removing Speed increases
            var j = 10 + 10;
            j += 25;
            i++;
        };
    }

    private static void ForLoop(int max)
    {
        for (int i = 0; i <= max; i++)
        {
            //Performe Some Operation. By removing Speed increases
            var j = 10 + 10;
            j += 25;
        }
    }

    private static void DisplayElapsedTime(TimeSpan ts)
    {
        string elapsedTime = String.Format("{0:00}:{1:00}:{2:00}.{3:00}", ts.Hours, ts.Minutes, ts.Seconds, ts.Milliseconds / 10);
        Console.WriteLine(elapsedTime, "RunTime");
    }
}

and these are the results of a live demo on DotNetFiddle:

Do While Loop:
00:00:00.06

While Loop:
00:00:00.13

For Loop:
00:00:00.27

Answer 2

I find the fastest loop is a reverse while loop, e.g:

var i = myArray.length;
while(i--){
  // Do something
}

Answer 3

Isn't a For Loop technically a Do While?

E.g.

for (int i = 0; i < length; ++i)
{
   //Code Here.
}

would be...

int i = 0;
do 
{
  //Code Here.
} while (++i < length);

I could be wrong though...

Also when it comes to for loops. If you plan to only retrieve data and never modify data you should use a foreach. If you require the actual indexes for some reason you'll need to increment so you should use the regular for loop.

for (Data d : data)
{
       d.doSomething();
}

should be faster than...

for (int i = 0; i < data.length; ++i)
{
      data[i].doSomething();
}

Answer 4

If that were a C program, I would say neither. The compiler will output exactly the same code. Since it's not, I say measure it. Really though, it's not about which loop construct is faster, since that's a miniscule amount of time savings. It's about which loop construct is easier to maintain. In the case you showed, a for loop is more appropriate because it's what other programmers (including future you, hopefully) will expect to see there.

Answer 5

I was wondering the same thing so i googled and ended up here. I did a small test in python (extremely simple) just to see and this is what I got:

For:

def for_func(n = 0):
    for n in range(500):
        n = n + 1

python -m timeit "import for_func; for_func.for_func()" > for_func.txt

10000 loops, best of 3: 40.5 usec per loop

While:

def while_func(n = 0):
    while n < 500:
        n = n + 1

python -m timeit "import while_func; while_func.while_func()" > while_func.txt

10000 loops, best of 3: 45 usec per loop

Answer 6

As for infinite loops for(;;) loop is better than while(1) since while evaluates every time the condition but again it depends on the compiler.