Which loop is faster, while or for?

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我寻月下人不归
我寻月下人不归 2020-11-27 07:07

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

While:

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


        
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  • 2020-11-27 07:19

    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

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  • 2020-11-27 07:20

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

    var i = myArray.length;
    while(i--){
      // Do something
    }
    
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  • 2020-11-27 07:20

    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();
    }
    
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  • 2020-11-27 07:21

    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.

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  • 2020-11-27 07:25

    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

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  • 2020-11-27 07:26

    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.

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