Which loop is faster while or for

Question

You can get the same output with for and while loops   While    i   0  while   i  lt   10     print  i   n      i         For   for   i   0   i  lt   10   i       print  i   n       But which one is faster

User · Answer

Some optimizing compilers will be able to do better loop unrolling with a for loop  but odds are that if you re doing something that can be unrolled  a compiler smart enough to unroll it is probably also smart enough to interpret the loop condition of your while loop as something it can unroll as well

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Isn t a For Loop technically a Do While   E g   for  int i   0  i  lt  length    i         Code Here      would be     int i   0  do        Code Here    while    i  lt  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  lt  data length    i          data i  doSomething

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I used a for and while loop on a solid test machine  no non-standard 3rd party background processes running   I ran a for loop vs while loop as it relates to changing the style property of 10 000  lt button gt  nodes   The test is was run consecutively 10 times  with 1 run timed out for 1500 milliseconds before execution   Here is the very simple javascript I made for this purpose  function runPerfTest          use strict        function perfTest fn  ns            console time ns           fn            console timeEnd ns              var target   document getElementsByTagName  button         function whileDisplayNone             var x   0          while  target length  gt  x                target x  style display    none               x                         function forLoopDisplayNone             for  var i   0  i  lt  target length  i                  target i  style display    none                        function reset             for  var i   0  i  lt  target length  i                  target i  style display    inline-block                        perfTest function             whileDisplayNone            whileDisplayNone         reset         perfTest function             forLoopDisplayNone            forLoopDisplayNone         reset          function        runPerfTest        runPerfTest        runPerfTest        runPerfTest        runPerfTest        runPerfTest        runPerfTest        runPerfTest        runPerfTest        setTimeout function            console log  cool run            runPerfTest           1500         Here are the results I got  pen js 8 whileDisplayNone  36 987ms pen js 8 forLoopDisplayNone  20 825ms  pen js 8 whileDisplayNone  19 072ms pen js 8 forLoopDisplayNone  25 701ms  pen js 8 whileDisplayNone  21 534ms pen js 8 forLoopDisplayNone  22 570ms  pen js 8 whileDisplayNone  16 339ms pen js 8 forLoopDisplayNone  21 083ms  pen js 8 whileDisplayNone  16 971ms pen js 8 forLoopDisplayNone  16 394ms  pen js 8 whileDisplayNone  15 734ms pen js 8 forLoopDisplayNone  21 363ms  pen js 8 whileDisplayNone  18 682ms pen js 8 forLoopDisplayNone  18 206ms  pen js 8 whileDisplayNone  19 371ms pen js 8 forLoopDisplayNone  17 401ms  pen js 8 whileDisplayNone  26 123ms pen js 8 forLoopDisplayNone  19 004ms  pen js 61 cool run pen js 8 whileDisplayNone  20 315ms pen js 8 forLoopDisplayNone  17 462ms   Here is the demo link  Update  A separate test I have conducted is located below  which implements 2 differently written factorial algorithms  1 using a for loop  the other using a while loop   Here is the code   function runPerfTest          use strict        function perfTest fn  ns            console time ns           fn            console timeEnd ns              function whileFactorial num            if  num  lt  0                return -1                    else if  num     0                return 1                    var factl   num          while  num--  gt  2                factl    num                    return factl             function forFactorial num            var factl   1          for  var cur   1  cur  lt   num  cur                  factl    cur                    return factl             perfTest function            console log  Result  100000    forFactorial 80            forFactorial100         perfTest function            console log  Result  100000    whileFactorial 80            whileFactorial100         function        runPerfTest        runPerfTest        runPerfTest        runPerfTest        runPerfTest        runPerfTest        runPerfTest        runPerfTest        runPerfTest        console log  cold run  1500ms timeout         setTimeout runPerfTest  1500           And the results for the factorial benchmark   pen js 41 Result  100000  7 15694570462638e 118 pen js 8 whileFactorial100  0 280ms pen js 38 Result  100000  7 156945704626378e 118 pen js 8 forFactorial100  0 241ms pen js 41 Result  100000  7 15694570462638e 118 pen js 8 whileFactorial100  0 254ms pen js 38 Result  100000  7 156945704626378e 118 pen js 8 forFactorial100  0 254ms pen js 41 Result  100000  7 15694570462638e 118 pen js 8 whileFactorial100  0 285ms pen js 38 Result  100000  7 156945704626378e 118 pen js 8 forFactorial100  0 294ms pen js 41 Result  100000  7 15694570462638e 118 pen js 8 whileFactorial100  0 181ms pen js 38 Result  100000  7 156945704626378e 118 pen js 8 forFactorial100  0 172ms pen js 41 Result  100000  7 15694570462638e 118 pen js 8 whileFactorial100  0 195ms pen js 38 Result  100000  7 156945704626378e 118 pen js 8 forFactorial100  0 279ms pen js 41 Result  100000  7 15694570462638e 118 pen js 8 whileFactorial100  0 185ms pen js 55 cold run  1500ms timeout  pen js 38 Result  100000  7 156945704626378e 118 pen js 8 forFactorial100  0 404ms pen js 41 Result  100000  7 15694570462638e 118 pen js 8 whileFactorial100  0 314ms   Conclusion  No matter the sample size or specific task type tested  there is no clear winner in terms of performance between a while and for loop  Testing done on a MacAir with OS X Mavericks on Chrome evergreen

User · Answer

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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Depends on the language and most likely its compiler  but they should be equivalent in most languages

User · Answer

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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Set the loop iterations to 10 000    Find the time in milliseconds Run Loop find time in milliseconds and subtract the first timer   Do it for both codes  what ever one has the lowest milliseconds it runs faster  You might want to run the test multiple times and average them out to reduce the likelihood of background processes influencing the test   You are likely to get really similar times on both of them  but I am interested to see if one is always just slightly faster

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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  lt  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

User · Answer

They should be equal  The for loop you wrote is doing exactly the same thing that the while loop is doing  setting  i 0  printing  i  and incrementing  i at the end of the loop

User · Answer

In C   the For loop is slightly faster     For loop average about 2 95 to 3 02 ms     The While loop averaged about 3 05 to 3 37 ms   Quick little console app to prove    class Program               static void Main string   args                        int max   1000000000              Stopwatch stopWatch   new Stopwatch                 if  args Length    1  amp  amp  args 0  ToString       While                                 Console WriteLine  While Loop                      stopWatch Start                    WhileLoop max                   stopWatch Stop                    DisplayElapsedTime stopWatch Elapsed                             else                               Console WriteLine  For Loop                      stopWatch Start                    ForLoop max                   stopWatch Stop                    DisplayElapsedTime stopWatch Elapsed                                    private static void WhileLoop int max                        int i   0              while  i  lt   max                                  Console WriteLine i                   i                                      private static void ForLoop int max                        for  int i   0  i  lt   max  i                                    Console WriteLine i                                    private static void DisplayElapsedTime TimeSpan ts                           Format and display the TimeSpan value              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

User · Answer

It shouldn t matter which is faster   If it does matter then benchmark it using your real code and see for yourself   The answers to this other question might be useful as well  How to write more efficient code

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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  lt   max              private static void WhileLoop int max                int i   0          while  i  lt   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  lt   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

User · Answer

As others have said  any compiler worth its salt will generate practically identical code  Any difference in performance is negligible - you are micro-optimizing   The real question is  what is more readable   And that s the for loop  at least IMHO

User · Answer

That will depend on the language implementation of said loop  compiler and what not     Most compiler will compile to the exact same executable code for example in CIL   NET  they definitely do     Source  vcsjones   http   forums asp net t 1041090 aspx   Either way  the body of the loop is where the processing time will be spent not the way you iterate

User · Answer

That clearly depends on the particular implementation of the interpreter compiler of the specific language   That said  theoretically  any sane implementation is likely to be able to implement one in terms of the other if it was faster so the difference should be negligible at most    Of course  I assumed while and for behave as they do in C and similar languages   You could create a language with completely different semantics for while and for

User · Answer

I find the fastest loop is a reverse while loop  e g   var i   myArray length  while i--        Do something

[performance] Which loop is faster, while or for?

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