Are loops really faster in reverse

Question

I ve heard this quite a few times  Are JavaScript loops really faster when counting backward  If so  why  I ve seen a few test suite examples showing that reversed loops are quicker  but I can t find any explanation as to why   I m assuming it s because the loop no longer has to evaluate a property each time it checks to see if it s finished and it just checks against the final numeric value   I e    for  var i   count - 1  i  gt   0  i--         count is only evaluated once and then the comparison is always on 0

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Love it   lots of marks up but no answer  D  Simply put a comparison against zero is always the fastest comparison  So  a  0  is actually quicker at returning True than  a  5   It s small and insignificant and with 100 million rows in a collection it s measurable   i e on a loop up you might be saying   where i  lt   array length and be incrementing i  on a down loop you might be saying where i    0 and be decrementing i instead   The comparison is quicker  Not the  direction  of the loop

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The best approach to answering this sort of question is to actually try it  Set up a loop that counts a million iterations or whatever  and do it both ways  Time both loops  and compare the results   The answer will probably depend on which browser you are using  Some will have different results than others

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It depends on placement of your array in memory and the hit ratio of memory pages while you are accessing that array    In some cases accessing array members in column order is faster than row order because of the increase in hit ratio

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Well  I don t know about JavaScript  it should really be just a matter of re-evaluation array length and maybe something to do with the associative arrays  if you only decrement  it is unlikely new entries would need to be allocated - if the array is dense  that is  someone may optimize for that    In low-level assembly  there is a looping instruction  called DJNZ  decrement and jump if non-zero   So the decrement and jump is all in one instruction  making it possibly ever-so-slightly faster than INC and JL   JB  increment  jump if less than   jump if below   Also  comparing against zero is simpler than comparing against another number  But all that is really marginal and also depends on target architecture  could make difference e g  on Arm in a smartphone    I wouldn t expect this low-level differences to have so great impact on interpreted languages  I just haven t seen DJNZ among the responses so I thought I would share an interesting thought

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The last time I bothered about it was when writing 6502 assembly  8-bit  yeah    The big gain is that most arithmetic operations  especially decrements  updated a set of flags  one of them was Z  the  reached zero  indicator   So  at the end of the loop you just did two instructions  DEC  decrement  and JNZ  jump if not zero   no comparison needed

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First  i   and i-- take exactly the same time on any programming language  including JavaScript   The following code take much different time   Fast   for  var i   0  len   Things length - 1  i  lt   len  i      Things i       Slow   for  var i   0  i  lt   Things length - 1  i      Things i       Therefore the following code take different time too   Fast   for  var i   Things length - 1  i  gt   0  i--    Things i       Slow   for  var i   0  i  lt   Things length - 1  i      Things i       P S  Slow is slow only for a few languages  JavaScript engines  because of compiler s optimization  The best way is to use   lt   instead   lt     or      and  --i  instead  i--

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In very simple words   i-- and i    Actually  they re both takes the same time    but in this case when you have incremental operation   processor evaluate the  length every time variable is incremented by 1 and in case of decrement   particularly in this case  it will evaluate  length only once till we get 0

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I don t think that it makes sense to say that i-- is faster that i   in JavaScript   First of all  it totally depends on JavaScript engine implementation    Secondly  provided that simplest constructs JIT ed and translated to native instructions  then i   vs i-- will totally depend on the CPU that executes it  That is  on ARMs  mobile phones  it s faster to go down to 0 since decrement and compare to zero are executed in a single instruction   Probably  you thought that one was waster than the other because suggested way is  for var i   array length  i--      but suggested way is not because one faster then the other  but simply because if you write  for var i   0  i  lt  array length  i      then on every iteration array length had to be evaluated  smarter JavaScript engine perhaps could figure out that loop won t change length of the array   Even though it looks like a simple statement  it s actually some function that gets called under the hood by the JavaScript engine   The other reason  why i-- could be considered  faster  is because JavaScript engine needs to allocate only one internal variable to control the loop  variable to the var i   If you compared to array length or to some other variable then there had to be more than one internal variable to control the loop  and the number of internal variables are limited asset of a JavaScript engine  The less variables are used in a loop the more chance JIT has for optimization  That s why i-- could be considered faster

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It s not that i-- is faster than i    Actually  they re both equally fast   What takes time in ascending loops is evaluating  for each i  the size of your array  In this loop   for var i   array length  i--     You evaluate  length only once  when you declare i  whereas for this loop  for var i   1  i  lt   array length  i      you evaluate  length each time you increment i  when you check if i  lt   array length   In most cases you shouldn t even worry about this kind of optimization

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vs  -- does not matter because JavaScript is an interpreted language  not a compiled language  Each instruction translates to more than one machine language and you should not care about the gory details   People who are talking about using --  or     to make efficient use of assembly instructions are wrong  These instruction apply to integer arithmetic and there are no integers in JavaScript  just numbers   You should write readable code

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It used to be said that --i was faster  in C    because there is only one result  the decremented value  i-- needs to store the decremented value back to i and also retain the original value as the result  j   i--    In most compilers this used up two registers rather than one which could cause another variable to have to be written to memory rather than retained as a register variable   I agree with those others that have said it makes no difference these days

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The way you re doing it now isn t faster  apart from it being an indefinite loop  I guess you meant to do i--   If you want to make it faster do   for  i   10  i--           super fast loop     of course you wouldn t notice it on such a small loop  The reason it s faster is because you re decrementing i while checking that it s  true   it evaluates to  false  when it reaches 0

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for var i   array length  i--    is not much faster  But when you replace array length with super puper function    that may be significantly faster  since it s called in every iteration   That s the difference   If you are going to change it in 2014  you don t need to think about optimization  If you are going to change it with  Search  amp  Replace   you don t need to think about optimization  If you have no time  you don t need to think about optimization  But now  you ve got time to think about it   P S   i-- is not faster than i

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Since none of the other answers seem to answer your specific question  more than half of them show C examples and discuss lower-level languages  your question is for JavaScript  I decided to write my own    So  here you go   Simple answer  i-- is generally faster because it doesn t have to run a comparison to 0 each time it runs  test results on various methods are below   Test results  As  proven  by this jsPerf  arr pop   is actually the fastest loop by far  But  focusing on --i  i--  i   and   i as you asked in your question  here are jsPerf  they are from multiple jsPerf s  please see sources below  results summarized   --i and i-- are the same in Firefox while i-- is faster in Chrome   In Chrome a basic for loop  for  var i   0  i  lt  arr length  i     is faster than i-- and --i while in Firefox it s slower   In Both Chrome and Firefox a cached arr length is significantly faster with Chrome ahead by about 170 000 ops sec   Without a significant difference    i is faster than i   in most browsers  AFAIK  it s never the other way around in any browser   Shorter summary  arr pop   is the fastest loop by far  for the specifically mentioned loops  i-- is the fastest loop   Sources  http   jsperf com fastest-array-loops-in-javascript 15  http   jsperf com ipp-vs-ppi-2  I hope this answers your question

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I made a comparison on jsbench   As alestani pointed out  one thing that takes time in ascending loops  is to evaluate  for each iteration  the size of your array  In this loop   for   var i   1  i  lt   array length  i       you evaluate  length each time you increment i  In this one   for   var i   1  l   array length  i  lt   l  i       you evaluate  length only once  when you declare i  In this one   for   var i   array length  i--      the comparison is implicit  it happens just before decrementing i  and the code is very readable  However  what can make a terrific difference  is what you put inside the loop    Loop with call to function  defined elsewhere    for  i   values length  i--        add  values i         Loop with inlined code   var sum   0  for   i   values length  i--        sum    values i       If you can inline your code  instead of calling a function  without sacrificing legibility  you can have a loop an order of magnitude faster     Note  as browser are becoming good at inlining simple functions  it really depends on how complex your code is  So  profile before optimizing  because   The bottleneck may be elsewhere  ajax  reflow       You may choose a better algorithm You may choose a better data structure   But remember       Code is written for people to read  and only incidentally for machines to execute

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Short answer For normal code  especially in a high level language like JavaScript  there is no performance  difference in i   and i--  The performance criteria is the use in the for loop and the compare statement  This applies to all high level languages and is mostly independent from the use of JavaScript  The explanation is the resulting assembler code at the bottom line  Detailed explanation A performance difference may occur in a loop  The background is that on the assembler code level you can see that a compare with 0 is just one statement which doesn t need an additional register  This compare is issued on every pass of the loop and may result in a measurable performance improvement  for var i   array length  i--     will be evaluated to a pseudo code like this   i array length   LOOP START  decrement i  if   i   0   goto  LOOP END      BODY CODE   LOOP END  Note that 0 is a literal  or in other words  a constant value  for var i   0   i  lt  array length  i      will be evaluated to a pseudo code like this  normal interpreter optimisation supposed    end array length  i 0   LOOP START  if   i  lt  end   goto  LOOP END  increment i      BODY CODE   LOOP END  Note that end is a variable which needs a CPU register  This may invoke an additional register swapping in the code and needs a more expensive compare statement in the if statement  Just my 5 cents For a high level language  readability  which facilitates maintainability  is more important as a minor performance improvement  Normally the classic iteration from array start to end is better  The quicker iteration from array end to start results in the possibly unwanted reversed sequence  Post scriptum As asked in a comment  The difference of --i and i-- is in the evaluation of i before or after the decrementing  The best explanation is to try it out  -  Here is a Bash example     i 10  echo  quot    --i   -- gt   i quot   9 -- gt  9    i 10  echo  quot    i--   -- gt   i quot   10 -- gt  9

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i-- is as fast as i    This code below is as fast as yours  but uses an extra variable   var up   Things length  for  var i   0  i  lt  up  i          Things i       The recommendation is to NOT evaluate the size of the array each time  For big arrays one can see the performance degradation

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Wouldn t the compiler cache  length  and therefore it makes no difference if you are comparing 0 or  length  I guess this is very specific to the compiler or interpreter you are dealing with   I would say if you are using a well optimised compiler or interpreter then you shouldn t worry about this  it is for the language developers to worry about

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Since  you are interested in the subject  take a look at Greg Reimer s weblog post about a  JavaScript loop benchmark  What s the Fastest Way to Code a Loop in JavaScript    I built a loop benchmarking test suite for different ways of coding loops in JavaScript  There are a few of these out there already  but I didn t find any that acknowledged the difference between native arrays and HTML collections   You can also do a performance test on a loop by opening https   blogs oracle com greimer resource loop-test html  does not work if JavaScript is blocked in the browser by  for example  NoScript   EDIT  A more recent benchmark created by Milan Adamovsky can be performed in run-time here for different browsers  For a Testing in Firefox 17 0 on Mac OS X 10 6 I got the following loop

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This guy compared a lot of loops in javascript  in a lot of browsers   He also has a test suite so you can run them yourself   In all cases  unless I missed one in my read  the fastest loop was   var i   arr length    or 10 while i--

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This is just a guess  but maybe it s because it s easier for the processor to compare something with 0   i    0   instead of with another value   i  lt  Things length

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It can be explained by JavaScript  and all languages  eventually being turned into opcodes to run on the CPU   CPUs always have a single instruction for comparing against zero  which is damn fast   As an aside  if you can guarantee count is always  gt   0  you could simplify to   for  var i   count  i--          whatever

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HELP OTHERS AVOID A HEADACHE --- VOTE THIS UP     The most popular answer on this page does not work for Firefox 14 and does not pass the jsLinter     while  loops need a comparison operator  not an assignment   It does work on chrome  safari  and even ie  But dies in firefox   THIS IS BROKEN   var i   arr length    or 10 while i--                THIS WILL WORK   works on firefox  passes the jsLinter   var i   arr length    or 10 while i gt -1              i   i - 1

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I want to contribute to this thread the fastest loop in JavaScript that is cross-browser    This loop yields over 500  improvement compared to the reverse while loop   My Blog  Fastest Loop in JavaScript

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The fastest way for decrement mode    var iteration   10000000      while iteration  gt  0           iteration--          FASTER THAN   var iteration   10000000      while iteration--     Javascript micro optimisation test

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It s not the -- or     it is the compare operation  With -- you can use a compare with 0  while with    you need to compare it with the length  On the processor  compare with zero is normally available  while compare with a finite integer requires a subtraction   a    lt  length  is actually compiled as  a   test  a-length    So it takes longer on the processor when compiled

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I ve seen the same recommendation in Sublime Text 2   Like it was already said  the main improvement is not evaluating the array s length at each iteration in the for loop  This a well-known optimization technique and particularly efficient in JavaScript when the array is part of the HTML document  doing a for for the all the li elements    For example    for  var i   0  i  lt  document getElementsByTagName  li   length  i      is much slower than   for  var i   0  len   document getElementsByTagName  li   length  i  lt  len  i     From where I m standing  the main improvement in the form in your question is the fact that it doesn t declare an extra variable  len in my example   But if you ask me  the whole point is not about the i   vs i-- optimization  but about not having to evaluate the length of the array at each iteration  you can see a benchmark test on jsperf

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Using the prefix increment operator is somewhat faster  With the postfix  the compiler must retain the previous value as the result of the expression   for  var i   0  i  lt  n    i      do stuff        A smart interpreter or compiler will see that the result of i   is not used and not store the result of the expression  but not all js engines do

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To cut it short  There is absolutely no difference in doing this in JavaScript  First of all  you can test it yourself   jsperf - is an excellent platform for all sorts of performance testing in JavaScript   http   jsperf com inc-vs-dec-2   Not only can you test and run any script in any JavaScript library  but you also have access to the whole bunch of previously written scripts  as well as the abilty to see differences between execution time in different browsers on different platforms  So as far as you can see  there is no difference between performance in any environment  If you want to improve performance of your script  things you can try to do   Have a var a   array length  statement so that you will not be calculating its value each time in the loop Do loop unrolling http   en wikipedia org wiki Loop unwinding  But you have to understand that the improvement you can gain will be so insignificant  that mostly you should not even care about it  My own opinion why such a misconception  Dec vs Inc  appeared A long  long time ago there was a common machine instruction  DSZ  Decrement and Skip on Zero   People who programmed in assembly language used this instruction to implement loops in order to save a register  Now this ancient facts are obsolete  and I am pretty sure you will not get any performance improvement in any language using this pseudo improvement  I think the only way such knowledge can propagate in our time is when you read another s person code  See such a construction and ask why was it implemented and here the answer   quot it improves performance because it compares to zero quot   You became bewildered of higher knowledge of your colleague and think to use it to be much smarter  -

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Not a lot of time is consumed by i-- or i    If you go deep inside the CPU architecture the    is more speedy than the --  since the -- operation will do the 2 s complement  but it happend inside the hardware so this will make it speedy and no major difference between the    and -- also these operations are considered of the least time consumed in the CPU     The for loop runs like this    Initialize the variable once at the start    Check the constraint in the second operand of the loop   lt    gt    lt    etc  Then apply the loop    Increment the loop and loop again throw these processes again      So     for  var i   Things length - 1  i  gt   0  i--        Things i        will calculate the array length only once at the start and this is not a lot of time  but  for var i   array length  i--       will calculate the length at each loop  so it will consume a lot of time

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Sometimes making some very minor changes to the way that we write our code can make a big difference to how quickly our code actually runs  One area where a minor code change can make a big difference to execution times is where we have a for loop that is processing an array  Where the array is of elements on the web page  such as radio buttons  the change has the biggest effect but it is still worth applying this change even where the array is internal to the Javascript code   The conventional way of coding a for loop to process an array lis like this   for  var i   0  i  lt  myArray length  i           The problem with this is that evaluating the length of the array using myArray length takes time and the way that we have coded the loop means that this evaluation has to be performed every time around the loop  If the array contains 1000 elements then the length of the array will be evaluated 1001 times  If we were looking at radio buttons and had myForm myButtons length then it will take even longer to evaluate since the appropriate group of buttons within the specified form must first be located before the length can be evaluated each time around the loop   Obviously we don t expect the length of the array to change while we are processing it so all of these recalculations of the length are just adding unnecessarily to the processing time   Of course if you have code inside the loop that adds or removes array entries then the array size can change between iterations and so we can t change the code that tests for it   What we can do to correct this for a loop where the size is fixed is to evaluate the length once at the start of the loop and save it in a variable  We can then test the variable to decide when to terminate the loop  This is much faster than evaluating the array length each time particularly when the array contains more than just a few entries or is part of the web page   The code to do this is   for  var i   0  var j   myArray length  i  lt  j  i           So now we only evaluate the size of the array once and test our loop counter against the variable that holds that value each time around the loop  This extra variable can be accessed much faster than evaluating the size of the array and so our code will run much faster than before  We just have one extra variable in our script   Often it doesn t matter what order we process the array in as long as all of the entries in the array get processed  Where this is the case we can make our code slightly faster by doing away with the extra variable that we just added and processing the array in reverse order   The final code that processes our array in the most efficient way possible is   for  var i   myArray length-1  i  gt  -1  i--         This code still only evaluates the size of the array once at the start but instead of comparing the loop counter with a variable we compare it with a constant  Since a constant is even more effective to access than a variable and since we have one fewer assignment statement than before our third version of the code is now slightly more efficient than the second version and vastly more efficient than the first

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This is not dependent on the -- or    sign  but it depends on conditions you apply in the loop   For example  Your loop is faster if the variable has a static value than if your loop check conditions every time  like the length of an array or other conditions   But don t worry about this optimization  because this time its effect is measured in nanoseconds

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I try to give a broad picture with this answer   The following thoughts in brackets was my belief until I have just recently tested the issue     In terms of low level languages like C C    the code is compiled so that the processor has a special conditional jump command when a variable is zero  or non-zero   Also  if you care about this much optimization  you could go   i instead of i    because   i is a single processor command whereas i   means j i 1  i j       Really fast loops can be done by unrolling them    for i 800000 i gt 0 --i      do it i     It can be way slower than  for i 800000 i gt 0 i- 8        do it i   do it i-1   do it i-2       do it i-7       but the reasons for this can be quite complicated  just to mention  there are the issues of processor command preprocessing and cache handling in the game    In terms of high level languages  like JavaScript as you asked  you can optimize things if you rely on libraries  built-in functions for looping  Let them decide how it is best done   Consequently  in JavaScript  I would suggest using something like  array forEach function i        do it i         It is also less error-prone and browsers have a chance to optimize your code     REMARK  not only the browsers  but you too have a space to optimize easily  just redefine the forEach function  browser dependently  so that it uses the latest best trickery      A M K  says in special cases it is worth rather using array pop or array shift  If you do that  put it behind the curtain  The utmost overkill is to add options to forEach to select the looping algorithm    Moreover  also for low level languages  the best practice is to use some smart library function for complex  looped operations if it is possible    Those libraries can also put things  multi-threaded  behind your back and also specialized programmers keep them up-to-date   I did a bit more scrutiny and it turns out that in C C    even for 5e9    50 000x100 000  operations  there is no difference between going up and down if the testing is done against a constant like  alestanis says   JsPerf results are sometimes inconsistent but by and large say the same  you can t make a big difference   So --i happens to be rather a  posh  thing  It only makes you look like a better programmer      On the other hand  for-unrolling in this 5e9 situation  it has brought me down from 12 nbsp sec to 2 5 nbsp sec when I went by 10s  and to 2 1 nbsp sec when I went by 20s  It was without optimization  and optimization has brought things down to unmeasureable little time      Unrolling can be done in my way above or using i    but that does not bring things ahead in JavaScript     All in all  keep i-- i   and   i i   differences to the job interviews  stick to array forEach or other complex library functions when available

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In many cases  this has essentially nothing to do with the fact that processors can compare to zero faster than other comparisons   This is because only a few Javascript engines  the ones in the JIT list  actually generate machine language code   Most Javascript engines build an internal representation of the source code which they then interpret  to get an idea of what this is like  have a look near the bottom of this page on Firefox s SpiderMonkey   Generally if a piece of code does practically the same thing but leads to a simpler internal representation  it will run faster   Bear in mind that with simple tasks like adding subtracting one from a variable  or comparing a variable to something  the overhead of the interpreter moving from one internal  instruction  to the next is quite high  so the less  instructions  that are used internally by the JS engine  the better

[javascript] Are loops really faster in reverse?

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