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Loops: while vs. for

  • #1
  • First Post: Jun 7, 2007 12:09pm Jun 7, 2007 12:09pm
I know the general distinction between the while and for loops is that for has a known number of iterations and while does not. I was wandering however if there are any other more subtle differences. Like which one is considered to be faster? Are there any situations when any of the 2 could be used but one of them would be better for the task at hand?
Just trying to get some general MQL understanding...
Stubbornly persistent
  • #2
  • Edited 1:14pm Jun 7, 2007 12:56pm | Edited 1:14pm
  •  tesla
  • | Joined Oct 2006 | Status: Friendly Neighborhood Programmer | 533 Posts
General consensus is that 'while' loops are faster, but 'for' loops are more readable. This depends on the compiler, and some compilers will end up with the same machine code for both which makes the performance issue irrelevant.

The performance hit is not an issue in 99% of the stuff I write, I tend to use 'for' loops as a matter of preference.

Some other interesting tidbits:
Decrementing is faster than incrementing, have your loops count down for maximum performance.
'==' is much faster than '<=' or '>=', typically between 15-30%.
  • #3
  • Jun 7, 2007 1:13pm Jun 7, 2007 1:13pm
Quoting tesla
General consensus is that 'while' loops are general faster, but 'for' loops are more readable. This depends on the compiler, and some compilers will end up with the same machine code for both which makes the performance issue irrelevant.

The performance hit is not an issue in 99% of the stuff I write, I tend to use 'for' loops as a matter of preference.

Some other interesting tidbits:
Decrementing is faster than incrementing, have your loops count down for maximum performance.
'==' is much faster than '<=' or '>=', typically between 15-30%.
Thanks tesla that's very interesting info!
Stubbornly persistent
  • #4
  • Jun 8, 2007 8:47am Jun 8, 2007 8:47am
tesla makes some very good points,

However i would say no matter how concerned you are with performance changing for's and while's is not a good idea.
Use a for loop when it makes sense and a while when it makes sense.

Sluggish performance is more like likely due to to many experts running, badly written indicators(re-calcing to many bars in the past/timeframes), order handling(checking/modifying every tick) and string handling.
Look to optimise those first.

Remember in any language, READABILITY is just as important, comments are free as is whitespace so make your code easy to maintain. you can always get a faster 'puter

have fun.
What if the Hokey Cokey is what its all about?
  • #5
  • Jun 8, 2007 10:11am Jun 8, 2007 10:11am
Telsa,

Concerning while vs for, your statement is true depending on how you right your for statements and what's contained in your while loop

when writing a for loop, you have the option of assignment, requirements and reassignment (most of the time this will be iteration).

when doing a while loop, you only have the option for requirement...meaning the while loop ONLY checks the requirements by default.

I believe that from a compiled perspective:

for(i = 0;i < 10;i++) {...}

would be the compiled equivalent of

i = 0;
while (i < 10) {
...
i++;
}

furthermore, you could write an assignment-less for loop that would be the compiled equivalent of a while loop:

for ( ;condition == true; ) {
if (SomethingHappens()) {
condition == false;
}
}

would be the same as

while (condition == true) {
if (SomethingHappens()) {
condition == false;
}
}

This is why I believe generally while loops are considered faster because often times we use them to assert only a condition (not necessarily for iteration) while we tend to use for loops almost soley for iterative purposes.

Make sense?

Quoting tesla
General consensus is that 'while' loops are faster, but 'for' loops are more readable. This depends on the compiler, and some compilers will end up with the same machine code for both which makes the performance issue irrelevant.

The performance hit is not an issue in 99% of the stuff I write, I tend to use 'for' loops as a matter of preference.

Some other interesting tidbits:
Decrementing is faster than incrementing, have your loops count down for maximum performance.
'==' is much faster than '<=' or '>=', typically between 15-30%.
  • #6
  • Jun 8, 2007 1:03pm Jun 8, 2007 1:03pm
I have looked at few indicators recently for learning purposes and it seems that almost all of them use the for loop to populate the indicator buffers and draw the indicators. I have also noticed that almost all of them use the an incrementing loop to do that:

for (int i=0; i < limit; i++)

With my limited MQL understanding it seems that such a loop would be plotting the indicator backwards (ie starting from the most recent candle). Am I corect? Does it also mean that the whole ind would be recalculated every time the start() function is initialized? Wouldn't it make more sense to use a decrementing loop?:

for (int i = limit; i > 0; i--)

Just trying to understand...
Stubbornly persistent
  • #7
  • Jun 8, 2007 1:35pm Jun 8, 2007 1:35pm
Quoting sergiu
I know the general distinction between the while and for loops is that for has a known number of iterations and while does not. I was wandering however if there are any other more subtle differences. Like which one is considered to be faster? Are there any situations when any of the 2 could be used but one of them would be better for the task at hand?
Just trying to get some general MQL understanding...
I've not delved into programming my own MQL, but I am a professional programmer and I can tell you that in my corporate life I rarely see while loops. Is this because there is anything wrong with a while loop? Not at all. It is largely a stylistic choice. However, this is why I tend to favor a for loop unless a while loop is just the obvious choice:

  1. The for loop gives you a built-in index to reference during its execution. I very frequently end up using the loop's index value inside the loop for further calculations.
  2. IMHO, it's easier to create an infinite loop with a while loop. Obviously, well-written code does not have this problem, and you can create infinite for loops by erroneously altering the index value during the loop's execution. But generally speaking, when someone tells me that they found an infinite loop in their code, I assume that it was a while loop before even seeing the offending code.

  • #8
  • Last Post: Jun 8, 2007 1:50pm Jun 8, 2007 1:50pm
Byte,

I agree, the most common loops I come across these days are For or For Each or For In loops...

It's not so much because the loop has a "built in" index as much as for loop is a lot like an abbreviation for the while loop. Obviously, to replicate the functionality of an iterative for loop, you need to add 2 extra lines of code (your initialization assignment, and your iterative assignment).

I agree, as well, with the fact that for loops tend to be focused on a KNOWN limited set of iterations, which makes them the loop of choice for iterations where the set of accessible values is known. While loops tend to be more common where the set of iterations is unknown (or the count is inaccessible).

Quoting Bytebodger
I've not delved into programming my own MQL, but I am a professional programmer and I can tell you that in my corporate life I rarely see while loops. Is this because there is anything wrong with a while loop? Not at all. It is largely a stylistic choice. However, this is why I tend to favor a for loop unless a while loop is just the obvious choice:

  1. The for loop gives you a built-in index to reference during its execution. I very frequently end up using the loop's index value inside the loop for further calculations.
  2. IMHO, it's easier to create an infinite loop with a while loop. Obviously, well-written code does not have this problem, and you can create infinite for loops by erroneously altering the index value during the loop's execution. But generally speaking, when someone tells me that they found an infinite loop in their code, I assume that it was a while loop before even seeing the offending code.

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