Going Loopy for Performance (C#)

I may have written about this before, and if so, I apologize.  Wait, no I don't.  I couldn't find it and this blog is for me anyway.  I retract my apology.  Anyway, if you've ever wondered whether your loop is as fast as it can be, I have some answers for you.

I read one time that a for loop is faster than a foreach loop in C#.  I can buy that, I suppose.  But in the same article it said that iterating backward through a for loop is faster than iterating forward and I had a hard time getting on board with that.  So I wrote some tests.  That author was right.

I wanted to test a variety of options, so I did.  That's how I roll.  I ran each test over 100,000,000 iterations.

The first test was a regular for loop:

...and the results:


Next up I did the same thing but without previously specifying the size of the loop (I use .Count as the upper limit of the loop):

...and the results:


You can see that of the two options, it is slightly faster to set the upper limit in a variable.  "Slightly" here meaning 11 milliseconds over 100,000,000 iterations, or .00000011 milliseconds per iteration.

Moving on.  Next we have the reason for my testing in the first place: the reverse for loop:

...and the results:


Whoa!  That's a bit of an improvement.  37 milliseconds over 100,000,000 iterations (compared to our previous best time), or .00000037 milliseconds per iteration.

This time the reverse loop without a variable:

...and the results:


Based on the earlier results of the forward for loop, this makes sense.  Next was a foreach loop over a list:

...and the results:


Wow.  That's a significant degradation.  Our previous worst time (a forward for loop with a variable as our upper bound) was 240 milliseconds.  The foreach on a list (which is, in my opinion, the easiest loop to write) takes nearly twice as long to run.

Last up is a foreach loop on an array instead of a list:

...and the results:


The overall results certainly point to a reverse for loop with a variable as the lower bound is the fastest we can iterate through an array.

BUT WAIT, THERE'S MORE!  I found this last bit while I was writing this up.  It's an alternative way to do a decrementing for loop that's actually slightly faster than anything else up there.

...and the results:


I learned a couple of lessons on this one.  First, don't always go with the easiest thing to do because it may not be the best thing to do.  Second, minor functions can have major performance implications.  In this example, I just incremented a variable during the iteration, but in a real world scenario I may do anything from build a complex object to make a service call.  This basic example showed us a difference of 279 milliseconds over 100,000,000 iterations.  That is a really small number per iteration, but keep in mind that the only thing we were doing was incrementing a counter.  If we had been doing any real work that would have been a major performance hit.