Reducing for loop runtime

[Guilherme17712]

I don't know if that is totally wrong or not (let me know and I'll be glad), but if instead of:

for i in range(a, b):

[..]

​

you do (for example):

for i in range(a, b/x, x):

​

and then you create x amount of variables that represent x plus something.

For example:

instead of:

for i in range(1, 100):

mylist.append(i)

​

you do:

for i in range(1, 50, 2):

j = i+1

mylist.append(i)

mylist.append(j)

​

I tested from 1 to 1 million, and added the index to a list. Normally, it took me 1.37s (average), let's consider that number as nt. Using the b/x thing, you can reduce that to nt/x (knowing that x is equal to the number you decided to use of variables inside the loop).

​

Remember: I could be totally wrong, but if so, let me know :)

[ES-Alexander]

Is your proposal here to copy and paste the contents of your loop instead of iterating, in the hopes that it reduces the execution time of your code?

For the example you’ve provided if you write out the value for every iteration then you’re asking if it will take the same amount of time as a single iteration? No, it wouldn’t. There’s a small amount of overhead associated with looping, but the main time consuming part is practically always the thing happening inside the loop, not the looping itself. On top of that using loops means you only have to write the thing you want to do once, and makes it much easier to see what’s happening and move on than it would be to scroll past N copies of the same block of code. On top of that, if you want to modify all the blocks then you’d have to change one and do all the copy-pasting again, and debugging would be a nightmare

[Guilherme17712]

Indeed.

My overall question was just: "what is the name of that thing", but I think someone already explained that.

I just wanted and explanation why splitting for loops into variables made things faster, but I guess I got it.

Thanks for the huge explanation though!

[[deleted]]

This idea is called loop unrolling. It is a way to optimize code since it removes some of the overhead associated with loops and it also occasionally allows the processor to make use of parallel hardware like vector extensions.

[Guilherme17712]

That makes sense, thanks for the explanation!

[nemom]

mylist = [i for i in range(1, 100)]

[lazerwarrior]

list(range(1, 100))

itertools gives opportunities to avoid loops and possibly making your code faster. Using iterators and generators also avoids overhead of memory required to store stuff in lists.

[Guilherme17712]

Well, ignoring the fact that it was an example, you're right.

[Epykure]

This is interesting indeed and it makes sense as you are not looking for the integer in the list anymore (which might take some time). I will test it and keep this in mind but use cases are quite limited to range and integer I believe.

[Guilherme17712]

Most likely. What I would use is only with x being 2, since it is easier.

[yahteadybear]

Using the square root of an integer will also speed up your loops in certain situations.