Loop through Python list with 2 variables

Question

In JavaScript, I can use two (2) pointers to get the values from a list at the beginning and end and then increment/decrement the pointers to continue. For example:

for (let i = 0, j = list.length - 1; i < j; i++, j--) {
    console.log(i, j);
}

How would I be able to accomplish the same in Python? I know that I can get the current index by doing:

for index in enumerate(list):
    print(index)

But how could I also initialize another pointer to start from the end?

Answer 1

I assume that you want a solution that can do anything with the indices, not just print them out. Python's for loops have other strengths. So use a while loop.

i = 0
j = len(alist)
while i < j:
    print(i, j)  # or console.log(i, j) or whatever you want here
    i += 1
    j -= 1

Here is something similar using zip, which is more pythonic. Here I illustrate doing something other than just printing the indices.

alist = [3, 1, 4, 1, 5, 9]
llen = len(alist)
for i, j in zip(range(llen // 2), range(llen - 1, -1, -1)):
    print(alist[i], alist[j])

But it is even more pythonic to ignore the indices and just use the items of the list, as in

alist = [3, 1, 4, 1, 5, 9]
llen = len(alist)
for u, v in zip(alist[:llen // 2], alist[::-1]):
    print(u, v)

Answer 2

Here's an example how you can do it. Take second index as function of length minus index minus one:

l = [1, 2, 3, 4]
for i, _ in enumerate(l):
    print(l[i], l[len(l)-i-1])

This will output

(1, 4)
(2, 3)
(3, 2)
(4, 1)

Not printing indexes themselves, but you can print them, if you chose to do so.