使用2个变量遍历Python列表

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.

Answer 3

The most pythonic way is to use list comprehension.

 In [9]: print([f'{i} {j}' for i, j in zip(named_list, reversed(named_list)) if i < j])
Out [9]: ['0 9', '1 8', '2 7', '3 6', '4 5', '5 4', '6 3', '7 2', '8 1', '9 0']

This list comprehension is shortcut for

results = []
for i, j in zip (named_list, reversed(named_list)):
   if i >= j:
      break
   results.append((i,j))

Answer 4

I am sure you will prefer this:

for i in zip(alist, alist[::-1]):
    print(i) 
    # use i[0] and i[1] if you need both in different vars

You can use i[0] and i[1] to access variable separately.

Output:

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

Answer 5

If you want two index lists, the one starting from lower to higher and the other is inverse, range() function is pretty much there to help.

For example if you have a list x = [1,2,3,4,5,6,7,8,9,10] then you can find both the indexes as:

low_to_high_idx = list(range(0, len(x)-1))
high_to_low_idx = list(range(len(x)-1, -1, -1))

You can get both indexes then using zip() function