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?
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)
console.log is for JS, not python. Use print(i, j) instead
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.
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))
i < j.range. Also, your loop can be quite expensive for large lists because it doesn't break once i exceeds j.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)
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
whileloop.