How to traverse a list in reverse order in Python (index-style: '... in range(...)' only)

Question

I'm new to Python so I am still getting used to looping in this language. So, for example, I have the following code (the element type in the list is array)

my_list = [array0, array1, array2, array3]
temp = my_list[-1]
for k in range(len(my_list) - 1, 0):
    temp = temp + my_list[k - 1]
return temp

I want to traverse the list from the end of the list and stop at the second element (k=1), but it looks like the for loop will never be entered and it will just return the initial temp outside the loop. I don't know why. Can someone help? Thanks.

Answer 1

Avoid looping with range, just iterate with a for loop as a general advice.

If you want to reverse a list, use reversed(my_list).

So in your case the solution would be:

my_list = reversed([array0,array1,array2,array3])

That gives you [array3, array2, ...]

For omitting the last array from the reversed list, use slicing:

my_list = reversed([array0,array1,array2,array3])[:-1]

So your code turns to a one liner :-)

Answer 2

Let's look at this part by part:

len(my_list) is 4. Thus range(len(my_list)-1, 0) is the same as range(3, 0), which is [] in Python 2. (In Python 3, try list(range(3, 0)) to see the same empty list.)

If you want a declining range, use a negative step (third argument):

range(3, 0, -1)

gives [3, 2, 1]. It doesn't include 0, because even for negative steps, the upper bound (second argument) is exclusive, i.e., not part of the result. It seems you've worked around this by initializing the temp variable with the last array in my_list already before then loop, and then subtracting 1 from the index when using it to access my_list contents in the loop.

my_list = [array0, array1, array2, array3]
temp = my_list[-1]
for k in range(len(my_list) - 1, 0, -1):
    temp = temp + my_list[k - 1]
return temp

would thus probably work.

However, if you need the indices only to access my_list, you can get rid of them by directly iterating over the elements of my_list in the (here reverse) order you want:

my_list = [array0, array1, array2, array3]
temp = empty_array  # Not quite sure what this has to be. (Depends on your array type.)
for array in reversed(my_list):
    temp = temp + array
return temp

I'm assuming your arrays are concatenated in the loop. If it's instead an actual addition (NumPy vector addition, for example — though I don't see why the order of addition would matter for that, except maybe for precision, if the values in my_list are sorted), use an appropriate zero-element instead of empty_array.

You can even get rid of the loop completely. If you want to add your arrays:

my_list = [array0, array1, array2, array3]
return sum(reversed(my_list))

If you want to concatenate them:

from itertools import chain

my_list = [array0, array1, array2, array3]
return list(chain.from_iterable(reversed(my_list)))

Answer 3

You can traverse a list in reverse like so:

for i in range(len(lis), 0, -1):
    print (i)

To stop at the second element edit your range() function to stop at 1:

for i in range(len(lis), 1, -1):
        print (i)