在python中循环访问下一次迭代的索引时索引超出范围

Question

There are several image files in a directory on which I need to iterate through, where need to access the next iteration value in the current iteration.

img_files_list = os.listdir(/path/to/directory)
new_list = []
for index, img in enumerate(img_files_list):
    if img in new_list:
        continue
    new_list.extend([img, img_files_list[index + 1], img_files_list[index + 2]])
    print(img)
    print(img_files_list[index + 1])
    print(img_files_list[index + 2])

I need to iterate over all the items of the img_files_list but when reached at the end of the loop need to properly come out of loop without index out of range exception . Anyone, please point out where do I missed the logic.

Answer 1

Seems more like Code Review than Stack Overflow.

When you reach the last item in your for image, img in enumerate(img_files_list) line, there is no item after the last one. This is what causes an IndexOutOfRangeException .

There are a few ways to approach this:

1. As Sruthi V. wrote, include a condition:

    print(img_files_list[i]) if i + 1 < len(img_files_list): print(img_files_list[i + 1]) if i + 2 < len(img_files_list): print(img_files_list[i + 2]) 

2. Include a try... except :

    try: print(img_files_list[i]) print(img_files_list[i + 1]) print(img_files_list[i + 2]) except IndexOutOfRangeException: pass 

3. Limit the range of your loop:

    i_img = list(enumerate(['a', 'b', 'c', 'd', 'e'])) for i, img in i_img [:len(i_img ) - 2]: print(img) print(i_img[i + 1]) print(i_img[i + 2]) 

Note that you have no reason to use enumerate — you're just using it to get indices. You also don't need your if img in new_list — you're just using it to skip the next two. You can make this more elegant and solve your problem using range (a modified solution 3).

imgs = os.listdir('/path/to/directory')
triplets = []
for i in range(0, len(imgs) - 2, 3):
    triplet = [imgs[i], imgs[i + 1], imgs[i + 2]]
    triplets.extend(triplet)
    print('\n'.join(triplet))

NB I'm not sure what you're even trying to do. Even your if img in new_list might be a mistake since it just skips the last 1-2 final items. And if that were gone, all you would be doing is making a copy of this list and printing its contents. In which case I would suggest:

imgs = os.listdir('/path/to/directory')
print('\n'.join(imgs))

But if you clarify what you're trying to do, I can edit this answer.

Answer 2

From my comment to your question:

index + 1 and index + 2 cause a problem for last 2 items. For example, if your list's size is 10, then indexes will be 0,2,...,9. When the index is 8 and 9, you are trying to execute img_files_list[10] and img_files_list[11] . So the index is out of range.

I assume that you want to access images as 3-item list, then I suggest you a little bit different way. I will use an integer array instead of images for a better understanding.

img_files_list = [0,1,2,3,4,5,6,7,8,9]
new_list = []  
n = 3
for i in range(0, len(img_files_list), n): # Respectively, i is 0,3,6,9,...
    temp = img_files_list[i:i+n]
    print(temp)
    new_list.extend(temp)

Output:

[0, 1, 2]
[3, 4, 5]
[6, 7, 8]
[9]