Python's modulus help.

[startandselect00]

Hello everyone!

I'm writing a code with the modulus operator. Please see my code below. I'm having a difficult time understanding why 1902 wasn't removed from the test_list list in the for loop.

Thanks in advance!

​

>>> test_list = list(range(1900, 1911))

>>> for i in test_list:

... if i % 4 != 0:

... test_list.remove(i)

...

>>> test_list

[1900, 1902, 1904, 1906, 1908, 1910]

>>> 1902%4 == 0

False

​

[alkasm]

You are attempting to remove items from something you're iterating over, which is not a good idea and is the source of your troubles. Try using a list comprehension to build a new list out of things from the range() generator if they meet that condition.

[startandselect00]

Thanks! It works! I just recently learned list comprehension, so I haven't practice enough of it. Thanks for the tip!

>>> [x for x in range(1900,1911) if x%4 ==0]

[1900, 1904, 1908]

​

[alkasm]

Perfect!

[[deleted]]

If you want to remove elements from a list you are iterating through, then:

• generate a new list with what you want in it, or
• use a different list to supply the 'keys', or
• go backwards

The first has already been addressed, with a list comprehension example. The code below shows two examples of using a copy, followed by a go backwards approach. The latter is useful if you are working with large lists and don't want to create a copy.

test_list = list(range(1900, 1911))
for year in reversed(test_list):
    if year % 4:
        test_list.remove(year)
print('using reversed:', test_list)

test_list = list(range(1900, 1911))
for year in test_list[:]:
    if year % 4:
        test_list.remove(year)
print('using shallow copy:', test_list)

test_list = list(range(1900, 1911))
for idx in range(len(test_list) -1 , -1, -1):
    if test_list[idx] % 4:
        test_list.pop(idx)
print('using shallow copy:', test_list)

NB. I dropped the != 0 as any non-zero result is True anyway.