Python：打印列表项不在单独的索引列表中

Question

I have two lists:
list = ["a","b","c","d"]
i_to_skip = [0,2]

I'd like to print everything in list except for the indices in i_to_skip . I've tried the following, which just returns a generator object:

print(x for x in list if x not in i_to_skip)

Answer 1

The reason your comprehension does work is that x is the value , like "a" , not the index , like 0 , and of course "a" is not in [0, 2] .

To get the index along with the value, you need enumerate . Then you can do this:

print([x for i, x in enumerate(list) if i not in i_to_skip])

Also, note that printing a generator expression (as you did) is just going to print something like <generator object <genexpr> at 0x1055fd8b8> ; that's why I converted your code to printing out a list comprehension, so you get ['b', 'd'] instead.

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If you instead wanted to print, say, one line at a time, you could loop over the generator expression:

for x in (x for i, x in enumerate(list) if i not in i_to_skip):
    print(x)

But really, it's easier to just collapse that into a single loop:

for i, x in emumerate(list):
    if i not in i_to_skip:
        print(x)

Or, even simpler, format the whole thing in a single expression, maybe like this:

print('\n'.join(x for i, x in enumerate(list) if i not in i_to_skip))

… or even let print do it for you:

print(*(x for i, x in enumerate(list) if i not in i_to_skip), sep='\n')

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Finally, as a side note, calling your list list is a bad idea; it hides the type/constructor function, which may want to use later on, and it also makes your code misleading.

Answer 2

Your print result is a generator , you need to put your list comprehension in a list sign ( [] ) or use list() function, also you need to check the indices not the values , so you can figure it out with enumerate() :

>>> print ([j for i,j in enumerate(list) if i not in i_to_skip] )
['b', 'd']

Answer 3

mylist = ["a","b","c","d"]
print([x for i,x in enumerate(mylist) if i not in i_to_skip])

You shouldn't call your list list or you're hiding the list type itself.

enumerate will give you the indexes and corresponding elements of the list.

Putting the comprehension in square brackets makes it into a list, which is easier to print than a generator.