[ Removing specific tuples from List ]

I've got a list

a = [(1,2),(1,4),(2,6),(1,8),(3,6),(1,10),(1,6)]

If I say that:

for x in a:
    if x[0]==1:
        print x

I get the expected result : (1,2) (1,4) (1,8) (1,10) (1,6)

However I want to remove all the occurrences of all the tuples in the format (1,x),So

for x in a:
   if x[0]==1:
       a.remove(x)

I thought that all the occurences should be removed.However when i say

Print a

I get [(1,4),(2,6),(3,6),(1,6)]

Not all the tuples were removed. How do I do it.?? Thanks

Answer 1

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I'd use list comprehension:

def removeTuplesWithOne(lst):
    return [x for x in lst if x[0] != 1]

a = removeTuplesWithOne([(1,2),(1,4),(2,6),(1,8),(3,6),(1,10),(1,6)])

For me it's more pythonic than built-in filter function.

P.S. This function does not change your original list, it creates new one. If your original list is huge, i'd probably use generator expression like so:

def removeTuplesWithOne(lst):
    return (x for x in lst if x[0] != 1)

Answer 2

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This isn't the same approach as yours but should work

a = filter(lambda x: x[0] != 1, a)

Answer 3

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You can use list comprehension like this, to filter out the items which have 1 as the first element.

>>> original = [(1, 2), (1, 4), (2, 6), (1, 8), (3, 6), (1, 10), (1, 6)]
>>> [item for item in original if item[0] != 1]
[(2, 6), (3, 6)]

This creates a new list, rather than modifying the existing one. 99% of the time, this will be fine, but if you need to modify the original list, you can do that by assigning back:

original[:] = [item for item in original if item[0] != 1]

Here we use slice assignment, which works by replacing every item from the start to the end of the original list (the [:]) with the items from the list comprehension. If you just used normal assignment, you would just change what the name original pointed to, not actually modify the list itself.

Answer 4

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You can do it with a generator expression if you're dealing with huge amounts of data:

a = [(1,2),(1,4),(2,6),(1,8),(3,6),(1,10),(1,6)]

# create a generator
a = ((x,y) for x, y in a if x == 1)

# simply convert it to a list if you need to...
>>> print list(a)
[(1, 2), (1, 4), (1, 8), (1, 10), (1, 6)]