CODEWITHSUNDEEP
pythonlist
lokheart
lokheart

Reputation: 24665

python for loop with "reducing" list

a = [1,2,3,4]
for loop in range(0,len(a)):
    if a[loop]%2==0:
        a.remove(a[loop])
        loop = loop - 1

I want to reduce the list by filtering numbers which can be divided by 2. There are two questions here:

update 01

Thanks for the prompt reply, first part of the question is solved, but what about the second part? What if I want to use for-loop to deal with it and make the loop to be reduced by ` if the condition of 'dividable by two' is fulfilled?

Upvotes: 1

Views: 1083

Answers (4)

Jamie Marshall
Jamie Marshall

Reputation: 2304

I found this question looking for an algorithm to do what the question says: "loop with 'reducing list'". However, it looks like all the answers are for implementing a filter, which is what OP really wanted. For anyone really interested in a reducing list:

def make_filter(comp_val):
    return lambda x: not (x.startswith(comp_val) and x != comp_val)

i = 0
l = list_1.copy()
while i < len(l):
    s_3 = list(filter(make_filter(s_3[0]), s_3))
    i += 1

The point of doing this would be to reduce timespace (O) and compute on many iterations through a list.

Upvotes: 0

thefourtheye
thefourtheye

Reputation: 239453

If you want to remove all the elements from the list, if it is not divisible by 2, then the best way is to create a new list without the numbers not divisible by 2, like this

[item for item in a if item % 2 != 0]

You can also use the filter function, like this

filter(lambda item: item % 2 != 0, a)

If you are using Python 3.x, then you need to generate the list of items from the filter object using list function, like this

list(filter(lambda item: item % 2 != 0, a))

Apart from these methods, if you want to do in-place replacement, then you might want to do it in reverse, like this

a = [1,2,3,4]
for loop in range(len(a) - 1, -1, -1):
    if a[loop] % 2 == 0:
        a.remove(a[loop])

Note: Whenever possible, prefer list comprehension method.

Upvotes: 2

Maroun
Maroun

Reputation: 95958

You can use filter:

filter(lambda x: x % 2 == 0, a)

Upvotes: 1

Guillaume Jacquenot
Guillaume Jacquenot

Reputation: 11717

A list comprehension can do the trick:

# Define data in variable a
a = [1,2,3,4]
# Apply the list comprehension to filter list a
a = [i for i in a if i%2==0]

Upvotes: 0

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