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Removing element from list - Printable Version

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Removing element from list - squall - Nov-21-2020

Hello guys,

I'm new on the forum. The reason I'm here is that I do not know how to fix my problem and hopefully you can help me.
I want to delete the maximum element from the list with the command "remove". I know that remove, removes the first element that I choose to remove, so I made a loop so if there are two or more maximum elements, all of them should be removed.
It doesn't work. It seems like the loop doesn't go through all elements.

def list(a):
    c=max(a)
    a.remove(c)

    for i in a:
        print(i)
        if i == c:
            a.remove(i)


    if a[len(a)-1] == c:
        a.pop(len(a)-1)

    return a



print(list([1,2,3,4,5,6,6,6,6,6,6,6,6,6]))


RE: Removing element from list - bowlofred - Nov-22-2020

list is already an object in python. It is recommended you use another name for your function.

It is a bad idea to remove objects from an object while you're iterating over it. It will mess with the iteration. You're doing this in lines 5-8.

An alternative is to make a copy without the objects you want removed, or to remove them outside of an iteration.

Remove by copy elements that don't match
a = [item for item in a if item != c]
Remove without iteration
while c in a:
    a.remove(c)
Remove while iterating over a copy
for i in list(a):
    if i == c:
        a.remove(i)


RE: Removing element from list - perfringo - Nov-22-2020

You can approach this from another angle: 'remove first occurence of maximum value n-times', where n is number of occurences of maximum value. It's simple but not efficient:

>>> list_ = [1,2,3,4,5,6,6,6,6,6,6,6,6,6]
>>> max_value = max(list_)                 # find maximum value
>>> repetitions = list_.count(max_value)   # find how many times maximum value occurs
>>> for repetition in range(repetitions):  # remove all maximum values
...     list_.remove(max_value)
...
>>> list_
[1, 2, 3, 4, 5]


RE: Removing element from list - squall - Nov-22-2020

Thank you guys. I have used option with while - it spoke to me the most.

Anyway I have a question about that: 
for i in list(a):
    if i == c:
        a.remove(i)
What does that list(a) there mean? It doesn't work for me.

RE: Removing element from list - jefsummers - Nov-22-2020

You want to use remove(), which has the problem of removing the first instance. The following removes that largest element until there are no longer any matching elements
a = [4,6,8,10,4,7,10,9]
c = max(a)
try :
    while True:
        a.remove(c)
except ValueError:
    pass
print(a)
Output:
[4, 6, 8, 4, 7, 9]


RE: Removing element from list - bowlofred - Nov-22-2020

(Nov-22-2020, 10:16 AM)squall Wrote: Anyway I have a question about that: 
for i in list(a):
    if i == c:
        a.remove(i)
What does that list(a) there mean? It doesn't work for me.

It creates a new (different) list from some other object. 
l = [1, 2, 3] # a list
a = l         # a is the *same* list
b = list(l)   # b is a *different* list
l.remove(1)   # removes the "1" from l (and from a since it's the same list)

print(l)
print(a)
print(b)
Output:
[2, 3]
[2, 3]
[1, 2, 3]
It probably doesn't work for you because you've named your function "list", so you've overriden the internal python "list" function.

RE: Removing element from list - jefsummers - Nov-22-2020

Another way to write line 3 above that may be a little more clear is
b=l.copy()