Newbie needs help on using for and lists

[diediealldie]

I'm really really newbie, almost no knowledge about python.

The problem I have is :
example)

 a = [ 1, 2, 3, 4, 5, 6, 7 ]
 for n in a:
     if a[n] < 4:
        del a[n]

if I code like this, an error occurs, because the number of elements is changed.
so, what should I do to do this withour any error.
Is there any way to access list datas simultaneously?

[Strider1066]

Rather than deleting items that fail the test, try copying the good'uns to a new list.

[ilikepython]

I'm really really newbie, almost no knowledge about python.

The problem I have is :
example)

 a = [ 1, 2, 3, 4, 5, 6, 7 ]
 for n in a:
     if a[n] < 4:
        del a[n]

if I code like this, an error occurs, because the number of elements is changed.
so, what should I do to do this withour any error.
Is there any way to access list datas simultaneously?

I don't get what you are trying to do. You have a list of indexes (??) that you use to acces that same list? Is this what you want:

a = range(1, 8) # generates a list like yours
new = []
for n in a:
    if n < 4:
        new.append(n)
 

Or you could use filter():

a = range(1, 8)
new = filter(lambda x: x < 4, a)
 

Or maybe a list comprehension is clearer:

a = range(1, 8)
new = [n for n in a if n < 4]
 

[bartonc]

I'm really really newbie, almost no knowledge about python.

The problem I have is :
example)

 a = [ 1, 2, 3, 4, 5, 6, 7 ]
 for n in a:
     if a[n] < 4:
        del a[n]

if I code like this, an error occurs, because the number of elements is changed.
so, what should I do to do this withour any error.
Is there any way to access list datas simultaneously?

Our new friend, Strider1066, has a good suggestion.
For a quick copy of a simple list, use a "slice":

>>> for i, item in enumerate(a[:]):  # A "slice" from beginning to end
...     if item < 4:
...         del a[i]
...         
>>> a
[2, 4, 6, 7]
>>> 

[ilikepython]

Our new friend, Strider1066, has a good suggestion.
For a quick copy of a simple list, use a "slice":

>>> for i, item in enumerate(a[:]):  # A "slice" from beginning to end
...     if item < 4:
...         del a[i]
...         
>>> a
[2, 4, 6, 7]
>>> 

Are you sure that works? I mean, 2 is not greater than 4. It works with a.remove() but I get the same results like yours when I use del. I never actually use del. What does it do?

[bartonc]

Are you sure that works? I mean, 2 is not greater than 4. It works with a.remove() but I get the same results like yours when I use del. I never actually use del. What does it do?

OOoooops! Can't rely on the index into a changing list.
Remove works because it doesn't rely on the index of the item.

>>> a = [ 1, 2, 3, 4, 5, 6, 7 ]
>>> for item in a[:]:  # A "slice" from beginning to end
...     if item < 4:
...         a.remove(item)
...         
>>> a
[4, 5, 6, 7]
>>> a = [ 1, 2, 3, 4, 5, 6, 7 ]
>>> for item in a[:]:  # A "slice" from beginning to end
...     if item < 4 or item == 7:
...         a.remove(item)
...         
>>> a
[4, 5, 6]
>>> 

Nice catch, my friend. Thank you.

[diediealldie]

Thank you everyone, it was wiser to copy and replace lists.

I'm going to like here. :) thx