[Tutor] list iteration [iteration and mutation?]

Karl Pflästerer sigurd at 12move.de
Mon May 3 15:26:07 EDT 2004

On  3 May 2004, Magnus Lycka <- magnus at thinkware.se wrote:

> Here is a little study to display the phenomena in a simple case. Remove
> all strings in a list that start with 'm'.

[some approaches to delete items from a list]

> I think there are three lessons we can learn from this:

> 1. Modifying a list that we are iterating over is error prone.
> 2. Iterating backwards, from the end to the beginning, typically
>    makes this problem go away.
> 3. Identifying objects by *what* they are, is often more robust
>    that to identify them based on *where* they are.

Right.  But there is a forth lesson to learn.

4. Try a functional approach. e.g.:

filter(lambda i: predicate(i), lst)


[i for i in lst if predicate(i)]

So with your example:

>>> filter(lambda s: not s.startswith('m'), l)
['polly', 'dolly', 'pike']
>>> [s for s in l if not s.startswith('m')]
['polly', 'dolly', 'pike']

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