[Tutor] for statement with addition ...

[Markus Hubig]

On Mon, Jul 13, 2009 at 2:08 PM, Dave Angel <davea at ieee.org> wrote:

> Markus Hubig wrote:
>
>> Hi @all,
>>
>> within diveintopython I often found a for-statement like this:
>>
>> f for f in bla:
>>    print f
>>
>> So what actually is the first f for ... is it just to declare f before
>> starting the for loop? I can't find any information on python.org
>> and it's hard to google this kinda stuff.
>>
>> - Markus
>>
>>
>>
> Please give us a reference, as to exactly where you saw that.  It gives a
> syntax error in Python 2.62, as I expected it would.  That's not how a for
> statement works.
>
>
> On the other hand, if you enclose that phrase in square brackets, it makes
> a list comprehension.
>
> mylist = [f for f in bla]
>
> A list comprehension builds a new list, where the first pieces says what
> goes into the list, and the second part describes how to generate it.  In
> this case, if bla is already a list, this just copies the list.  But
> consider
> mylist = [f*f  for f in bla]
>
> That builds a new list from the old, where each item is the square of the
> corresponding old item.
>
> List comprehension is something you can look up in the help.   Also look up
> generator expression, which uses similar syntax.
>
>
> DaveA
>

Yes your right, the examples are enclosed in square brackets ...
so this is the explanation I was looking for, thank you.
And now I also found the explanation in
diveintopython<http://diveintopython.org/native_data_types/mapping_lists.html>!
;-)

- Markus

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