list comprehensions whats happening here
Jeremy Hylton
jeremy at digicool.com
Fri Apr 13 18:15:54 EDT 2001
>>>>> "CR" == Carlos Ribeiro <cribeiro at mail.inet.com.br> writes:
CR> I also lost. I did a small change on your example to better
CR> illuminate what is happening:
>>>> [[i,j] for i in range(3), for j in 'abc']
CR> [[[0, 1, 2], 'a'], [[0, 1, 2], 'b'], [[0, 1, 2], 'c']]
>>>> [[i,j] for i in range(3), for j in 'abc',]
CR> [[[0, 1, 2], 'abc']]
>>>> [[i,j] for i in range(3) for j in 'abc']
CR> [[0, 'a'], [0, 'b'], [0, 'c'], [1, 'a'], [1, 'b'], [1, 'c'], [2,
CR> 'a'], [2, 'b'], [2, 'c']]
CR> And also, a fourth case for completeness:
>>>> [[i,j] for i in range(3) for j in 'abc',]
CR> [[0, 'abc'], [1, 'abc'], [2, 'abc']]
CR> So it seems that the idiom
CR> for i in range(x),
CR> with the comma right after the for clause, "reduces" the
CR> iterated list to the original list again, and then apply it to
CR> all further iterations of the list comprehension. Now, I don't
CR> know if this is by design or by accident; all I know is that
CR> this is something that may be exploited as a useful idiom. For
CR> instance, neither map or zip have this behavior, of applying the
CR> same list to all members of the other.
I think it would be hard to say that this was "by design" but it makes
sense after you stare at it for a while <0.5 wink>.
The "range(x)," is being interpreter as the length-1 tuple containing
the value of range(x). The for loop iterates over the (single) value
in the tuple.
Jeremy
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