Loop in list.

[Fredrik Lundh]

Stephen Thorne wrote:

>> '>>> a = [i*2*b for i in range(3) for b in range(4)]
>> '>>> a
>> [0, 0, 0, 0, 0, 2, 4, 6, 0, 4, 8, 12]
>>
>> Might take you a while to correlate the answer with the loop, but you
>> should be able to see after a while that this nesting is the same as
>>
>> '>>> a = []
>> '>>> for b in range(4):
>> '>>>     for i in range(3):
>> '>>>         a.append(i*2*b)
>
> There is a subtle error in this explanation.

if you run the example, you'll notice that it's not so subtle.  read on.

> The equivilence actually looks like:
>
> '> a = []
> '> l1 = range(4)
> '> l2 = range(3)
> '> for b in l1:
> '>     for i in l2:
> '>         a.append(i*2*b)

really?

    def myrange(x):
      print "RANGE", x
      return range(x)

    print [i*2*b for i in myrange(3) for b in myrange(4)]

    a = []
    for b in myrange(4):
      for i in myrange(3):
        a.append(i*2*b)
    print a

    a = []
    l1 = myrange(4)
    l2 = myrange(3)
    for b in l1:
      for i in l2:
        a.append(i*2*b)
    print a

prints

    RANGE 3
    RANGE 4
    RANGE 4
    RANGE 4
    [0, 0, 0, 0, 0, 2, 4, 6, 0, 4, 8, 12]
    RANGE 4
    RANGE 3
    RANGE 3
    RANGE 3
    RANGE 3
    [0, 0, 0, 0, 2, 4, 0, 4, 8, 0, 6, 12]
    RANGE 4
    RANGE 3
    [0, 0, 0, 0, 2, 4, 0, 4, 8, 0, 6, 12]

(to translate a list comprehension to nested statements, remove
the result expression, insert colons and newlines between the for/if
statement parts, and put the append(result expression) part inside
the innermost statement)

</F>