Python iteration

Re:Python iteration by MaxGabriel on Wednesday April 25, 2012 @03:36PM (#3890633)

Can you control the ordering of map/filter/reduce? As in, I want to iterate through a matrix by iterating through the columns and rows in reverse order? As in:

  [9,6,3]
  [8,5,2]
  [7,4,1]

I've just started using map/filter/reduce so I'm not sure.

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Re:Python iteration by dalke on Wednesday April 25, 2012 @04:33PM (#3890926)

No. They start from the beginning and go to the end. Conceptually it could work on any iterable, and not just a list container.

You can, however, use the 'reversed()' function, as in "for row in reversed(rows): for col in reversed(row): print col"

There is a special double-underscore method, which lists and other data structures support, to allow iteration in reverse with the normal performance of forward iteration.

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Re:Python iteration by kisielk on Wednesday April 25, 2012 @04:19PM (#3890847)

What data structure are you using to represent your matrix? A list of lists? If so, it's not terribly difficult:

    >>> a = [[1,2,3], [4,5,6], [7,8,9]]
    >>> b = zip(*map(reversed, reversed(a))
    >>> b
    [(9, 6, 3), (8, 5, 2), (7, 4, 1)]
    >>> reduce(operator.add, b) # could also use a lambda with + instead of operator.add...
    (9, 6, 3, 8, 5, 2, 7, 4, 1)

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