assignment in a for loop

Ben Finney bignose+hates-spam at benfinney.id.au
Wed May 17 07:05:29 CEST 2006


"MackS" <mackstevenson at hotmail.com> writes:

> >>> l = [1,2]
> >>> for i in l:
> ...     i = i + 1
> ...
> >>> l
> [1, 2]
> 
> I understand (I think!) that this is due to the fact that in Python
> what looks like "assignment" really is binding a name to an
> object. The result is that inside the loop I am creating an object
> with value (i+1) and then "pointing" the name i at it. Therefore,
> the object to which i previously pointed (an element of list l)
> remains unchanged.

That's a fair explanation, yes.

> Two brief questions:
> 
> 1) Is what I wrote above (minimally) correct?

Correct for what? You can tell if it's *syntactically* correct by
simply running it.

As for any other "correct", define that. Does it do what you want it
to do?

> 2) Independently of the answer to 1, is there a way for me to assign
> to elements of a list inside a loop and without resorting to C-style
> ugliness of
> 
> for i in range(len(l))
>      l[i] = l[i] + 1

You can build a new list from your operations on the old one.

    new_list = []
    for x in old_list:
        new_list.append(x+1)

You can also do it more succinctly with a list comprehension
expression.

> (Note: not using a list comprehension.)

What's preventing the use of list comprehensions?

    new_list = [x+1 for x in old_list]

-- 
 \      "Smoking cures weight problems. Eventually."  -- Steven Wright |
  `\                                                                   |
_o__)                                                                  |
Ben Finney




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