Change value of currently iterated element in list

Question

problem: when you use construction

for a in _list_:     print a 

it prints every item in array. But you can't alter array. Is it possible to alter value of array (something like a=123, but that ain't working) I know it's possible (for example in while loop), but I want to do it this way (more elegant)

In PHP it would be like

foreach ($array as &$value) {     $value = 123; } 

(because of & sign, is passed as reference)

Answer 1

for idx, a in enumerate(foo):     foo[idx] = a + 42 

Note though, that if you're doing this, you probably should look into list comprehensions (or map), unless you really want to mutate in place (just don't insert or remove items from iterated-on list).

The same loop written as a list comprehension looks like:

foo = [a + 42 for a in foo] 

Answer 2

Because python iterators are just a "label" to a object in memory, setting it will make it just point to something else.

If the iterator is a mutable object (list, set, dict etc) you can modify it and see the result in the same object.

>>> a = [[1,2,3], [4,5,6]] >>> for i in a: ...    i.append(10) >>> a [[1, 2, 3, 10], [4, 5, 6, 10]] 

If you want to set each value to, say, 123 you can either use the list index and access it or use a list comprehension:

>>> a = [1,2,3,4,5] >>> a = [123 for i in a] >>> a [123, 123, 123, 123, 123] 

But you'll be creating another list and binding it to the same name.