Question

let’s say I have a list

a = [1,2,3]

I’d like to increment every item of that list in place. I want to do something as syntactically easy as

for item in a:
    item += 1

but in that example python uses just the value of item, not its actual reference, so when I’m finished with that loop a still returns [1,2,3] instead of [2,3,4]. I know I could do something like

a = map(lambda x:x+1, a)

but that doesn’t really fit into my current code and I’d hate to have to rewrite it :-\

Answer 1

Here ya go:

# Your for loop should be rewritten as follows:
for index in xrange(len(a)):
    a[index] += 1

Incidentally, item IS a reference to the item in a, but of course you can’t assign a new value to an integer. For any mutable type, your code would work just fine:

>>> a = [[1], [2], [3], [4]]
>>> for item in a: item += [1]
>>> a
[[1,1], [2,1], [3,1], [4,1]]