Python Referenced For Loop

I'm playing with for loops in Python and trying to get used to the way they handle variables. Take the following piece for code: a=[1,2,3,4,5] b=a b[0]=6 After doing this, the zer

Solution 1:

Everything in python is treated like a reference. What happens when you do b[0] = 6 is that you assign the 6 to an appropriate place defined by LHS of that expression.

In the second example, you assign the references from the array to i, so that i is 1, then 2, then 3, ... but i never is an element of the array. So when you assign 6 to it, you just change the thing i represents.

http://docs.python.org/reference/datamodel.html is an interesting read if you want to know more about the details.

Solution 2:

That isn't how it works. The for loop is iterating through the values of a. The variable i actually has no sense of what is in a itself. Basically, what is happening:

# this is basically what the loop is doing:# beginning of loop:i = a[0]
i = 6# next iteration of for loop:i = a[1]
i = 6# next iteration of for loop:i = a[2]
i = 6# you get the idea.

At no point does the value at the index change, the only thing to change is the value of i.

You're trying to do this:

for i in xrange(len(a)):
    a[i] =6# assign the value at index i

Solution 3:

Just as you said, "The = sign points a reference". So your loop just reassigns the 'i' reference to 5 different numbers, each one in turn.