Weird output of python array

Question

I'm newbie in python. I don't understand the output of following python program.

arr = []
for i in range(3):
 arr.append(i * 2)
for arr[i] in arr:
 print arr[i]

Output:

0
2
2 // Why also print 2?? 

Here, An array elements print twice time 2. This is really weird. Please someone help me to clear my doubt. Why program print twice time 2?

• Antti Haapala: because you're using the arr[i] - i.e. - arr[2] as the loop variable! You want to do for x in arr: print(x)
• Stuart Buckingham: What happens if you change the second loop to for j in arr: print j? Not sure what you're trying to achieve by having arr[i] as the iterating value.

Answer 1

This happens because your second loop is using the array itself as the loop variable, overwriting the last value. It should have been written like this:

for x in arr:
 print x

PS. Since you're just starting in Python: Switch to Python 3 today!

• msc: python 3 also, print same output.
• alexis: That's right. There is no difference between Python 2 and 3 with respect to your question, but you should switch because of several important things that do differ.
• zdim: @rsp I would suggest to keep a keen eye on "both" versions of Python (they aren't different languages!). There is a lot of code in Python 2, and there will be yet more, for a long while. But it may well be a good idea to focus on 3, unless you have an immediate need for 2.

Answer 2

What for arr[i] in arr: does is that it reassigns the first and then second element of arr to arr[i] which at that point is arr[2]. That is why your first and second elements of arr are unchanged but the last one has the value of the second.

A for loop in Python loops through elements of an iterable, i.e. your loop will literally fetch element by element like @Alexis explained. In this case it means it will return each element of arr and assign it to arr[i]. It will do that for the last element too - leaving it unchanged but already equal to the second element of arr.