I am new to Python language and used C++ for the previous 2 years. I am trying to understand why these two codes produce different results:
The first for loop:
l1=[1,2,3,4,5]
for i in l1:
i+=10
print l1
# [1,2,3,4,5]
The second for loop:
l1=[1,2,3,4,5]
for i in range(len(l1)):
l1[i]+=10
print l1
# [11,12,13,14,15]
Why doesn't the first loop produce the same result? I thought that the "for i in object" does the dereferencing for you and gives you access to the object's value?
In the first example you are altering the list at all, instead you are iteratng through it and you are altering the i variable you made
however, in the second example you index li so you are directly acsessing its items, instead of a variable assigned the the items value
The first example doesn't give the same results as the second because you're only operating on the value that's stored in the list, not on the list element itself. When you use the slice notation l1[i]+=10 you are performing an operation on the list element, and changing the values stored in the list.
l1 as an iterator to assign to the variable i, the value that was referenced in the list now becomes an integer value referenced by i, and is separated from the list reference. Does that make sense?i would not be changed. So, say you have for i in l1:, then do l1[0] = 42, then print the value of i, i will still be 1, even though the list element changed. Hopefully I'm not further confusing you :)i in first loop is a scalar value and is copied from l1, however l1[i] in second loop directly modifies data in array.
1st for loop: i is the iteration variable that is taking value from the l1 in each iteration. In the first iteration i is 1. Inside the loop and after the
i+=10
i will be equal to 11.
At the second iteration i is given the value 2 (2nd value on the l1 list) that replaces value 11. and so on... The list is not modified that way.
2nd for loop: Again i takes value from the values inside l1. Now you are using:
l1[i]+=10
Tha actually modifies the list content.
That explains the different output in the second for loop
