Why such an output?

Question

a=[1,2,3,4,1]
for n in a:
a[n]=0
print (a)
output:
0 0 3 0 1

I thought that
a[1]=0, so 2=0
a[2]=0, so 3=0
a[3]=0, so 4=0
a[4]=0, so 1=0

isn't it?

Answer 1

First iteration a[1] = 0 causes a = [1,0,3,4,1].
Second iteration a[0] = 0 causes a = [0,0,3,4,1]. # here is a[0] because values of the array have already been changed above in the first iteration
Third iteration a[3] = 0 causes a = [0,0,3,0,1].
Respectively fourth and fifth iterations set 0 to 1st and 2nd element of the array that have already been set.
This code does not set 0's for all the elements of the array. You should make a loop for a range 0...5 to do so. These are different things.

Answer 2

The contents of a change, while the loop is running, so it is:

a[1]=0, so 2=0
a[0]=0, so 1=0
a[3]=0, so 4=0
a[0]=0, so 0=0
a[1]=0, so 0=0

Answer 3

Finally got it! Thank u so much!