Why I am getting such strange behavior value for loop?-CodePudding

a = []
for i in range(10):
    a.append (i*i)

for a[i] in a:
    print(a[i])

For the above-mentioned code I am getting the output as follows:

I am not able to understand why 64 is getting repeated twice. If anyone knows the proper reason please explain me in detail.

CodePudding user response：

So basically when the 1st loop finishes, the value of i is 9(because its in range 10)

Hence, When you start the 2nd loop, every iteration changes the value of a[i] ie the last element which was earlier 9.

So,

a = []
for i in range(10):
    a.append (i*i)
# Here: i = 10 and a = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
    print(a[i])
# While iterating through this, the value of a[i] changes every item and is equal
# the current element of a
# When it reaches the last element, it's already a[9] so the previous value is 
# printed.

So in every Iteration:

currentElement     a[i]     a
0                  0        [0, 1, 4, 9, 16, 25, 36, 49, 64, 0]
1                  1        [0, 1, 4, 9, 16, 25, 36, 49, 64, 1]
4                  4        [0, 1, 4, 9, 16, 25, 36, 49, 64, 4]
9                  9        [0, 1, 4, 9, 16, 25, 36, 49, 64, 9]
16                 16       [0, 1, 4, 9, 16, 25, 36, 49, 64, 16]
25                 25       [0, 1, 4, 9, 16, 25, 36, 49, 64, 25]
36                 36       [0, 1, 4, 9, 16, 25, 36, 49, 64, 36]
49                 49       [0, 1, 4, 9, 16, 25, 36, 49, 64, 49]
64                 64       [0, 1, 4, 9, 16, 25, 36, 49, 64, 64]
64                 64       [0, 1, 4, 9, 16, 25, 36, 49, 64, 64]

Causing this behviour

CodePudding user response：

With a suggestion that the loop should be like for item in a:, let's try to know the behavior for for a[i] in a:

a = []
for i in range(10):
    a.append (i*i)


print("Values before loop")
print(a, i)
for a[i] in a:
    print(a[i])
print("Values after loop")
print(a, i)

Values before loop
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81] 9
0
1
4
9
16
25
36
49
64
64
Values after loop
[0, 1, 4, 9, 16, 25, 36, 49, 64, 64] 9

When you are iterating in above loop, i is always 9, so the iteration for a[i] in a assigns the subsequent values from a to a[i] i.e. a[9], hence, at second final iteration, the value a[9] becomes a[8], i.e. 64