为什么程序中的列表会给出意外的输出？

Question

I am having a problem with my list in python.

I am printing out the list (working), a number that shows the line number (working) and an item in the list that should change every time the list is printed(not working?)

a = ["A", "B", "C", "D", "E"]
b = 0

for x in a:
    while b <= 10:

    print(a, x, b)
    b += 1

My current program output is

['A', 'B', 'C', 'D', 'E'] A 0
['A', 'B', 'C', 'D', 'E'] A 1
['A', 'B', 'C', 'D', 'E'] A 2
['A', 'B', 'C', 'D', 'E'] A 3

so on

the output I would like

['A', 'B', 'C', 'D', 'E'] A 0
['A', 'B', 'C', 'D', 'E'] B 1
['A', 'B', 'C', 'D', 'E'] C 2
['A', 'B', 'C', 'D', 'E'] D 3

and so on Although, when I try a different program it works perfectly?

list = ["a", "b", "c"]

for a in list:
    print(a)

Why does this happen and how can I fix it?

Answer 1

That is because you have the while loop inside the outer for loop (that iterates over the elements of the list. So the inner while loop only exists when b becomes greater than 10 , and till then the value of x is A .

For what you want I would suggest using itertools.cycle() . Example -

>>> a = ["A", "B", "C", "D", "E"]
>>>
>>> b = 0
>>> import itertools
>>> acycle = itertools.cycle(a)
>>> for i in range(11):
...     print(a,next(acycle),i)
...
['A', 'B', 'C', 'D', 'E'] A 0
['A', 'B', 'C', 'D', 'E'] B 1
['A', 'B', 'C', 'D', 'E'] C 2
['A', 'B', 'C', 'D', 'E'] D 3
['A', 'B', 'C', 'D', 'E'] E 4
['A', 'B', 'C', 'D', 'E'] A 5
['A', 'B', 'C', 'D', 'E'] B 6
['A', 'B', 'C', 'D', 'E'] C 7
['A', 'B', 'C', 'D', 'E'] D 8
['A', 'B', 'C', 'D', 'E'] E 9
['A', 'B', 'C', 'D', 'E'] A 10

Answer 2

You have a double loop here ( while inside for ) and you never reset the b to 0. To get the result you expected, you should use enumerate :

for idx, x in enumerate(a):
    print(a, x, idx)