How to get one number specific times in an array python

Question

I have one array like A = [1,2,3] and another array B = [4,5,6]. Now, I need another array C so that the elements in C should be the same elements of B having occurrence in the order of element A. Like, C = [4, 5, 5, 6, 6, 6]

Answer 1

A = [1,2,3]
B = [4,5,6]
C = [b_item for a_item, b_item in zip(A,B) for _ in range(a_item)]
print C

Result:

[4, 5, 5, 6, 6, 6]

This is a one-line equivalent to:

C = []
for a_item, b_item in zip(A,B):
    for _ in range(a_item):
        C.append(b_item)

... Which is roughly equivalent to

C = []
for i in range(min(len(A), len(B))):
    a_item = A[i]
    b_item = B[i]
    for _ in range(a_item):
        C.append(b_item)

(N.B. Don't get tripped up by the underscore. It's an ordinary variable. It is conventionally used when you don't actually have to refer to the variable's value, as in this example)