python从两个列表中创建一个新列表，第二个列表表示重复第一个列表中的元素的次数

Question

I have data like this:

df
col1 col2
3      4
1      2
2      2
4      7

Right now the data is a pandas df, but I could conceivably make the columns lists or whatever is needed.

I want the output to be this.

[3,3,3,3,1,1,2,2,4,4,4,4,4,4,4]

Answer 1

if col1 and col2 can be represented as lists then :

ans = []
for i in xrange(len(col1)):
    ans+=[col1[i]]*col2[i]

print ans

Assuming the len of col1 and col2 would be equal

Answer 2

([ a  for a, b in zip(df.col1,df.col2) for _ in xrange(b)])
[3, 3, 3, 3, 1, 1, 2, 2, 4, 4, 4, 4, 4, 4, 4]

Or using a normal loop:

res = []
for a, b in zip(df.col1, df.col2):
    res.extend([a]* b)
print(res)
[3, 3, 3, 3, 1, 1, 2, 2, 4, 4, 4, 4, 4, 4, 4]

Or simply use repeat :

print(df.col1.repeat(df.col2).tolist())

[3, 3, 3, 3, 1, 1, 2, 2, 4, 4, 4, 4, 4, 4, 4]

Answer 3

Here's another

>>> sum(([x]*y for (x, y) in zip(col1, col2)), [])
[3, 3, 3, 3, 1, 1, 2, 2, 4, 4, 4, 4, 4, 4, 4]

Answer 4

In [218]: col1 = [3,1,2,4]

In [219]: col2 = [4,2,2,7]

In [220]: list(itertools.chain.from_iterable(itertools.repeat(n,k) for n,k in zip(col1, col2)))
Out[220]: [3, 3, 3, 3, 1, 1, 2, 2, 4, 4, 4, 4, 4, 4, 4]