Duplicate each member in a list

Question

I want to write a function that reads a list [1,5,3,6,...] and gives [1,1,5,5,3,3,6,6,...]. Any idea how to do it?

Answer 1

>>> a = range(10)
>>> [val for val in a for _ in (0, 1)]
[0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9]

N.B. _ is traditionally used as a placeholder variable name where you do not want to do anything with the contents of the variable. In this case it is just used to generate two values for every time round the outer loop.

To turn this from a list into a generator replace the square brackets with round brackets.

Answer 2

>>> a = [1, 2, 3]
>>> b = []
>>> for i in a:
    b.extend([i, i])


>>> b
[1, 1, 2, 2, 3, 3]

or

>>> [a[i//2] for i in range(len(a)*2)]
[1, 1, 2, 2, 3, 3]

Answer 3

numpy.repeat does what you want:

import numpy as np
yourList = [1,5,3,6]
n = 2
list(np.repeat(yourList, n))

result:

[1, 1, 5, 5, 3, 3, 6, 6]

If you don't mind using numpy arrays you can also omit the list() call in the last line.

Answer 4

If you already have the roundrobin recipe described in the documentation for itertools—and it is quite handy—then you can just use

roundrobin(my_list, my_list)