How to zip each element from a 1D array with the elements from each row of a 2D array?

Question

I am trying to plot a dataset using matplotlib which contains multiple y coordinates per x . To plot them I have to combine these arrays to show them in a single plot. How do I zip each element from a 1D array with the elements from each row of a 2D array (corresponding with the index for the element of the 1D array)? Without using explicit for-loops. Using built-ins (eg. zip/list comprehension) or even better: numpy ?

Turning:

x = [1, 2, 3]
y = [[4, 5], [6, 7], [8, 9]]

Into:

r = [(1, 4), (1, 5), (2, 6), (2, 7), (3, 8), (3, 9)]

Answer 1

I thought of following:

1. Extend x to be of same size as y

2. Flatten y

3. Zip x and y

as follows

>>> x = [1, 2, 3]
>>> y = [[4, 5], [6, 7], [8, 9]]
>>> x = np.array(x).repeat(2)
>>> y = np.array(y).reshape(-1)
>>> list(zip(x, y))
[(1, 4), (1, 5), (2, 6), (2, 7), (3, 8), (3, 9)]

I would love to learn how more efficiently this can be done.

Please comment or answer more efficient approach.

Answer 2

x = [1, 2, 3]
y = [[4, 5], [6, 7], [8, 9]]

result = [(i[0], i[1][0]) for i in zip(x,y)] + [(i[0], i[1][1]) for i in zip(x,y)]

[(1, 4), (2, 6), (3, 8), (1, 5), (2, 7), (3, 9)]

Answer 3

Inspired by approach 5 here :

from functools import reduce

x = [1, 2, 3]
y = [[4, 5], [6, 7], [8, 9]]

def listOfTuples(l1, l2): 
    l = list(map(lambda x, y:[(x,y[0]), (x,y[1])], l1, l2))
    m = reduce(lambda x, y: x + y, l)
    return(m)

r = listOfTuples(x, y)

Gives

[(1, 4), (1, 5), (2, 6), (2, 7), (3, 8), (3, 9)]

Answer 4

A variation on your repeat and reshape approach:

In [89]: x = [1, 2, 3] 
    ...: y = [[4, 5], [6, 7], [8, 9]]                                                          
In [90]: res = np.zeros((3,2,2),int)                                                           
In [91]: res[:,:,1]=y                                                                          
In [92]: res[:,:,0]=np.array(x)[:,None]                                                        
In [93]: res                                                                                   
Out[93]: 
array([[[1, 4],
        [1, 5]],

       [[2, 6],
        [2, 7]],

       [[3, 8],
        [3, 9]]])
In [94]: res.reshape(6,2)                                                                      
Out[94]: 
array([[1, 4],
       [1, 5],
       [2, 6],
       [2, 7],
       [3, 8],
       [3, 9]])

It's using broadcasting to map the x and y onto the res array.

Answer 5

This is making use of iterators only, using itertools.chain.from_iterable

import itertools

x = [1, 2, 3]
y = [[4, 5], [6, 7], [8, 9]]

list(zip(list(itertools.chain.from_iterable(itertools.repeat(n, 2) for n in x)),
     list(itertools.chain.from_iterable(y))))

Output

[(1, 4), (1, 5), (2, 6), (2, 7), (3, 8), (3, 9)]

This should be faster in longer runs, IMHO.