How to replicate a row of an array with numpy?

Question

I want to replicate the last row of an array in python and found the following lines of code in the numpy documentation

>>> x = np.array([[1,2],[3,4]])
>>> np.repeat(x, [1, 2], axis=0)

in the above code what does the second parameter "[1,2]" in np.repeat do? if i want to replicate a row in a 3*3 array how will this second parameter change.

Second argument of repeat specify how many times each element should be repeated. Can you provide your sample output? I would be much easier to provide an answer if output is there — Sohaib Farooqi
– Sohaib Farooqi, Commented Dec 24, 2017 at 13:33
Dark's answer nicely elaborate the behaviour of np.repeat. However you should also have a look at np.tile and decide how you are willing to repeat rows. — Sohaib Farooqi
– Sohaib Farooqi, Commented Dec 24, 2017 at 13:45

Scaramouche · Accepted Answer · 2019-06-18 21:52:19Z

6

It's the repeats parameter

repeats : int or array of ints

The number of repetitions for each element. repeats is broadcasted to fit the shape of the given axis.

It's the number of times you want to repeat a row or column based on the parameter axis.

x = np.array([[1,2],[3,4],[4,5]])
np.repeat(x, repeats = [1, 2, 1 ], axis=0)

This would lead to repetition of row 1 once, row 2 twice and row 3 once.

array([[1, 2],
       [3, 4], 
       [3, 4],
       [4, 5]])

Similarly, if you specify the axis = 1. Repeats can take maximum of 2 elements in the list,and below code lead to repetition of column 1 once and column 2 twice.

x = np.array([[1,2],[3,4],[4,5]])
np.repeat(x, repeats = [1, 2 ], axis=1)

array([[1, 2, 2],
       [3, 4, 4],
       [4, 5, 5]])

If you want to repeat only last row, repeat only last row and stack i.e

rep = 2
last = np.repeat([x[-1]],repeats= rep-1 ,axis=0)

np.vstack([x, last])

array([[1, 2],
       [3, 4],
       [4, 5],
       [4, 5]])

edited Jun 18, 2019 at 21:52

Scaramouche

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answered Dec 24, 2017 at 13:37

Bharath M Shetty

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1 Comment

CS Thakur Over a year ago

I have another question suppose i have a very large array say 100*100 then if i want to replicate the last row how would i do so would it be like [1,1,1,1....2] or is there a shorter way of doing so?

firesurfing · Accepted Answer · 2017-12-24 13:39:16Z

2

I have test it using following code

    >>> a
    array([[1, 2],
           [3, 4]])
    >>> np.repeat(a, [2,3], axis = 0)
    array([[1, 2],
           [1, 2],
           [3, 4],
           [3, 4],
           [3, 4]])
    >>> np.repeat(a, [1,3], axis = 0)
    array([[1, 2],
           [3, 4],
           [3, 4],
           [3, 4]])

The second parameter seems mean how many times the i-th elements in a will be repeat. As my code shown above, [2,3] repeats a[0] 2 times and repeats a[1] 3 times, [1,3] repeats a[0] 1 times and repeats a[1] 3 times

answered Dec 24, 2017 at 13:39

firesurfing

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