Quick numpy array extension

Question

I wonder if there is a trick to extending a numpy array with consecutive numbers in between each original values, up to a user controlled default length. Perhaps there is already a built-in function that turns x into y

x=np.array([4,8,4,10])

y=np.array([4,5,6,7,8,9,10,11,4,5,6,7,10,11,12,13])

Here the length between each element that I added was three. Speed is of the essence here. I need something like this for my column data to work in creating a sparse matrix.

Also, if I have an array such as

s=np.array([0,1])

is there a quick way to extend each element an arbitrary amount of times, lets say 4 for example:

s=np.array([0,0,0,0,1,1,1,1])

For the second half of your question, have a look at numpy.repeat or numpy.tile. — user707650
– user707650, Commented Oct 11, 2016 at 0:34

hpaulj · Accepted Answer · 2016-10-11 00:38:44Z

2

Broadcasted addition is probably the fastest

In [241]: (x[:,None]+np.arange(4)).ravel()
Out[241]: array([ 4,  5,  6,  7,  8,  9, 10, 11,  4,  5,  6,  7, 10, 11, 12, 13])

It gets trickier if adding different amounts for each sublist.

repeat is useful:

In [242]: np.repeat(np.array([0,1]),[3,4])
Out[242]: array([0, 0, 0, 1, 1, 1, 1])

tile is another good tool.

answered Oct 11, 2016 at 0:38

hpaulj

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1 Comment

Canuck Over a year ago

All great methods guys, this one was fastest though. Mahalo!

user707650 · Accepted Answer · 2016-10-11 00:39:11Z

0

I don't know a built-in function that does precisely that, but with some creativity you could do:

>>> x=np.array([4,8,4,10])
>>> np.array([x+i for i in range(4)]).T.ravel()
array([ 4,  5,  6,  7,  8,  9, 10, 11,  4,  5,  6,  7, 10, 11, 12, 13])

For the second half of your question, have a look at numpy.repeat and numpy.tile.

answered Oct 11, 2016 at 0:39

user707650

Comments

Mateen Ulhaq · Accepted Answer · 2016-10-11 00:39:39Z

0

This works on lists:

def extend(myList, n):
    extensions = [range(x, x + n) for x in myList]
    return [item for sublist in extensions for item in sublist] # flatten

Use as follows:

extend([4, 8, 4, 10], 4)

answered Oct 11, 2016 at 0:39

Mateen Ulhaq

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