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Is possible to construct a NumPy array from a python list?

yatu
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Hossein
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9 Answers9

169

First of all, I'd recommend you to go through NumPy's Quickstart tutorial, which will probably help with these basic questions.

You can directly create an array from a list as:

import numpy as np
a = np.array( [2,3,4] )

Or from a from a nested list in the same way:

import numpy as np
a = np.array( [[2,3,4], [3,4,5]] )
yatu
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Bryce Siedschlaw
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    import numpy as np; and add np. before array(np.array([]); for someone who might be confused – Abhi Sep 04 '16 at 21:33
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    I would have put that syntax a little differently. How about `import numpy as np` then `a = np.array ( [[2,3,4],[3,4,5]] )` ? – SDsolar Sep 12 '17 at 23:12
42

you mean something like this ?

from numpy  import array
a = array( your_list )
Cédric Julien
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19

Yes it is:

a = numpy.array([1,2,3])
Glorfindel
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Felix Kling
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16

You want to save it as a file?

import numpy as np

myList = [1, 2, 3]

np.array(myList).dump(open('array.npy', 'wb'))

... and then read:

myArray = np.load(open('array.npy', 'rb'))
eumiro
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8

You can use numpy.asarray, for example to convert a list into an array:

>>> a = [1, 2]
>>> np.asarray(a)
array([1, 2])
Bilal
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4

I suppose, you mean converting a list into a numpy array? Then,

import numpy as np

# b is some list, then ...    
a = np.array(b).reshape(lengthDim0, lengthDim1);

gives you a as an array of list b in the shape given in reshape.

Hadamard
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0

Here is a more complete example:

import csv
import numpy as np

with open('filename','rb') as csvfile:
     cdl = list( csv.reader(csvfile,delimiter='\t'))
     print "Number of records = " + str(len(cdl))

#then later

npcdl = np.array(cdl)

Hope this helps!!

rahul ranjan
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SDsolar
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0
import numpy as np 

... ## other code

some list comprehension

t=[nodel[ nodenext[i][j] ] for j in idx]
            #for each link, find the node lables 
            #t is the list of node labels

Convert the list to a numpy array using the array method specified in the numpy library.

t=np.array(t)

This may be helpful: https://numpy.org/devdocs/user/basics.creation.html

ThermoRestart
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0

maybe:

import numpy as np
a=[[1,1],[2,2]]
b=np.asarray(a)
print(type(b))

output:

<class 'numpy.ndarray'>
Raazescythe
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