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list building - consecutive

asked 2013-06-23 20:09:10 +0200

mresimulator gravatar image

updated 2013-06-23 20:39:07 +0200

calc314 gravatar image

Hi experts!

How can i create N empty NUmpy arrays or lists (L1, L2, L3, ..., LN) in a 'for' cycle, i.e.:

for j in srange(N):

(A= numbers of element in the array).

Waiting for your answers.

Thanks a lot.

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5 Answers

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answered 2013-06-23 21:06:45 +0200

Volker Braun gravatar image
sage: [np.empty(A) for j in range(N)]
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answered 2013-07-16 10:49:45 +0200

Volker Braun gravatar image

It seems that the question is about generating new global variables in code. The correct way to do that is to add them to the globals() dictionary (has nothing to do with numpy)

sage: import numpy
sage: for i in range(1, 5):
....:     variable_name = 'L'+str(i)
....:     globals()[variable_name] = numpy.empty(3)
sage: L1
array([  1.26596386e-312,   1.26596386e-312,   6.90349880e-310])
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answered 2013-06-24 11:06:53 +0200

tmonteil gravatar image

Actually, the answer of @volker-braun was quite satisfactory, since it creates a list that you can call:

# data you should add in your question to ease reproduction:
sage: import numpy as np
sage: N = 4
sage: A = 3

sage: L = [np.empty(A) for j in range(N)]
sage: L[0]
array([  6.89920558e-310,   6.89920558e-310,   0.00000000e+000])
sage: L[3]
array([  6.89920558e-310,   6.89920558e-310,   0.00000000e+000])

But you should notice that elements of a list L are numbered from 0 to len(L)-1.

Perhaps are you looking for a dictionary:

sage: L = dict()
sage: for j in srange(1, N+1):
sage:     L[j]=np.empty(A)
sage: L
{1: array([  6.89920558e-310,   5.92371271e-316,   5.43231038e-310]),
 2: array([  6.89920558e-310,   5.91607169e-316,   5.40520702e-316]),
 3: array([  6.89920558e-310,   5.91582347e-316,   5.43231038e-310]),
 4: array([  6.89920558e-310,   5.91594758e-316,   1.09178468e-316])}
sage: L[1]
array([  6.89920558e-310,   5.92371271e-316,   5.43231038e-310])
sage: L[4]
array([  6.89920558e-310,   5.91594758e-316,   1.09178468e-316])
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answered 2013-07-16 06:41:08 +0200

jack77 gravatar image

updated 2013-07-16 16:49:46 +0200

Hello mresimulator,

I do not know, why you want to encode the index j in the name,
but you could use 'exec()' for this:

import numpy as np
N = 4
A = 3
for j in srange(N):

Then L1 to L4 gets defined:

>>> L1
array([  9.79748720e-251,   2.90930926e-252,   2.34745402e-251])
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answered 2013-06-24 10:28:44 +0200

mresimulator gravatar image

Hi! Thank for the answer...

Maybe i didnt express me right...

In my script i wanna write a 'for' cycle that create N arrays separated, named L1, L2, ..., LN.

I mean: how can i introduce this successives numbers (1,2,...,N) in the array (or list) names whis that cycle?

Waiting for your answers.

Thanks a lot.

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Asked: 2013-06-23 20:09:10 +0200

Seen: 652 times

Last updated: Jul 16 '13