Assigning complex values to numpy arrays?

Question

This gives the expected result

x = random.rand(1) + random.rand(1)*1j
print x.dtype
print x, x.real, x.imag

and this works

C = zeros((2,2),dtype=complex)
C[0,0] = 1+1j
print C

but if we change it to

C[0,0] = 1+1j + x

I get "TypeError: can't convert complex to float".

If we now omit the explicit dtype = complex, I get "ValueError: setting an array element with a sequence".

Can someone explain what's going on, and how to do this without errors? I'm lost.

Lenka42 · Accepted Answer · 2015-11-09 20:18:36Z

53

Actually, none of the proposed solutions worked in my case (Python 2.7.6, NumPy 1.8.2). But I've found out, that change of dtype from complex (standard Python library) to numpy.complex_ may help:

>>> import numpy as np
>>> x = 1 + 2 * 1j
>>> C = np.zeros((2,2),dtype=np.complex_)
>>> C
array([[ 0.+0.j,  0.+0.j],
       [ 0.+0.j,  0.+0.j]])
>>> C[0,0] = 1+1j + x
>>> C
array([[ 2.+3.j,  0.+0.j],
       [ 0.+0.j,  0.+0.j]])

answered Nov 9, 2015 at 20:18

Lenka42

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Comments

Fred Foo · Accepted Answer · 2014-02-26 15:04:09Z

6

To insert complex x or x + something into C, you apparently need to treat it as if it were an array, so either index into x or assign it to a slice of C:

>>> C
array([[ 0.+0.j,  0.+0.j],
       [ 0.+0.j,  0.+0.j]])
>>> C[0, 0:1] = x
>>> C
array([[ 0.47229555+0.7957525j,  0.00000000+0.j       ],
       [ 0.00000000+0.j       ,  0.00000000+0.j       ]])
>>> C[1, 1] = x[0] + 1+1j
>>> C
array([[ 0.47229555+0.7957525j,  0.00000000+0.j       ],
       [ 0.00000000+0.j       ,  1.47229555+1.7957525j]])

It looks like NumPy isn't handling this case correctly. Consider submitting a bug report.

edited Feb 26, 2014 at 15:04

answered Feb 25, 2014 at 14:37

Fred Foo

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8 Comments

zhangxaochen Over a year ago

but why a=np.arange(4).reshape((2,2)); b=np.array([100]); a[0,0]=b does not give such an error?

Fred Foo Over a year ago

@zhangxaochen Good Q, maybe the behavior of complex arrays is even buggier than I though. I never use those :)

gibson Over a year ago

Interesting. C[0,0] = random.rand(1) is fine, but C[0,0] = random.rand(1)+random.rand(1)*1j is not. Why does x become an array when we add the imaginary part to it? I thought complex numbers were a native data type in python, not just implemented through arrays?

Fred Foo Over a year ago

@gibson random.rand(1) is an array.

gibson Over a year ago

@larsmans If that's the case, C[0,0] = random.rand(1) shouldn't work, but it does. I'm still confused.

|

SYuan · Accepted Answer · 2020-11-16 16:00:00Z

3

I used astype to change the type to complex and it worked in my case (Python 3), although I am not sure whether it is the best way. One example:

import numpy as np
c2 = np.empty([2,2]).astype(complex)
c2[0] = 5j+2

answered Nov 16, 2020 at 16:00

SYuan

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Comments

bruce · Accepted Answer · 2021-12-30 06:27:12Z

0

For example:

x = 1.0
y = 1.5

C[i,j] = complex(x, y)

answered Dec 30, 2021 at 6:27

bruce

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