Python: two questions about a "numpy.ndarray" - python

I created a numpy.ndarray with value
import numpy as np
from numpy import nonzero
data = np.zeros((5, 5))
data
array([[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.]])
i wish to change some values with 1
data[0,0] = 1
data[4,4] = 1
data
array([[ 1., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 1.]])
if i change 0 with 5 using negative values i have
data[-5,-5] = 5
data[-4,-4] = 5
>>> data
array([[ 5., 0., 0., 0., 0.],
[ 0., 5., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.]])
1- I don't understand why i have not an error message as
>>> data[10,10] = 5
Traceback (most recent call last):
File "<interactive input>", line 1, in <module>
IndexError: index (10) out of range (0<=index<5) in dimension 0
2- it's not clear why with data[-5,-5] = 5 and data[-4,-4] = 5 the value 5 is inserted in position 0,0 and 1,1


From the documentation:
Negative indices are interpreted as counting from the end of the array
This is the standard Python indexing behavior (used in Python lists, etc.).

Related

CVXPY: Using two sets of constraints of different format returns None

I am new to CVXPY. I have been trying to solve an easy feasibility problem. Here is the code:
from cvxpy import *
import numpy as np
dim = np.shape(Bs[0])[1]
X = Variable(dim)
objective = Minimize(0)
constraintsA = [X.T * M * X + B * X + C == 0 for M, B, C in zip(Ms, Bs, Cs)]
constraintsB = [A * X - b == 0 for A, b in zip(As, bs)]
constraints = constraintsA + constraintsB
prob = Problem(objective, constraints)
result = prob.solve()
R = X.value
print R
Below, you can find the input matrices:
Ms = [array([[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.]]),
array([[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.]]),
array([[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0.]])]
Bs = [array([[-18., 0., 0., 18., 0., 0.]]),
array([[ 0., 0., 0., 0., -36., 0.]]),
array([[ 0., 0., -18., 0., 0., -18.]])]
Cs = [270.0, 540.0, 810.0]
As = [array([[ 0., -1., 1., 0., 0., 0.],
[ 0., 0., 0., 0., -1., 1.]]),
array([[-1., 1., 0., 0., 0., 0.],
[ 0., 0., 0., -1., 1., 0.]]),
array([[-1., 0., 1., 0., 0., 0.],
[ 0., 0., 0., -1., 0., 1.]])]
bs = [array([[ 0],
[ 1.00000000e+01]]),
array([[ 20.],
[ 10.]]),
array([[ -2.00000000e+01],
[ 0]])]
Running it only for constraintsA, the code is executed and returns back a result vector as expected. However, when considering the constraintsB set too, my result is None. Any ideas where I might have messed it up? Thanks a lot!

Appending matrix A with matrix B

Say I have two matrices A and B. For example,
A = numpy.zeros((5,5))
B = np.eye(5)
Is there a way to append A and B?

It sounds to me like you're looking for np.hstack:
>>> import numpy as np
>>> a = np.zeros((5, 5))
>>> b = np.eye(5)
>>> np.hstack((a, b))
array([[ 0., 0., 0., 0., 0., 1., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0., 1., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0., 0., 1., 0., 0.],
[ 0., 0., 0., 0., 0., 0., 0., 0., 1., 0.],
[ 0., 0., 0., 0., 0., 0., 0., 0., 0., 1.]])
np.vstack will work if you want to stack them downward:
>>> np.vstack((a, b))
array([[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 1., 0., 0., 0., 0.],
[ 0., 1., 0., 0., 0.],
[ 0., 0., 1., 0., 0.],
[ 0., 0., 0., 1., 0.],
[ 0., 0., 0., 0., 1.]])

Replace values in bigger numpy array with smaller array

I have 2 numpy arrays: The bigger one is a 10 x 10 numpy array and the smaller one is a 2 x 2 array.
I would like to substitute the values in the bigger array with those from the smaller array, at a user specified location. E.g. Replace the values of the 10 x 10 array starting from its center point by replacing 4 values with the 2 x 2 array.
Right now, I am doing this by using a nested for loop, and figuring out which pixels in the bigger array overlap those of the smaller array. Is there a more pythonic way to do it?

In [1]: import numpy as np
In [2]: a = np.zeros(100).reshape(10,10)
In [3]: b = np.ones(4).reshape(2,2)
In [4]: a[4:6, 4:6] = b
In [5]: a
Out[5]:
array([[ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 1., 1., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 1., 1., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.]])

Printing FULL contents of numpy array [duplicate]

This question already has answers here:
How do I print the full NumPy array, without truncation?
(22 answers)
Closed 9 years ago.
I am working with image processing in python and I want to output a variable, right now the variable b is a numpy array with shape (200,200). When I do print b all I see is:
array([[ 0., 0., 0., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 0., 0., 0.],
...,
[ 0., 0., 0., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 0., 0., 0.]])
How do I print out the full contents of this array, write it to a file or something simple so I can just look at the contents in full?

Of course, you can change the print threshold of the array as answered elsewhere with:
np.set_printoptions(threshold=np.nan)
But depending on what you're trying to look at, there's probably a better way to do that. For example, if your array truly is mostly zeros as you've shown, and you want to check whether it has values that are nonzero, you might look at things like:
import numpy as np
import matplotlib.pyplot as plt
In [1]: a = np.zeros((100,100))
In [2]: a
Out[2]:
array([[ 0., 0., 0., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 0., 0., 0.],
...,
[ 0., 0., 0., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 0., 0., 0.]])
Change some values:
In [3]: a[4:19,5:20] = 1
And it still looks the same:
In [4]: a
Out[4]:
array([[ 0., 0., 0., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 0., 0., 0.],
...,
[ 0., 0., 0., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 0., 0., 0.]])
Check some things that don't require manually looking at all values:
In [5]: a.sum()
Out[5]: 225.0
In [6]: a.mean()
Out[6]: 0.022499999999999999
Or plot it:
In [7]: plt.imshow(a)
Out[7]: <matplotlib.image.AxesImage at 0x1043d4b50>
Or save to a file:
In [11]: np.savetxt('file.txt', a)

to_print = "\n".join([", ".join(row) for row in b])
print (to_print) #console
f = open("path-to-file", "w")
f.write(to_print) #to file
In case it's numpy array: Print the full numpy array

Using numpy to Obtain Submatrix

I'm trying to do the following with numpy (python newbie here)
Create a zeroed matrix of the rigth dimensions
num_rows = 80
num_cols = 23
A = numpy.zeros(shape=(num_rows, num_cols))
Operate on the matrix
k = 5
numpy.transpose(A)
U,s,V = linalg.svd(A)
Extract sub-matrix
sk = s[0:(k-1), 0:(k-1)]
Results on error
Traceback (most recent call last):
File "tdm2svd.py", line 40, in <module>
sk = s[0:(k-1), 0:(k-1)]
IndexError: too many indices
What am I doing wrong?

to answer your question s is only a 1d array ... (even if you did actually transpose it ... which you did not)
>>> u,s,v = linalg.svd(A)
>>> s
array([ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0.])
>>>
for selecting a submatrix
I think this does what you want ... there may be a better way
>>> rows = range(10,15)
>>> cols = range(5,8)
>>> A[rows][:,cols]
array([[ 0., 0., 0.],
[ 0., 0., 0.],
[ 0., 0., 0.],
[ 0., 0., 0.],
[ 0., 0., 0.]])
or probably better
>>> A[15:32, 2:7]
array([[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.]])

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