How to Select Elements of an Array Given Condition

How do I select elements of an array given condition?

Your expression works if you add parentheses:

>>> y[(1 < x) & (x < 5)]
array(['o', 'o', 'a'], 
      dtype='|S1')

Selecting elements from an array based on a condition

Since you included Numpy in your tags, I'm assuming you're OK with a solution with Numpy?

import numpy as np

a = np.array([0,10,20])

np.min(a[a > 0])

Out[1]: 10

How do I select elements of an two array given condition?

Use numpy.where:

np.where(y, x2, x1)

Output:

array([ 5,  2, 22,  1,  0,  7])

How to conditionally select elements in numpy array

You can index directly like:

sampleArr[sampleArr > 0.5]

Test Code:

sampleArr = np.array([0.725, 0.39, 0.99])

condition = (sampleArr > 0.5)
extracted = np.extract(condition, sampleArr)  # returns [0.725 0.99]

print(sampleArr[sampleArr > 0.5])
print(sampleArr[condition])
print(extracted)

Results:

[ 0.725  0.99 ]
[ 0.725  0.99 ]
[ 0.725  0.99 ]

Assign new value to a condition selected element of an array

As others mentioned in their answers Yr[Xg>5] is a new array and any change on it does not apply on the original array, but you can do it by using np.put like this:

np.put(Yr, np.where(Xg>5)[0][2], 10)

Or simply do this:

Yr[np.where(Xg>15)[0][2]] = 10

Numpy: Selecting Rows based on Multiple Conditions on Some of its Elements

Elegant is np.equal

Z[np.equal(Z[:, [0,1]], 1).all(axis=1)]

Or:

Z[np.equal(Z[:,0], 1) & np.equal(Z[:,1], 1)]

JavaScript - get array element fulfilling a condition

In most browsers (not IE <= 8) arrays have a filter method, which doesn't do quite what you want but does create you an array of elements of the original array that satisfy a certain condition:

function isGreaterThanFive(x) {
     return x > 5;
}

[1, 10, 4, 6].filter(isGreaterThanFive); // Returns [10, 6]

Mozilla Developer Network has a lot of good JavaScript resources.

how to select values from two arrays according to a value condition from another array?

You can create an array with your conditions and use it to select from a and c.

import matplotlib.pyplot as plt

select = (b == 0) & (a > 0)

plt.plot(a[select], c[select])
plt.xlabel('a')
plt.ylabel('c');

Output

Selecting elements of a vector for which a matrix satisfies a condition

What you can do is create an "elimination array". Loop over all x,y pairs, check the corresponding entry in S and set elimination to true if the condition is fulfilled. Then, at the end, remove all elements which should be eliminated.

x=[1,3,2,3];
y=[2,4,1,3];

S=[1,3,4,5;1,3,4,5;1,2,0,0.001;12,21,2,5];

% Preallocate to no elimination taking place
eliminate = zeros(size(x),'logical');
for ii = 1:numel(x)  % Loop over all elements
   if S(x(ii),y(ii)) < 0.1  % If the condition holds
       eliminate(ii) = true;  % Set elimination to true
   end
end

% Remove elements to be eliminated
x(eliminate) = []
y(eliminate) = []

x =
     1     2
y =
     2     1

A one-liner to do this a bit more elegant

eliminate = diag(S(x,y))<0.1;
x(eliminate) = []
y(eliminate) = []

S(x,y) creates a matrix of where x and y are considered to be permutable, i.e. it will be 4x4 in this example. We only need the pairs you present, which will be on the diagonal of this matrix; use diag() to obtain those. Then, do the logical check with <0.1 and finally use the same removal trick as above.

---

Related Topics