How do you print elements that are less than a variable from a numpy array

Question

Hi so Im fairly new to python and an assignment require me to print elements that are less than a variable from a numpy array.

I made a 20x10 numpy array of random integers between -5 and 50

x = np.random.randint (-5, 50, (20, 10))
x

array([[17, 23, 15, 13, -1, 17, 30, 14,  2,  3],
   [ 8,  0, -5,  3, 10, 10, 48,  6, -1, 34],
   [23, 40, 21,  5, 47, 41, 44, 22, 46, 30],
   [36, 13, 48, 29, 46, 25, 48, 38, 13, 40],
   [18, -4,  1, 37, 48, 43, 25, 11, 21, 30],
   [44, 37,  4, 39,  8,  1, 33, 34,  3,  8],
   [ 2, 11, 17, 10, 20,  3, 30,  1, 12,  2],
   [15, 20, -3, 11, 45, 40, 18, 19, -1, 31],
   [39, 44, 18, 25, 49, 20, 15, 28, 32, 18],
   [22, 24, 28, 46, 48, 46, 17, 49,  2, 36],
   [44,  4, 49, -5, 14, 31, 12, 15, 48, 43],
   [-2, 37, -4, 15, 31, -1, 11, 43, 42,  5],
   [40, 35, 25, 22, 38, 26, 15,  1,  4, 22],
   [42, 30, 14,  7, 13, 44,  5, 29, 28, 38],
   [-2,  7, 31, -4, 44, -5, 34, 19, 31, 30],
   [ 0,  1, -2, 29, 35, 28, 23, -1, 21, 27],
   [40, 46,  4, 48,  0, 28,  2, 25,  3, 49],
   [15,  2, -2, 16, 22, 39, -2, 33, 15,  2],
   [14, 26, -5,  0, 22, 38, 25,  4, 14,  2],
   [16, 32, 23,  3, 38, 41, -5, 35, 46, 33]])

above is the result. Now i want to print the number of elements that are less than 5 in each row.

I managed to do this

print (x[0, :] < 5)

[False False False False  True False False False  True  True]

the result is as shown above but what i wanted was for it to show the number of elements that is less than 5. I wanted for it to give me 3 since there are 3 elements.

Can anyone help me with this? Thank you

mathfux · Accepted Answer · 2020-02-27 15:12:11Z

2

It's possible to use np.sum for arrays of type bool like yours. So, at first I have tried the following:

[np.sum(n<5) for n in x]

This gives me a list [3, 4, 0, 0, 2, 3, 4, 2, 0, 1, 2, 3, 2, 0, 3, 4, 4, 4, 4, 2] which is correct but the bad thing is that you need to avoid list comprehensions in numpy actions. Here is the best way to do this in numpy:

np.sum(x<5, axis=1)

This command makes bool array out of x and then calculates True values for each row along y axis (axis number 1)

answered Feb 27, 2020 at 15:12

mathfux

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3 Comments

Carlo Silanu Over a year ago

Thank you so much man this is the answer that I am looking for

Carlo Silanu Over a year ago

Hi, if you dont mind me asking again. why does this code that you give np.sum(x<5, axis=1) works perfectly but when i try to do np.sum ((x>=0) and (x<=50), axis=1) it gives me the error?

mathfux Over a year ago

x<5 is a mask array. It contains Trues and Falses. Such kind of arrays doesn't support and operator unlike bools True and False. Try to use arr1 & arr2 or np.logical_and(arr1, arr2) instead.

chrissi_gsa · Accepted Answer · 2020-02-27 15:07:01Z

1

You can use your boolean mask to index the array and then count the elements. Alternatively, you can use numpy.where(). Similar to your approach, it will give you a boolean mask where a certain condition is met.

For your example:

indices = numpy.where(x < 3)
values_greater_than_3 = x[indices]
count = len(values_greater_than_3)
print(count)

answered Feb 27, 2020 at 15:07

chrissi_gsa

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