python how to create an array based on condition

Question

I want to create an array with a format, and the values originate from another array. My input array consists out of three columns. I want to create an array with in the first row all values from the third column if the second column is equal. So in this example the first three values in the second column are equal, so in the new array i want the third value of each row in the new array.

a = 
[[1,  1,  4],
[2,  1,  6],
[3,  1,  7],
[4,  2,  0],
[5,  2,  7],
[6,  3,  1]]

result:

b = 
[[4, 6 , 7],
[0, 7],
[1]]

I tried:

c = []    
x = 1
for row in a:
    if row[0] == x
        c.extend[row[2]]
    else:
        x = x + 1
        c.append(row[2])

But the result is a list of all 3rd values

Jep, they are all integer, and the values in the second column will only increase. However, the amount of values that are the same will vary — S Verhoef
– S Verhoef, Commented Aug 16, 2017 at 14:43

Jürg W. Spaak · Accepted Answer · 2017-08-16 14:44:44Z

1

a = np.asarray(a)
c = []
for i in range(a[-1,1]): #a[-1,1] is the maximum that will occur
    save = a[a[:,1]==i] # take all the ones that have i in the second entry
    c.append(save[:,2]) # of those add the last entry

It's important, that ais converted to a np.array for this.

answered Aug 16, 2017 at 14:44

Jürg W. Spaak

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Comments

akuiper · Accepted Answer · 2017-08-16 14:49:14Z

1

If the second column is sorted, you can use np.diff to find out the index where the value changes and then split on it:

np.split(a[:,2], np.flatnonzero(np.diff(a[:,1]) != 0)+1)
# [array([4, 6, 7]), array([0, 7]), array([1])]

answered Aug 16, 2017 at 14:49

akuiper

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3 Comments

S Verhoef Over a year ago

This results in a list of arrays, is it also possible to create an array immediately?

akuiper Over a year ago

You can wrap the result in np.array(...) to get an array, but Since the result arrays have different lengths, it will be an array of objects. Normal useful numpy array operations won't apply here.

Jürg W. Spaak Over a year ago

I would avoid wrapping it into an np.array as it will not behave as usual arrays. In case you really need it I would recommend creating a array with np.nan and then filling in your values, this will use to much information, but will be usable like a normal array.

M3RS · Accepted Answer · 2017-08-16 14:58:24Z

1

The below works for me:

import numpy as np

c = [[]]    
x = 1
for row in a:
    if row[1] == x:
        c[-1].append(row[2])
    else:
        x = x + 1
        c.append([row[2]])

c = np.asarray(c)

edited Aug 16, 2017 at 14:58

answered Aug 16, 2017 at 14:48

M3RS

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4 Comments

S Verhoef Over a year ago

The format is correct, but this results in a list consiting of lists, but because I need to do calculations with the result i prefer an array.

M3RS Over a year ago

you can add import numpy as np and c = np.asarray(c)

S Verhoef Over a year ago

This creates an object array. If i try to open it to preview i get the error object arrays are currently not supported (Spyder). My other arrays are the type float 64 and won't show this error..

M3RS Over a year ago

I guess as @Psidom writes, the three arrays are unequal length. It's not possible to have one (float 64) array that contains the information presented in the three of them. So either you look at each array separately or go with the object array (array of three arrays), but that can't be displayed in Spyder.

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