CODEWITHSUNDEEP
arrayspython-3.xnumpy
Steve
Steve

Reputation: 505

Appending pairs of coordinates to numpy array

I need to append 1D arrays (coordinates) into a 2d array using numpy in python 3.6.

I can do this with lists using append, such as in the first example below.

mastlist =[]
i=0
for i in  range (10):
    i=i+1
    coor = []
    xcoor = i
    ycoor =i*2
    coor.append(xcoor)
    coor.append(ycoor)
    mastlist.append(coor)
print(mastlist)

But I want a more performant approach using numpy arrays. When I attempt to convert the list approach to an array (second example),

import numpy as np
i=0
for i in  range (10):
    i=i+1
    centroid =np.append(i,i*2)
    masterarray=np.append([centroid],axis=0)
    print(masterarray)

print(masterarray)

I get the error below.

My error is:

TypeError: append() missing 1 required positional argument: 'values'

I would of expected an array such as:

[[1, 2], [2, 4], [3, 6], [4, 8], [5, 10], [6, 12], [7, 14], [8, 16], [9, 18], [10, 20]]

I have also fumbled with attempts using extend, vstack, and concatenate.

Any advice would be welcome.

Upvotes: 2

Views: 5382

Answers (2)

haofeng
haofeng

Reputation: 651

I recommend you get single coordinate data firstly , then concatenate them. To my best knowledge, I dont think it can be done by np.append

The common method is np.concatenate, which I see it from cs231n class.

My sample codes are as follows:

import numpy as np
xcoor = np.arange(1,11,1).reshape(-1,1)
ycoor = np.arange(2,22,2).reshape(-1,1)
xycoor = np.concatenate((xcoor,ycoor),axis = 1)
print(xycoor)

Output:

[[ 1  2]
 [ 2  4]
 [ 3  6]
 [ 4  8]
 [ 5 10]
 [ 6 12]
 [ 7 14]
 [ 8 16]
 [ 9 18]
 [10 20]]

Upvotes: 3

sentence
sentence

Reputation: 8903

Why not just use list comprehension?

import numpy as np

masterarray = np.array([[i,2*i] for i in range(1,11)])

output

array([[ 1,  2],
       [ 2,  4],
       [ 3,  6],
       [ 4,  8],
       [ 5, 10],
       [ 6, 12],
       [ 7, 14],
       [ 8, 16],
       [ 9, 18],
       [10, 20]])

Upvotes: 1

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