Append function in numpy array

Question

I need help for my project. I have an array that look like this?

rndm = [[0 1]
        [0 0]
        [0 0]
        [0 1]]

Now, I want to add par_1 = [[1 0]], par_2 = [[0 0], ch1 = [[1 1]], and ch2 = [[0 1]] to rndm.

My code looks like this:

new_rndm = []
new_rndm.append(par_1)
new_rndm.append(par_2)
new_rndm.append(ch1)
new_rndm.append(ch2)
# add them to rndm
rndm = numpy.append(rndm, [new_rndm])
print(rndm)

The output gives me something like this:

rndm = [0 1 0 0 0 0 0 1 1 0 0 0 1 1 0 1]

What I am expecting as my out put is:

rndm = [[0 1]
        [0 0]
        [0 0]
        [0 1]
        [1 0]
        [0 0]
        [1 1]
        [0 1]]

I think the problem is that append cannot be used in arrays. If correct, anyone help me what other function I could try? If not, kindly educate me. I am very much willing to learn. Thank you!

we are able to see it is an array but is it python nested list or a numpy array ? — sushanth
– sushanth, Commented Jun 4, 2020 at 13:18

Pygirl · Accepted Answer · 2020-06-04 14:02:25Z

3

Use np.append(<array>, <elem to append>, axis=0)

rndm = np.array([[0, 1],
        [0, 0],
        [0, 0],
        [0, 1]])

par_1 = [[1, 0]]; par_2 = [[0, 0]]; ch1 = [[1, 1]]; ch2 = [[0, 1]]

rndm = np.append(rndm, par_1, axis=0)
rndm = np.append(rndm, par_2, axis=0)
rndm = np.append(rndm, ch1, axis=0)
rndm = np.append(rndm, ch2, axis=0)

array([[0, 1],
       [0, 0],
       [0, 0],
       [0, 1],
       [1, 0],
       [0, 0],
       [1, 1],
       [0, 1]])

Edit:

Reshape:

x = np.array([2,1])
y = x.reshape(-1,1) # <------------ you have to do this
x.shape, y.shape

((2,), (2, 1))

edited Jun 4, 2020 at 14:02

answered Jun 4, 2020 at 13:30

Pygirl

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13 Comments

Acee Over a year ago

Hello! Thank you so much for your answer! But why is it that I am getting an error trying this solution? Error is: ValueError: all the input arrays must have same number of dimensions, but the array at index 0 has 2 dimension(s) and the array at index 1 has 1 dimension(s)

Pygirl Over a year ago

What numpy version do you use? I use 1.18.4

Pygirl Over a year ago

Are you using the whole code or some part of it. Because I am not getting any error.

Acee Over a year ago

I am using the same version of numpy. This is just a part of my code. rndm is generated randomly as well as par_1 and the others.

Pygirl Over a year ago

This (2, ) is the cause of this issue. reshape it to (2,1)

|

sugarfi · Accepted Answer · 2020-06-04 13:16:47Z

0

You can use .append to add an array to the end of another array. The problem here is that numpy.append flattens the array first, ie. numpy.append([1 0], [0 1]) is [1 0 0 1]. See the numpy docs on .append.

answered Jun 4, 2020 at 13:16

sugarfi

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2 Comments

Acee Over a year ago

It is because rndm is a numpy array. So would it be better to make it as list then append those values?

Pygirl Over a year ago

The reason why it is appending like that because shape of those two are (2,) They are flattened that's why you will get that answer only but if you have shape like this (2,1) then you can append row wise.

Gustav Rasmussen · Accepted Answer · 2020-06-04 13:31:44Z

0

You can use ordinary list appending to generate the desired nested list structure:

rndm = [[0, 1],
        [0, 0],
        [0, 0],
        [0, 1]
        ]

par_1 = [[1, 0]]
par_2 = [[0, 0]]
ch1 = [[1, 1]]
ch2 = [[0, 1]]

new_rndm = []

new_rndm.append(par_1)
new_rndm.append(par_2)
new_rndm.append(ch1)
new_rndm.append(ch2)

new_rndm = [i for k in new_rndm for i in k]

for data in new_rndm:
    rndm.append(data)

for data in rndm:
    print(data)

Outputs:

[0, 1]
[0, 0]
[0, 0]
[0, 1]
[1, 0]
[0, 0]
[1, 1]
[0, 1]

answered Jun 4, 2020 at 13:31

Gustav Rasmussen

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