Multi Dimension Python Lists-CodePudding

I am new to python 3 and trying to work with a list but struggling. I have read things but some are quite confusing, so I have been playing with code but getting nowhere fast.

I want to store 3 elements (Count,Angle,Speed) for every second of the day which is 86400 entries.

To make things easier for this example lets have a list of 2 entries of 3 elements I created a list and initially want to set every element to 0

a = [[0, 0, 0], [0, 0, 0]]
print(a[0])
print(a[0][0])
print(a[0][1])
print(a[0][2])
print(a[1])
print(a[1][0])
print(a[1][1])
print(a[1][2])
print("** Change a[0][2] to 7 from 0")
a[0][2] = 7
print(a[0])
print(a[0][0])
print(a[0][1])
print(a[0][2])
print("-----------")
print(a[1])
print(a[1][0])
print(a[1][1])
print(a[1][2])
print("-----------")

This works great and for example I am able to access and change a[0][2]

[0, 0, 0] 0 0 0 [0, 0, 0] 0 0 0 ** Change a[0][2] to 7 from 0 [0, 0, 7] 0 0 7

[0, 0, 0] 0 0 0

However I want to be able to pre-fill the list with 0 as it will be tedious to do it for 86400 entries but again for this example lets go with 2

So I read up and seen how to do it using... a = [[0,0,0]] * 2

a = [[0,0,0]] * 2
print(a[0])
print(a[0][0])
print(a[0][1])
print(a[0][2])
print(a[1])
print(a[1][0])
print(a[1][1])
print(a[1][2])
print("** Change a[0][2] to 7 from 0")
a[0][2] = 7
print(a[0])
print(a[0][0])
print(a[0][1])
print(a[0][2])
print("-----------")
print(a[1])
print(a[1][0])
print(a[1][1])
print(a[1][2])
print("-----------")

However why does a[1][2] also change to 7

[0, 0, 0] 0 0 0 [0, 0, 0] 0 0 0 ** Change a[0][2] to 7 from 0 [0, 0, 7] 0 0 7

[0, 0, 7] 0 0 7

I have obviously done something wrong with pre-filling the list or even defining the list, so if anybody can help I would be grateful.

Thanks

CodePudding user response：

[[0,0,0]]*2

means that you evaluate [0,0,0], that is build such a list. And then create an array of 2 elements whose each element is this [0,0,0]. So it is the same one!

It is like doing

arr=[]
arr0=[0,0,0]
for i in range(2):
    arr.append(arr0)

So, since arr[0] and arr[1] are the same list, altering one, alters the other.

Note that changing arr[0] does not alter arr[1]. Because that is changing the content of arr not of arr[0].

arr=[[0,0,0]]*2
arr[0]=12
# arr = [12, [0,0,0]], not [12,12]

What you need to do is to build a new [0,0,0] for each line.

For example like this

arr=[ [0]*3 for _ in range(2) ]
arr[0][2]=7
# [[0, 0, 7], [0, 0, 0]]

Or, alternative, using numpy

import numpy as np
arr=np.zeros((86400, 3))
arr[0,2]=7

CodePudding user response：

You can simply use numpy to create array of dimension 86400, 3

import numpy as np
a=np.zeros((86400, 3))

CodePudding user response：

It's a good idea to use the numpy module for big numerical arrays as access is very efficient - worth reading up on. You can easily create a numpy array of any shape and fill with zeros (ints or floats)

import numpy as np

x = np.zeros((86400, 3), dtype = int)

CodePudding user response：

You can create a list of list 86400 x 3 like this.

l = [[0]*3 for _ in range(86400)]