Creating a 2d array based on two different arrays

Question

I receive constant data, for ease of life I get both data values that I receive and put them into each into a different array, the idea of this is so that I can link a value of Variable2 to a value of Variable1. Now, Variable1 will always be a value between 0 and 7, which I want to use this value to index the other array containing all values from variable2, data is polled serially inside a for loop, so I get data for both variables constantly. an example of data I get:

Variable1:  6 Variable2:  1499.6
Variable1:  6 Variable2:  1186.5
Variable1:  6 Variable2:  570.4
Variable1:  1 Variable2:  405.9
Variable1:  1 Variable2:  754.0
Variable1:  1 Variable2:  197.2
Variable1:  1 Variable2:  726.0
Variable1:  1 Variable2:  959.1
Variable1:  1 Variable2:  709.6

An example of two arrays of data for variable1 and variable 2 is below:

  Variable2 ={  [1499.6, 1186.5, 570.4, 405.9, 754.0, 197.2, 726.0, 959.1, 709.6]}
  Variable1 ={  [[6], [6], [6], [1], [1], [1], [1], [1], [1]]}

So my question is how would I grab these two arrays and make it so for example if I where to do print(VariableArray[6][2]) results in it printing : 570.4 which is the 3rd variable2 corresponding to Variable1 = 6

Paul · Accepted Answer · 2020-10-12 06:11:30Z

4

I'd go with a dictionary of lists:

{
    6: [1499.6, 1186.5, 570.4],
    1: [405.9, 754.0, 197.2, 726.0, 959.1, 709.6]
}

As an example in the Python REPL:

>>> d = {6: [1499.6, 1186.5, 570.4], 1: [405.9, 754.0, 197.2, 726.0, 959.1, 709.6]}
>>> d[6][2]
570.4

The idea is: Keep a key for your 0 to 7 values (initially Variable1) and for each of those, store a list of values where you push the new data.

You could initialize this dictionary like so:

d = {
    0: [],
    1: [],
    2: [],
    3: [],
    4: [],
    5: [],
    6: [],
    7: [],
}

Or you could use a defaultdict, but first, make sure you understand lists and dictionaries.

In order to append data to a list you'd do (assuming 42.0 is your new value and the dictionary already contains the data you posted in the question):

d[6].append(42.0)

Which yields:

{6: [1499.6, 1186.5, 570.4, 42.0], 1: [405.9, 754.0, 197.2, 726.0, 959.1, 709.6]}

Or more generally: d[Variable1].append(Variable2).

Note: make sure your list will not grow indefinitely.

Please read the general theory and usage on dictionaries and lists in Python here (minimal reading for better understanding):

edited Oct 12, 2020 at 6:11

answered Oct 12, 2020 at 5:49

Paul

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4 Comments

Victor J Mytre Over a year ago

in code how would I do this? I attempted to do a dictionary append, but it would always just keep the last value of each key, so in my case i would end up with: { 6:[670.4], 1:[709.6] }

Paul Over a year ago

Edited the post to include the appending.

Victor J Mytre Over a year ago

Thanks, the issue is that the list will grow a lot, im building both lists above constantly, after a certain number of datapoints I will clear the list and create 2 new lists after getting the data of the list, I will probably post a questio nif I am unable to find the lowest value for each key since the idea I have is to build around a list of 100 elements, create the dictionary, pick the lowests values per key, to create a new array of 8 elements where the index position is equal to the key of the dictionary , and the values are the lowest per key. then start again from 0. Thanks!

Paul Over a year ago

Yes, it's better to post that question separately. Please make sure to click the check mark icon to the left of the answer if it solves the problem posted here.

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