Python Numpy Array Modification

Question

I have a plot y vs x. Based on some boundary conditions, I have obtained a list of x-values and corresponding y-values

x = [2, 4, 8, 11, 18, 24, 31]

the corresponding y-values are

y = [180, 139, 144, 90, 81, 109, 153]

the output is an array that returns twice the value of x output = [4, 8, 16, 22, 36, 48, 62]

My task is to limit the final output array to 5 values maximum. These values should correspond to the 5 highest values in the array y.

So, I should get

output = [4, 8, 16, 48, 62]

Also, different data sets will give different no. of x and y values. So, if the original output array has less than or equal to 5 values, it should be printed as it is.

I am trying to find a way to do this with a couple of lines of code.

So, have you done this already with more than the couple of lines you're aiming for (which would be a refactoring issue), or are you having problems implementing this at all? What have you attempted so far? — Andrew
– Andrew, Commented Dec 6, 2018 at 11:51

offeltoffel · Accepted Answer · 2018-12-06 11:59:11Z

1

Your problem basically comes down to the question on how to obtain n highest values of an array. You will need the argsort() for this.

if len(y) > 4:
    n_highest = 5
else:
    n_highest = len(y)

y_highest = y.argsort()[-n_highest:][::-1] # this gives you the indices of the five highest values of y

x_out = x[y_highest] * 2

If you don't want x_out to be sorted, but rather reflect the original sorting, you'd have to iterate through eatch item, asking if its index is found in y_highest:

x_out = [x[i]*2 for i in range(len(x)) if i in y_highest]

answered Dec 6, 2018 at 11:59

offeltoffel

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Juan D. · Accepted Answer · 2018-12-06 12:06:55Z

1

You can reorder the array with

array = sorted(array)

After that you just need to take the 5 first terms:

output = array[0:4]

Remember that we need to write 4 instead of 5 because on Python array's dimension starts ar 0. You can do this with less code:

output = sorted(array)[0:4]

answered Dec 6, 2018 at 12:06

Juan D.

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1 Comment

offeltoffel Over a year ago

Only problem is, rahul needs the indices of the n highest values of y and extract those from x. Hence I suggest argsort instead of sorted.

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