Index multiple arrays

Question

Given I have four lists (in reality I have many more, read in from a CSV as numpy arrays) such that:

a = [54,35,67...]
b = [45,21,87...]
c = [32,58,52...]
d = [4,78,43...]
# In reality I have plant % cover surveyed at a location

and four lists like:

A = ['foo', 'bar', 'ham'...]
B = ['eggs', 'spam', 'bar'...]
C = ['ham', 'eggs', 'foo'...]
D = ['eggs', 'spam', 'bar'...]
# These are plant species

I can find the max of a, b, c and d using:

max = [a, b, c, d].max(axis=0)
# The max plant % cover between a, b, c and d on the 0 axis

to get:

max = [54, 78, 87...]
# the plant species that accompanies the max plant % cover

but how do I now get the corresponding text value, so my output looks like:

Text = ['foo', 'spam', 'bar'...]

Just to clarify. You want the letter/string that is at the same X/Y location as the numbers in Max? — Colin Nichols
– Colin Nichols, Commented Mar 10, 2017 at 22:54
Yes, I want the letter that corresponds with the max value only — GeoMonkey
– GeoMonkey, Commented Mar 10, 2017 at 22:57
No, you don't have four arrays, you have four lists. At least, that's what you have in your code. And your code would throw an AttributeError — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Mar 10, 2017 at 22:58

akuiper · Accepted Answer · 2017-03-10 23:48:05Z

1

You can use argmax to get the index of the maximum values and advanced indexing to pick up the names from the names array:

import numpy as np

names = np.array([A, B, C, D])
values = np.array([a, b, c, d])

# use argmax to find out the index of the max values
index_max = values.argmax(0)

# use index to pick up corresponding names using advance indexing
names[index_max, np.arange(names.shape[1])]

# array(['foo', 'spam', 'bar'], 
#       dtype='<U4')

edited Mar 10, 2017 at 23:48

answered Mar 10, 2017 at 22:53

akuiper

216k33 gold badges362 silver badges379 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

GeoMonkey Over a year ago

What if A, B, C and D are not so straightforward and contain lots of different words that I won't be able to enter manually as "ABCD"? In my actual data its plant species names

akuiper Over a year ago

Your example is a little vague to me. Do you mean you really have four lists where each list contains only one unique value?

GeoMonkey Over a year ago

Apologies, my error. Yes, I'll edit my question to reflect this

Collectives™ on Stack Overflow

Index multiple arrays

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related