How to find the index of n largest elements in a list or np.array, Python

Question

Is there a built-in function or a very simple way of finding the index of n largest elements in a list or a numpy array?

K = [1,2,2,4,5,5,6,10]

Find the index of the largest 5 elements?

I count the duplicates more than once, and the output should be a list of the indices of those largest numbers

Answer 1

Consider the following code,

 N=5
 K = [1,10,2,4,5,5,6,2]
 #store list in tmp to retrieve index
 tmp=list(K)
 #sort list so that largest elements are on the far right
 K.sort()
 #To get the 5 largest elements
 print K[-N:]
 #To get the 5th largest element
 print K[-N]
 #get index of the 5th largest element
 print tmp.index(K[-N])

If you wish to ignore duplicates, then use set() as follows,

 N=5
 K = [1,10,2,4,5,5,6,2]
 #store list in tmp to retrieve index
 tmp=list(K)
 #sort list so that largest elements are on the far right
 K.sort()
 #Putting the list to a set removes duplicates
 K=set(K)
 #change K back to list since set does not support indexing
 K=list(K)
 #To get the 5 largest elements
 print K[-N:]
 #To get the 5th largest element
 print K[-N]
 #get index of the 5th largest element
 print tmp.index(K[-N])

Hopefully one of them covers your question :)

Answer 2

This should work:

K = [1,2,2,4,5,5,6,10]
num = 5
print 'K %s.' % (sorted(K, reverse=True)[:num])

Answer 3

import headq

Then use function nlargest()

Answer 4

For efficiency, numpy partition is much more efficient for large array. There is no need to sort fully.

"All elements smaller than the k-th element are moved before this element and all equal or greater are moved behind it. The ordering of the elements in the two partitions is undefined."