Finding indexes of maximum values of an array

Question

How do I find the index of the 2 maximum values of a 1D array in MATLAB? Mine is an array with a list of different scores, and I want to print the 2 highest scores.

Answer 1

I'll go for an O(k*n) solution, where k is the number of maximum values you're looking for, rather than O(n log n):

x = [3 2 5 4 7 3 2 6 4];
y = x; %// make a copy of x because we're going to modify it
[~, m(1)] = max(y);
y(m(1)) = -Inf;
[~, m(2)] = max(y);

m =

   5   8

This is only practical if k is less than log n. In fact, if k>=3 I would put it in a loops, which may offend the sensibilities of some. ;)

Answer 2

You can use sort, as @LuisMendo suggested:

[B,I] = sort(array,'descend');

This gives you the sorted version of your array in the variable B and the indexes of the original position in I sorted from highest to lowest. Thus, B(1:2) gives you the highest two values and I(1:2) gives you their indices in your array.

Answer 3

To get the indices of the two largest elements: use the second output of sort to get the sorted indices, and then pick the last two:

x = [3 2 5 4 7 3 2 6 4];
[~, ind] = sort(x);
result = ind(end-1:end);

In this case,

result =
     8     5