MATLAB: How to find 3 maximum element in array and their indices without sorting

Question

Hi everybody

I have an array like this(its data type is double):

a = {1,4,2,7,1,2,9,11,4,3,1,7,12,3,5,6,2,1}

I want to return 3 or more maximum element with their indices without any changes in the original form of "a" (like sorting it)

thanks in advance

Answer 1

You can call max three times, each time removing the element you found, but see my comment above, you are effectively sorting the set

a = [1,4,2,7,1,2,9,11,4,3,1,7,12,3,5,6,2,1];
elemrequired = 3;
sorteda = zeros(1, elemrequired)
sortedindices = zeros(1, elemrequired)
for iter = 1:elemrequired
  [sorteda(iter), sortedindices(iter)] = max(a);
  a(sortedindices(iter)) = nan;
end

You are effectively emulating the first elemrequired steps of an inefficient sort algorithm (selection sort).

This is of course simpler, and probably faster:

a = [1,4,2,7,1,2,9,11,4,3,1,7,12,3,5,6,2,1];
elemrequired = 3;
[sorteda, sortedindices] = sort(a, 'descend');
sorteda = sorteda(1:elemrequired)
sortedindices =  sortedindices(1:elemrequired)