Matlab，找到最小值的索引，条件是它必须是负数

Question

In a given array, I need to find the index of the minimum value in an array, but only if it is negative.

For example : [1, 2, 3, 4] would return no indices

and [1, 4, -7, -2] would return 3

I was thinking that it must be simple with the find() command, but I couldn't figure out how to use it for this specific situation.

Answer 1

Suppose the input matrix is A , this should do the trick:

find(A==min(A) & A<0)

For example:

>> A = [1, 2, 3, 4];
>> B = [1, 4, -7, -2];
>> find(A==min(A) & A<0)

ans =

   Empty matrix: 1-by-0

>> find(B==min(B) & B<0)

ans =

     3

Answer 2

Sometimes, throwing everything into one complicated vector expression isn't optimal.

In this instance, I expect it to be much faster to avoid a call to find .

function [i] = most_negative_index(x)
   [mn, i] = min(x);
   if mn >= 0
       i = [];
   end
end