A function called small_elements that takes as input an array named X that is a matrix or a vector. Could you help me to understand the meaning?

Write a function called small_elements that takes as input an array named X that is a matrix or a vector. The function identifies those elements of X that are smaller than the product of their two indexes. For example, if the element X(2,3) is 5, then that element would be identified because 5 is smaller than 2 * 3. The output of the function gives the indexes of such elements found in column-major order. It is a matrix with exactly two columns. The first column contains the row indexes, while the second column contains the corresponding column indexes. For example, the statement indexes = small_elements([1 1; 0 4; 6 5], will make indexes equal to [2 1; 1 2; 3 2]. If no such element exists, the function returns an empty array.

I'm not native english speaker so I can't understand very well. My question is..why the function returns a "3"?

Thank you for your help!

Answer 1

[Walter Roberson]

For a 3 x 2 input named x, the logic is like

if x(1,1) < 1*1
  disp([1, 1])
end
if x(1,2) < 1*2
  disp([1, 2])
end
if x(2,1) < 2*1
  disp([2, 1])
end
if x(2,2) < 2*2
  disp([2, 2])
end
if x(3, 1) < 3*1
  disp([3, 1])
end
if x(3, 2) < 3*2
  disp([3, 2])
end

Answer 2

[RAMAKANT SHAKYA]

function ind=small_elements(v)
[m,n]=size(v);
j=0;
ind=[];
for c=1:n
    for r=1:m
        if (r*c) > v(r,c)
            j=j+1;
            s=[r c];
            ind=vertcat(ind,s); %adding matrix vertically
        end
    end
end
end

[Stephen23]

Vectorized code is much simpler and most likely more efficient:

>> X = [1,1;0,4;6,5]
X =
   1   1
   0   4
   6   5
>> S = size(X);
>> [R,C] = find(X<((1:S(1)).' .* (1:S(2))));
>> Z = [R,C]
Z =
   2   1
   1   2
   3   2