How to add values to matrix in a loop given an extra condition.

I have a 101 x 2 matrix and i need to make n x 1 matrix where n = A(i,1)*A(i,2) and n has to be negative.

I.E : A = [1 -2

3 3

4 -5 ]

to A2 = [-2

-20]

heres my code at the moment:

A(:,1) = []
A(1:101,:) = []
A
i = 1;
H = [];
%length(A(:,1))
for i = 1: length(A(:,1))
    if A(i,1)*A(i,2) < 0
        H2=H(A(i,1)*A(i,2));
    else
        
    i=i+1;
    end
end
H2

[Jan]

What is the purpose of these lines:

A(:,1) = []
A(1:101,:) = []

This deletes elements.

The FOR loop cares for the loop counter i already. Then te initial i=0 and i=i+1 are useless. Simply omit them.

Answer 1

[Jan]

A = [1  -2; ...
     3   3; ...
     4   -5 ];

A2 = zeros(size(A, 1), 1);  % Pre-allocate maximum number of elements
iA2 = 0;                    % Index inside A2
for iA = 1:size(A, 1)       % Cleaner and faster than: length(A(:,1))
    num = A(iA, 1) * A(iA, 2);
    if  num < 0
       iA2     = iA2 + 1;
       A2(iA2) = num; 
   end
end
A2 = A2(1:iA2);             % Crop unused elements

The efficient matlab'ish way is:

A2 = A(:, 1) .* A(:, 2);
A2 = A2(A2 < 0);

or:

index = (A(:, 1) < 0) ~= (A(:, 2) < 0);
A2    = A(index, 1) .* A(index, 2)

[Stefan Juhanson]

Thanks, this works perfectly. The lines

A(:,1) = []

A(1:101,:) = []

were just to take wanted values from a bigger N x 2 matrix.