Not overwriting array in if-elseif loop

Hi,

I have the following code

        for k = 1:length(yarray)-1
            if yarray(k+1) <= A & yarray(k) >= A
        M = [yarray(k) yarray(k+1) k k+1]
            elseif yarray(k+1) >= A & yarray(k) <= A
        M = [yarray(k) yarray(k+1) k k+1]
            end
        end

Where yarray is an array of y values in a plot, and A is the y value that I want to find corresponding x values to. I am expecting the if and elseif conditions to be satisfied 4 times, resulting in 4 M arrays. However, each time this is satisfied, M is overwritten. How can I come up with 4 different arrays which I can recall later?Let me know if this needs more clarification. Thanks!

Answer 1

[Oleg Komarov]

Preallocate M before the loop and index it inside the loop:

M = zeros(length(yarray)-1,4);

for...

M(k,:) = 

end

Answer 2

[Jan]

y_le_A = (yarray <= A);
y_ge_A = (yarray >= A);
index  = or(and(y_le_A(2:end), y_ge_A(1:end-1)), ...
            and(y_ge_A(2:end), y_le_A(1:end-1)));
k      = find(index);
M      = transpose([yarray(k); yarray(k+1); k; k+1]);

The last line needs yarray to be a row vector.

I have the impression, that this can be simplified using a DIFF trick.