If loop issue for assigning new variable

Hello everyone,

I got an incomprehensible issue. In the code below, I want to make a new matrix with maximum on each row but keep remain the same index as the original matrix. However, when <j> runs, I cannot assign its value to variable <mark>. The value of <mark> is unchangable, it still remains 1.

Please help me, I don't understand what happening.

Thank you in advance.

close all; clc; clear
[x,y] = meshgrid(-2:2,-2:2)

[xlength,ylength] = size(x);
r = zeros(xlength,ylength)
s = zeros(xlength,ylength)
Pnpos = zeros(xlength,ylength)
Pnpo = 3*x.*x.*y - x.*y + 2*y + x - 7
maxRow = 0;
for i = 1:xlength
    maxRow = max(Pnpo(i,:))
    mark = 1;
    for j = 1:ylength
        if maxRow < Pnpo(i,j)
            maxRow = Pnpo(i,j);
            mark = j
        end
    end
    Pnpos(i,mark) = maxRow;
end

[Walter Roberson]

maxRow = max(Pnpo(i,:))

OK it starts as the largest value in the row

        if maxRow < Pnpo(i,j)

maxRow is the largest value in the row. It cannot be less than any element in the row, only equal to one or more copies of the maximum value. (If the row is all nan then it would never be equal so do not count on equal). So < is never true so the statements inside the if are never done.

[Nuec Tah]

@Walter Roberson: excuse me, I think the condition is not really a problem because maxRow has value for each loop. So, after the comparison between maxRow and Pnpo(i,j) is true condition, the value of maxRow will be established in the first task

maxRow = Pnpo(i,j);

but the next task isn't executed

mark = j

and that is the point.

[Ameer Hamza]

@Nuec, since maxRow is already the maximum value in the current row, therefore the condition

maxRow < Pnpo(i,j)

never becomes true, and the execution never reaches

mark = j

[Walter Roberson]

If you are looking for the index of the maximum then use the two-output form of max()

[Nuec Tah]

@Walter Roberson: Thank you. That is better idea for this situation.

Answer 1

[Ameer Hamza]

You can solve the issue mentioned by @Walter in the comment by initializing the maxRow with the lowest possible value. Change

maxRow = max(Pnpo(i,:))

to

maxRow = -inf;

and you code should run fine.

You can also avoid the for loop altogether by shown below

close all; clc; clear
[x,y] = meshgrid(-2:2,-2:2);
[xlength,ylength] = size(x);
r = zeros(xlength,ylength);
s = zeros(xlength,ylength);
Pnpos = zeros(xlength,ylength);
Pnpo = 3*x.*x.*y - x.*y + 2*y + x - 7;

[~, subs] = max(Pnpo, [], 2);
index = sub2ind(size(Pnpo), 1:xlength, subs');
Pnpos(index) = Pnpo(index);

[Nuec Tah]

@Ameer Hamza: thank you, I got it.