分别查看矩阵中的每一行（Matlab）

Question

I have a matrix in Matlab(2012) with 3 columns and X number of rows, X is defined by the user, so varies each time. For this example though I will use a fixed 5x3 matrix.

So I would like to perform an iterative function on each row within the matrix, while the value in the third column is below a certain value. Then store the new values within the same matrix, so overwrite the original values.

The code below is a simplified version of the problem.

M=[-2 -5 -3 -2 -4]; %Vector containing random values

Vf_X=M+1; %Defining the first column of the matrix
Vf_Y=M+2; %Defining the secound column of the matrix
Vf_Z=M; %Defining the third column of the matrix

Vf=[Vf_X',Vf_Y',Vf_Z']; %Creating the matrix

while Vf(:,3)<0 
Vf=Vf+1;
end
disp(Vf)

The result I get is

 1     2     0
-2    -1    -3
 0     1    -1
 1     2     0
-1     0    -2

Ideally I would like to get this result instead

 1     2     0
 1     2     0
 1     2     0
 1     2     0
 1     2     0

The while will not start if any value is above zero to begin with and stops as soon as one value goes above zero.

I hope this makes sense and I have supplied enough information

Thank you for your time and help.

Answer 1

Your current problem is that you stop iterating the very moment any of the values in the third row break the condition. Correct me if I'm wrong, but what I think you want is to continue doing iterations on the remaining rows, until the conditions are broken by all third columns.

You could do that like this:

inds = true(size(Vf,1),1);
while any(inds)
    Vf(inds,:) = Vf(inds,:)+1;
    inds = Vf(:,3) < 0;
end

Of course, for the simple addition you provide, there is a better and faster way:

inds = Vf(:,3)<0; 
Vf(inds,:) = bsxfun(@minus, Vf(inds,:), Vf(inds,3));

But for general functions, the while above will do the trick.