Determining Quadrants in a Matrix

matrix=load('Data');
x=matrix(:,1);
y=matrix(:,2);
if x>0 & y>0
    matrix(:,3)=1
elseif x<0 & y>0
    matrix(:,3)=2
elseif x<0 & y<0
    matrix(:,3)=3
elseif x>0 & y<0
    matrix(:,3)=4
elseif x<-128 & y<-128 & x>128 & y>128
'Invalid Values'
end

With this code, I'm trying to put separate points into a matrix. However, my code doesn't do anything with the values as far as I have seen. I was trying to put the quadrant values into a third column so that I could code the script to count all points in each quadrant, so there is a reason for it. If you have any suggestions I would greatly appreciate it.

Answer 1

[Jan]

The if command evaluates the condition as a scalar. This is logical, because what could this mean:

if [true, false]

? Is this true or not? Therefore Matlab inserts a all() in the condition, if the user forgot this. So you have:

if all(x>0 & y>0)

And this is never true. What you want is:

index = (x>0 & y>0);
matrix(index, 3) = 1;
...

The last condition is magic:

elseif x<-128 & y<-128 & x>128 & y>128

You are looking for numbers which are smaller than -128 and larger than 128? You want an or.

[TS]

Thank you!

Answer 2

[Stephen23]

Actually you do not need these if statements at all: just use logical indexing directly, like this:

matrix = load('Data');
x = matrix(:,1);
y = matrix(:,2);
matrix(x>0 & y>0,3) = 1;
matrix(x<0 & y>0,3) = 2;
matrix(x<0 & y<0,3) = 3;
matrix(x>0 & y<0,3) = 4;

[Yong Cho]

This is ausome answer I ever read!