MATLAB: Determining Quadrants in a Matrix

matrixquadrants

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.

Best Answer

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.

Related Solutions

MATLAB: Color for points in scatter plot based on quadrants

You actually have several syntax errors in that script, but they're never being reached because of the way you've structured your if statements.

An if-statement evaluated on a vector only executes if all values in that vector are true. So, for example, when you write

if x < -128

it's the same as

if all(x < -128)

Because that statement is false, the rest of the line isn't evaluated (which is why you don't immediately get an error, since the rest of the statement isn't valid command). You should use any instead.

To get what you want, simply pass the third column as your color data, and set the colormap accordingly:

matrix = randn(50,2);
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;
if any(x < -128 | x > 128 | y < -128 | y > 128)
    error('Value exceeds limit');
end
scatter(x,y,[],matrix(:,3),'filled');
colormap([1 0 0; 0 0 1; 1 0 0; 0 0 1]);
set(gca, 'clim', [0.5 4.5]);
title('Variables and Their Quadrants')
xlabel('X Values')
ylabel('Y Values')

MATLAB: I keep getting a syntax error with the program??

in the If statement you need to use '==' Your first if statement would be

if x==0 & y==0 
 disp('This point is located at the origin.')

Your entire code

x=input('Enter value for x: '); 
y=input('Enter value for y: '); 
if x==0 & y==0 
    disp('This point is located at the origin.')
elseif x>0 & y==0 
    disp('This point is located on the x-axis.') 
elseif x==0 & y>0 
    disp('This point is located on the y-axis.') 
elseif x>0 & y>0 
    disp('This point is located in the first quadrant.') 
elseif x<0 & y>0 
    disp('This point is located in the second quadrant.') 
elseif x<0 & y<0 
    disp('This point is located in the third quadrant.') 
elseif x>0 & y<0 
    disp('This point is located in the fourth quadrant.') 
end

Best Answer

Related Solutions

MATLAB: Color for points in scatter plot based on quadrants

MATLAB: I keep getting a syntax error with the program??

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