MATLAB Answers

## HOW Calculate the distance of points form one center in 2-D space and display output in a distance matrix?

Asked by sona nil

### sona nil (view profile)

on 7 Sep 2019
Latest activity Commented on by sona nil

### sona nil (view profile)

on 8 Sep 2019
Accepted Answer by Rik

### Rik (view profile)

Hello all, I am new to Matlab and I am having quite a bit of difficulty understanding how to calculate the distance between points with equal distances in a 2-D space. I need to develop a function to create an array of 5 points and find the distances between them. The output of the function will be a distance matrix wich size is 5*5.
I know about "pdist2" but I canot develop this for my problem. I need this matrix(look at the picture). The out put matrix should be a 5*5 Symmetric matrix and the elemet of diagonal should be zero( because the distance of every point with each self will be zero)

sona nil

### sona nil (view profile)

on 8 Sep 2019
Did you see my other comment?
Because one of the assumptions of my question is that the distance is equal. look at this picture.
Rik

### Rik (view profile)

on 8 Sep 2019
That is a totally different question. You never said you needed to generate the coordinates as well. Our functions work for any set of points. If you want to randomly generate a point cloud with exactly the shape you describe, you can do this:
x_center=rand;
y_center=rand;
h=randi([2 5]);
X=x_center+[0 0 h 0 -h];
Y=y_center+[0 h 0 -h 0];
Or this:
x_center=rand;
y_center=rand;
h=randi([2 5]);
phi_offset=2*pi*rand;
phi=[0.5 0 -0.5 -1]*pi;
X=x_center+[0 h*cos(phi_offset+phi)];
Y=y_center+[0 h*sin(phi_offset+phi)];
sona nil

### sona nil (view profile)

on 8 Sep 2019
fine, I think this is what I need. Thank you Rik.

Sign in to comment.

R2015a

## 2 Answers

Answer by Rik

on 7 Sep 2019
Edited by Rik

### Rik (view profile)

on 7 Sep 2019
Accepted Answer

This should do the trick:
X=2*(rand(5,5)-0.5);
Y=2*(rand(5,5)-0.5);
x=0.3;y=0.3;
d=point2pointcloud(X,Y,x,y)
function d=point2pointcloud(X,Y,x,y)
%X and Y are the coordinates of the point cloud
%x and y are the coordinates of the point
if ~isequal(size(X),size(Y)) || ~strcmp(class(X),class(Y))
error('X and Y must be same size and type')
end
if numel(x)~=1 || numel(y)~=1 || ~strcmp(class(x),class(y))
error('x and y must be scalar and same type')
end
if ~strcmp(class(x),class(X))
error('all inputs must be same type')
end
d=sqrt((X-x).^2+(Y-y).^2);
end
Edit:
To make the function easier to find, I'll paste it here. This function seems to return the required matrix.
function d=pointcloud_dist(X,Y)
%X and Y are the coordinates of the point cloud
if ~isequal(size(X),size(Y)) || ~strcmp(class(X),class(Y))
error('X and Y must be same size and type')
end
[ind1,ind2]=ndgrid(1:numel(X));
X1=X(ind1);X2=X(ind2);
Y1=Y(ind1);Y2=Y(ind2);
d=sqrt((X1-X2).^2+(Y1-Y2).^2);
end

sona nil

### sona nil (view profile)

on 7 Sep 2019
yes you are right,you suppose that we dont consider center. your code is worked . your code obtains a Symmetric matrix and the elemet of diagonal is zero(my target). but the matrix size is 25*25.
but I need 5*5 matrix(the first row shows the distance from point 1 from others(other four points), the second row shows the distance from point 2 to others,.....) so we will have 5 rows and five columns.
Rik

### Rik (view profile)

on 7 Sep 2019
My input was 25 points, so if you don't provide my 25 randomly generated points but put in your 5 points instead, you will get a 5*5 matrix.
sona nil

### sona nil (view profile)

on 8 Sep 2019
Thanks rik. I learned applicable information from you.
I did your suggestion about the release of matlab.
you answered all of my question, but one thing is wrong.
If you saw the picture of the points. They have equal distances(distance is contsant) but when we use "rand " it generates some random number with unequal distances.
Is there a code in matlab that generates random number with equal distance in (0 1)?
for example (0 , 0.1) , (0.1 , 0.1) , (0.2 , 0.1),(0.1 , 0),(0.1 , 0.2)
x=[0 0.1 0.2]
y=[0 0.1 0.2]

Sign in to comment.

### Andrei Bobrov (view profile)

Answer by Andrei Bobrov

### Andrei Bobrov (view profile)

on 7 Sep 2019
Edited by Andrei Bobrov

### Andrei Bobrov (view profile)

on 8 Sep 2019

X = rand(5,1)
Y = rand(5,1)
D = squareform(pdist([X,Y]))
or
XY = [X, Y];
D = sqrt(squeeze(sum((XY - permute(XY,[3,2,1])).^2,2)))
%For old MATLAB
D = sqrt(squeeze(sum(bsxfun(@minus,XY,permute(XY,[3,2,1])).^2,2)))

sona nil

### sona nil (view profile)

on 8 Sep 2019
Thanks andrei for your useful and brief answer.
Is there a code in matlab that generates random number with equal distance in (0 1)?
for example (0 , 0.1) , (0.1 , 0.1) , (0.2 , 0.1),(0.1 , 0),(0.1 , 0.2)
x=[0 0.1 0.2]
y=[0 0.1 0.2]

Sign in to comment.