How do I write a for loop to calculate the difference between two points?

Here's what I have so far:

% Write a program using a for loop to get user input for a nX2 array, then find the difference between point 1 and the rest.

n=input('how many points do you have? ');

A=zeros (n,2);

for row=1:n
   for column=1:2
        usernumber=input('enter a number: ');
        A(row, column)=usernumber;
    end
end

A

for row=1:length(A(n,2))
    Dist=sqrt((A(n,1)-A(n+1,1))^2)+((A(n+1,2)-A(n,2))^2)
end

Dist

I'm not sure how to find the differences through the loop and then store them

Answer 1

[Andrei Bobrov]

Dist = zeros(size(A,1),1);
for jj = 2:numel(Dist)
    Dist(jj) = sqrt(sum((A(jj,:) - A(1,:)).^2,2));
end

or

Dist = sqrt(bsxfun(@minus,A,A(1,:)).^2*[1;1]);

or

A1 = num2cell(bsxfun(@minus,A,A(1,:)),1);
Dist = hypot(A1{:});

[Simon]

Hi!

There is a typo in " - A(1,jj)", you should write " - A(1,:)"

[Andrei Bobrov]

Thank you Simon! I corrected.

[S.]

thanks everyone for your help. Andrei's answer was the one that worked for my application.

S.

Answer 2

[Michael]

Try using cell arrays by using something like:

for row =1:n
    for col =1:2
        dist{row,col} = ... %pythogrean thm
    end
end 
% Define, by hand, as it would be zero (I think)
dist{1,1} = 1;

Good Luck

[S.]

so my distance formula:

d=sqrt((x2-x1)^2+(y2-y1)^2)

what would I call x2,x1, y2,y1? maybe row(n) and column(x)?

or would the for loop take care of the rows and columns?

thanks

Answer 3

[Simon]

Hi!

You store your entered user input in a matrix, say N. As you wrote N is of dimension n*2.

The distance of a point from the first one is like you wrote

d=sqrt((x2-x1)^2+(y2-y1)^2)

You can write a loop to calculate this

for p = 1:n
  d(p) = sqrt((N(n,1)-N(1,1))^2+(N(n,2)-N(1,2))^2)
end

This gives you the distance from each point to the first one, with "d(0) = 0" of course.

[Simon]

One comment to my solution: try to avoid loops! Matlab is best used with matrix calculations. Rewrite the loop like this:

% create matrix as big as N, with only the first point in all lines
Nfirst = ones(n,1) * N(1,:);

% create the difference (in x and y) between all point to the first one
Ndiff = N - Nfirst;

% take the square of each difference (this is x^2 and y^2)
Nsquare = Ndiff.^2;

% sum along dimension 2 (this is x^2+y^2)
Nsum = sum(Nsquare, 2);

% take the square root
d = sqrt(Nsum)

Or write it in one row

d =  sqrt(sum((N-(ones(n,1) * N(1,:))).^2, 2));