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

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S.
S. on 24 Oct 2013
Edited: Randy Souza on 24 Oct 2013
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

Accepted Answer

Andrei Bobrov
Andrei Bobrov on 24 Oct 2013
Edited: Andrei Bobrov on 24 Oct 2013
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{:});
  3 Comments
S.
S. on 24 Oct 2013
thanks everyone for your help. Andrei's answer was the one that worked for my application.
S.

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More Answers (2)

Michael
Michael on 24 Oct 2013
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
  1 Comment
S.
S. on 24 Oct 2013
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

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Simon
Simon on 24 Oct 2013
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.
  1 Comment
Simon
Simon on 24 Oct 2013
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));

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