# How do I get a loop to calculate for each value of a formula, not just the last n of the input(N)?

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David Prendergast on 23 Nov 2020
Commented: Rik on 23 Nov 2020
Take a table T, where the columns are x and y coordinates, and each row represents a point. The length is calculated from the first two values, i.e (1,2) for each N.
Whenever I run this, it goes through the iteration for length, but it only uses length at j=5 to calculate force.
My question is how do I get the loop to calculate length, put that through the force formula, and then add to the force sum, then repeat for next iteration?
T = [1 2 ; 3 4 ; 5 6 ; 7 8 ; 9 10];
N=5;
force=0;
length=0;
for F = 1:(N-1)
for j=(F+1):N
length = sqrt((T(j,1)-T(1,1))^2+(T(j,2)-T(1,2))^2);
force = force + 2.2*((10/length)^3-(10/length)^2);
end
end
disp(force)

#### 1 Comment

Rik on 23 Nov 2020
Why did you try to delete your question? That is a very rude thing to do. If you want private consulting, hire a consultant. I have restored your edit from the Google cache, which unforturnately didn't contain the comments you posted.
Don't be a jerk. Leave your question as it is now so other people can benefit from the question and the answer.

Jon on 23 Nov 2020
Edited: Jon on 23 Nov 2020
You must provide an array to hold your output force. You keep overwriting it with each loop iteration
So, depending upon what you want
F(i,j) = ...
or
F(i) = ...

Jon on 23 Nov 2020
Also you should not use "length" as a variable name, length is the MATLAB function that gives array dimensions (largest array dimension). You will have problems in other parts of your code if you then try to use this function. It is also confusing to people who might look at your code and expect that length is a function not a variable name.
I think you could probably do this whole calcuation without loops (vectorize) it.
Jon on 23 Nov 2020
If I understand your calculation correctly you should be able to do it like this without loops at all
T = [1 2 ; 3 4 ; 5 6 ; 7 8 ; 9 10];
% get components of vector going from start point to each row
delta = T - T(1,:)
% calculate the magnitude of each vector
L = sqrt(delta(:,1).^2 + delta(:,2).^2) % note .^ does element by element
% calculate the force
force = 2.2*(10./L).^3 - (10 ./L).^2 % note ./ and .^ for element by element operation
% the first row will be NaN as the magnitude is zero in the denominator
% you can get rid of this row if you want
force = force(2:end)
Jon on 23 Nov 2020
Sorry, I didn't notice on my first look that you had force on the right hand side of the equation also.
If you could please further explain, what you are trying to calculate (maybe with an example of what you are expecting for a result for a particular case) I might be able to help further. I'm not clear now what might just be errors in how you are coding it and what are things your are really trying to do.

VBBV on 23 Nov 2020
Edited: VBBV on 23 Nov 2020
%if true
for j=1:N
You need to use above loop structure. You don't need one for loop

#### 1 Comment

VBBV on 23 Nov 2020
There is extra for loop with F = 1:N-1 which may not be necessary

R2020b

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