How to record the first value every time it occurs?

I cant figure out how to record n when the value in the matrix dist is first greater than 10 in each column. I should end up with 50 values for my out_of_bounds variable but I currently end up with only one for the first column of dist.

close all; clear all; clc; format compact;

% m= particle
% dr = 1 %(delta)r = length of stride
% n = 1:1000 %n = number of steps
dr= 1;

%hold on             %Allow for multiple graphs to be drawn

%Zeroes function is an empty vector that stores data
m=zeros(1000,50);
y=zeros(1000,50);

out_of_bounds=zeros(1,50); %since there is only 1 row it should only 
    %record the fist time it crosses the boundary

dist=zeros(1000,50);

for n = 2:50000;                 %1000 steps, 50 runs
    a = 360 * rand;              % random angle in [0,360]

      %Each step taken at random from last position
      m(n) = m(n-1) + dr * cosd(a); 
      y(n) = y(n-1) + dr * sind(a);
      m(1,2:50) = 0;                 %Each particle starts at 0,0
      y(1,2:50) = 0;

      dist(n)=sqrt(((m(n)).^2)+((y(n)).^2)); %r is the radius equation

      B=10; %Boundary is 10

      if dist(n) >= B  & out_of_bounds==0 %when the particle is greater than the distance of 10
      %then it should somehow record the number of steps into the out of
      %bounds array

      out_of_bounds =n ;
      end

end

Answer 1

[Star Strider]

I don’t completely understand what you want to do. If you want to loop through N particles for M iterations each, use two nested for loops, the outer for each particle and the inner for the random walk of each particle.

Then:

B = 10;
for k1 = 1:N
    count = 0;
    for k2 = 1:M
        ... CODE ...
        m(k1, k2) = ...
        y(k1, k2) = ...
        dist = hypot(m(k1,k2), y(k1,k2));
        if dist <= B
            count = count+1;
            out_of_bounds(k1,k2) = count;
        end
   end
end

This is simply sketching out the code, but it may be the only way to do what you want. It isn’t possible to do all particles simultaneously, you have to iterate through each particle individually.

When the loop completes, I would also save all the variables of interest that it generated in a .mat file so you can use them again without having to re-run your code.

[Kathryn Jordan]

The code I have gives us 50 columns of a 1000 rows called dist. I want to be able to record the row number for the first time the output is greater than 10. For example if for column 1 in the matrix dist hits 10 at 107 i want but column two hits 10 at 96 i want to record those numbers. The columns are the amount of runs I am doing and the rows are the number of steps.

[Star Strider]

In that situation, change the if block to:

if dist >= B
    out_of_bounds(k1) = k2;
    break
end

That will break out of the ‘k2’ loop, stop the random walk for that particle, and go on to the next particle.

If you want to only record the first time the particle goes out-of-bounds — and no other times — and then continue with its random walk, this would work:

B = 10;
for k1 = 1:N
    flag = false;
    for k2 = 1:M
        ... CODE ...
        m(k1, k2) = ...
        y(k1, k2) = ...
        dist = hypot(m(k1,k2), y(k1,k2));
        if (dist >= B) & (flag == false)
            flag = true;
            out_of_bounds(k1,k2) = count;
        end
   end
end

This is untested but should work, so experiment with it in your code.

[Lilly Hoodenpyle]

What does the "..." mean in your version of the code?

[Star Strider]

The ‘...’ simply stands for code lines I didn’t want to type out. The ‘... CODE ...’ line stands for several lines of code. I’m sketching out the general idea of two nested loops.