How to run a while loop 1000 times

Here is my matlab code, I try to run this while loop 1000 times, but i have no idea how to do it.

Could you give me some ideas abouut it.

T = 400;
x = 150; %initial amount of money.
xx = [150]; tt = [0];
tstart=0;
ev_list = inf*ones(3,2);         %record time, type
ev_list(1,:) = [7 + 3*rand, 1];  %schedule type 1 event
ev_list(2,:) = [25 + 10*rand,2]; %schedule type 2 event
ev_list(3,:) = [-log(rand),3];
ev_list = sortrows(ev_list,1);
for i = 1:1000
    t=tstart
    while t < T 
        t = ev_list(1,1);
        ev_type = ev_list(1,2);
        switch ev_type
            case 1
                x = x + 16*-log(rand);
                ev_list(1,:) = [7 + 3*rand + t, 1];
            case 2
                x = x + 100;
                ev_list(1,:) = [25 + 10*rand + t, 2];
            case 3
                x = x - (5 + randn);
                ev_list(1,:) = [-log(rand) + t, 3];
        end
        ev_list = sortrows(ev_list,1); % sort event list
        xx = [xx,x];
        tt = [tt,t]; 
    end
    XX{i} = xx;
    TT{i} = tt;
    xx = [];
    tt = [];   % reset t to a value < T        
end

Answer 1

[Torsten]

iter = 0;
while t < T && iter < 1000
    iter = iter + 1;
    ...
end

[Yifei Yang]

Thank you so much! But it seems no change for my result. It will produce around 400 xx and tt from my code. For every run, the value of xx and tt are different. So I try to take the while loop 1000 times to get like 400*1000 answers.

[Torsten]

for i = 1:1000
  while t < T
        ...
  end
  XX{i} = xx;
  TT{i} = tt;
  xx = [];
  tt = [];
  t = tstart;   % reset t to a value < T     
end

[Yifei Yang]

Thank you! But what is the tstart? It does not work here.

[Torsten]

The following code should work:

for i = 1:1000
  Ev_list = ev_list;
  t = Ev_list(1,1);
  while t < T
    t = Ev_list(1,1);
    Ev_type = Ev_list(1,2);
    switch Ev_type
      case 1
        x = x + 16*-log(rand);
        Ev_list(1,:) = [7 + 3*rand + t, 1];
      case 2
        x = x + 100;
        Ev_list(1,:) = [25 + 10*rand + t, 2];
      case 3
        x = x - (5 + randn);
        Ev_list(1,:) = [-log(rand) + t, 3];
     end
     Ev_list = sortrows(Ev_list,1); % sort event list
     xx = [xx,x];
     tt = [tt,t]; 
  end
  XX{i} = xx;
  TT{i} = tt;
  xx = [];
  tt = [];
end

[Walter Roberson]

In your original code you must have initialized t to something before the while loop. Perhaps you had something like

t = 0;

Change that slightly to

tstart = 0;   %or as appropriate
for i = 1:1000
  t = tstart;
  while t < T
        ...
  end
  XX{i} = xx;
  TT{i} = tt;
  xx = [];
  tt = [];
end

[Yifei Yang]

I tried again and i found only the first value of XX is a list and the others are numbers. But I think all elements in lists XX should be a list.

[Torsten]

I changed different things within the while loop. So you will have to copy the complete code.

Answer 2

[John D'Errico]

You use a for loop when you know how many iterations you will need. So I'm not sure why you want to use a while loop for a specific number of iterations. The point being, using a while loop with a counter takes more work. Why would you want to do more work? So...

for i = 1:1000
    stuff
end

As opposed to something qualitatively like this:

counter = 1;
while counter <= 1000
    stuff
    
    counter = counter + 1;
end

So not too much more work, but now you need to maintain the counter, to test for when you are done. You need to be careful about making sure the loop terminates properly. Should you have incremented the counter immediately after the while, or at the end? (Note that sometimes you might do it in one place or the other, and then you need to be careful, else your code might terminate at the 999th iteration, or go one too far, etc. This is the stuff bugs are made from.)

The foor loop is easier to visualize, to recognize that the loop will continue for exactly 1000 iterations, etc. The for loop makes for better code here. Good code is easy to read, easy to follow, easy to debug, easy to maintain.

Use a while loop mainly when you don't know when the loop will terminate.

[Yifei Yang]

Thank you so much! Since I am new for matlab and this is one request of my project. I have no idea to change the while loop because I am not good at it. The request is run the code 1000 times.