Creating a counter in matlab.

Wed, 15 Feb 2012 20:21:01 -0700

Hello. what im trying to do is create a random walk simulation 1000 times where i am tracking the movement of an object in 48 steps. i figured out what i need to do, but the last thing im trying to do is to count how many times the walk ends at a number where y>12. So for example if the walk ends at y=13, it adds 1 to the counter, but if it ends on y=8, it will not add 1 to the counter.
The reason im doing this is so i can find a percentage of how many times the object ends at y>12 which i want to do by finding 'number of times object went over twelve divided by 1000
(ans=counter/1000). heres my code:

>> %n= # of fungus sources
>> %t= # of time steps (inch)
>> %N= # of simulation runs
>> %alfa= random variables for angle
>> %d= random variable for distance
>> %M1= x coordinate of a point
>> %M2= y coordinate at a point
>> %maxy= max y value at each step
>> %count= counter that stores # of times maxy is > 12 inches
>> n=1;
>> t=48;
>> N=1000;
>> count=0;
>> for n=1:N;
m1(1,1:n)=1:n;
m2(1,1:n)=(1:n)*0;
maxy=0;
for i=1:t;
alfa=rand(1,n)*pi;
d=randn(1,n);
m1(i+1,1:n)=m1(i,1:n)+(d.*cos(alfa));
m2(i+1,1:n)=m2(i,1:n)+(d.*sin(alfa));
if m2(i+1,1:n)<0;
m2(i+1,1:n)=-m2(i+1,1:n);
end;
end;
maxy=max(maxy,m2(i+1,1:n));

end;
>> if maxy>12
count=count+1
end;
>> ans=count/N

ans =

0
....
Looking at my workspace i see there are times when y>12 but my counter is not recording it, so i keep getting an answer of 0. Any suggestions on how to get my counter working? Much appreciated.

Re: Creating a counter in matlab.

Thu, 16 Feb 2012 00:40:12 -0700

You have to index maxy, something like this:
maxy(n)=max(maxy(n-1),m2(i+1,1:n));
then out of the for loop
pos=find(maxy>12)
finds the positions of the elements >12
then
values=maxy(pos)
finds the values of the elements >12

Re: Creating a counter in matlab.

Thu, 16 Feb 2012 07:47:20 -0700

as soon as i replace the line to 'maxy(n)=max(maxy(n-1),m2(i+1,1:n))' it gives me this error:'??? Attempted to access maxy(0); index
must be a positive integer or logical.'
'

Re: Creating a counter in matlab.

Thu, 16 Feb 2012 23:52:38 -0700

Yes, you have to shift the indices. Because when n=1, n-1=0 and maxy(0) doesn't exist.

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Creating a counter in matlab.

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