assigning a probability to a loop
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Hello,
I am making a random walk code, and I want to assign a probability to the direction chosen. I found a way to do this with IF loops, as in the code below, using the variable J:
x=2; %starting poition in X and Y
y=3;
v=50; %number of steps
X=[];
Y=[];
X(1)= x;
Y(1)= y;
for i=1:v
J=rand; % Random value to decide on direction.
if J<0.3 % Go North
X(i+1)=X(i)+1;
Y(i+1)=Y(i);
elseif (J>0.3) && (J<0.45) % Go NW
X(i+1)=X(i)+1;
Y(i+1)=Y(i)-1;
elseif (J>0.45) && (J<0.6) %Go NE
X(i+1)=X(i)+1;
Y(i+1)=Y(i)+1;
elseif (J>0.6) && (J<0.7) % Go East
X(i+1)=X(i);
Y(i+1)=Y(i)+1;
elseif (J>0.7) && (J<0.8) % Go West
X(i+1)=X(i);
Y(i+1)=Y(i)-1;
elseif (J>0.80) && (J<0.875) % Go SE
X(i+1)=X(i)-1;
Y(i+1)=Y(i)+1;
elseif (J>0.875) && (J<0.95) % Go SW
X(i+1)=X(i)-1;
Y(i+1)=Y(i)-1;
else %Go south
X(i+1)=X(i)-1;
Y(i+1)=Y(i);
end
plot (X, Y)
end
I am wondering if there is an easier way to assign a probability of an event happening. Ideally I'm looking for a way to just enter a %. e.g:
for i=1:v
In 30% of cases, do this: %North
X(i+1)=X(i)+1;
Y(i+1)=Y(i);
In 15% of cases, do this: % Go NW
X(i+1)=X(i)+1;
Y(i+1)=Y(i)-1;
and so on.
Thank you.
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Walter Roberson
2011-11-23
Create your probability vector with the per-state probabilities, and then cumsum() the vector to get the boundaries for the range rand() has to fall in.
If you want, you could combine that with a find() to determine the state number, and then switch on the state number rather than on the probabilities.
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