MATLAB: Assigning a probability to a loop

probability event

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.

Best Answer

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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