vector is not overwritten after certain interation during loop

Hi guys,

I tried to write a small function to perform a random walk.

I've struggled with a strict vector formulation, so I created some mixture.

However, it works until the an certain interation loop, after that, the step directions (here stpx and stpy) are not overwritten whereas this is the case during the first interations.

The effect appears randomly between the 5th and 10th interation step.

Although k is changing, stpx and stpy are not.

"num" is my matrix with random numbers between 1 and 4

k in the loop takes for each iteration values from "num" and generates a new 5x1 vector for each interation

the idea was that stpx/stpy is computed each time again using the valid k-vector for the interation step.

function [x,y,num]=random_walk(n,x0,y0)

num=randi([1 4],5,n);

x(5,1)=x0;

y(5,1)=y0;

stpx(1)=-1;

stpx(2)=0;

stpx(3)=0;

stpx(4)=1;

stpy(1)=0;

stpy(2)=1;

stpy(3)=-1;

stpy(4)=0;

stpx_0=0;

stpy_0=0;

x(5,n+1)=0;

y(5,n+1)=0;

k0=0;

for a=1:n

k=k0+num(:,a);

stpx= stpx_0+[stpx(k(1,1));stpx(k(2,1));stpx(k(3,1));stpx(k(4,1));stpx(k(5,1))];

stpy= stpy_0+[stpy(k(1,1));stpy(k(2,1));stpy(k(3,1));stpy(k(4,1));stpy(k(5,1))];

x(:,a+1)=x(:,a)+stpx;

y(:,a+1)=y(:,a)+stpy;

end

end

Is this an issue because stpx and stpy have no indices?

Thank you all for the your help with this issue.

Kind regards, Dennis

Answer 1

[Dennis Trapp]

update: closed matlab, restarted laptop, now it works

...classic trouble shooting :-D

anyway, thanks for the support (great community here) :)

[KALYAN ACHARJYA]

@Dennis

Glad to know that you resolved the issue. We're so happy you're here!

Answer 2

[KALYAN ACHARJYA]

for a=1:n
    k=k0+num(:,a);
    stpx(a)= stpx_0+[stpx(k(1,1));stpx(k(2,1));stpx(k(3,1));stpx(k(4,1));stpx(k(5,1))];
    stpy(a)= stpy_0+[stpy(k(1,1));stpy(k(2,1));stpy(k(3,1));stpy(k(4,1));stpy(k(5,1))];
    x(:,a+1)=x(:,a)+stpx;
    y(:,a+1)=y(:,a)+stpy;
end

Please note this iteration replace the stpx and stpy value ((Initializations)) until 1 to 4

[Dennis Trapp]

Hi Kalyan,

thanks for the fast reply.

num is a 5xn matrix with values between 1 and 4

k calls for each iteration step 1:n (n=input, e.g. 10) the n-th colum from num

k is always a 5x1 vector with values from 1-4

this works fine within the loop (I checked it with section run) until the n-th iteration is reached