Hello, this is the Question:
Suppose you want to know how often a single source of fungus will spread more than one foot from a fallen tree. You will be using a computer simulation to answer the question.

i found this code using the m file ranwalk2d.m:

2-dimensional random walk
ranwalk2d.m

Plots the points (u(k),v(k)), k=1:n, in the (u,v)-plane, where (u(k)) and (v(k)) are one-dimensional random walks. The sample code plots four trajectories of the same walk, four different colors.

n=100000;
colorstr=['b' 'r' 'g' 'y'];
for k=1:4
z=2.*(rand(2,n)<0.5)-1;
x=[zeros(1,2); cumsum(z')];
col=colorstr(k);
plot(x(:,1),x(:,2),col);
hold on
end
grid



The problem is i don't want the graph to go below 0 on the y axis. In other words pertaining to the problem, the fungus cant go back to its starting point.

I'm not a computer science person so i'm not that knowledgeable. Can Someone please guide me on how to do random walks on matlab?

Change this line
plot(x(:,1),x(:,2),col);

to this
plot(x(:,1), abs(x(:,2)), col);

that will eliminate the negative values.

I really don't know what you want to do, but the function 'abs'
gets the absolute value of any number. If you have negative values, they will become positive on the same plot.

i guess I'm trying to figure out how far it will end up from its initial starting point using the random walk process or what have you. So it can go down, but it cannot go below 0. I'm really trying to learn this stuff but ive only used this program to solve linear algebra and differential equations so its difficult for me.

These are some good resources that could help you...
(obviously, the best resource is a good book, not a site)

http://www.mathworks.com/products/matlab/demos.html

http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-094-introduction-to-matlab-january-iap-2010/

http://matrixlab-examples.com/matlab-books.html