CODEWITHSUNDEEP
matlab
Federico
Federico

Reputation: 33

Matlab function works erratically, and some other times returns error

function [N] = rand_walk1(n)
j = zeros(1,n); %j will be the output vector
i=2;
for i= 2:n        %This 'for' loop will randomly add or subtract 1 to each entry of the vector j, starting from j(2).
   if rand >= 0.5 %This 'if' statement returns 1 or -1 with equal probability (it uses the uniform distribution with parameters [0,1]).
      v = 1;
   elseif rand <0.5
      v = -1;
   end
   j(i)=j(i-1)+v;
end

N = j;
end

This is my function, sometimes it works and some other times it returns the following error:

Error in rand_walk1 (line 10)
j(i)=j(i-1)+v;

Any ideas of why I'm randomly getting this error, while some other times the function works?

Upvotes: 0

Views: 39

Answers (1)

beaker
beaker

Reputation: 16791

Occasionally, sometimes, v is unassigned. Let's look at your random number generation:

if rand >= 0.5 
   v = 1;
elseif rand <0.5
   v = -1;
end

The first if sets v = 1 half of the time. The other half of the time, you want to set v = -1. But what you're doing is creating a new random number and checking that. If that new random number is also less than 0.5, you get a good result. If it's greater than 0.5, you'll just fall through and v will be unchanged. If this is your first time through the loop, there's nothing to change, so v will remain unassigned. That's when you get an error.

Instead, you want to do this:

if rand >= 0.5 
   v = 1;
else
   v = -1;
end

This way, if your result is < 0.5, you set v = -1 every time. It's got to be one or the other. You don't have to check again.

I'd like to note that one of the reasons this bug was a bit trickier to find is that you called the function rand without any parentheses after it. rand could be mistaken for a variable name while rand() is obviously a function call with no arguments. Not a big deal, and perfectly legal MATLAB, but every little bit helps when it comes to code clarity.

Upvotes: 3

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