How do I repeat a for loop for all columns?

I have a for loop that runs through all of my 1,000 rows. The next step is to get this code to run exactly the same way but for each of the 10,000 columns. I don't know if there is a simple way to do this--I tried adding "for j=1:n_trials" but that didn't seem to work. Any help is appreciated. Thanks!

n_steps = 1000 n_trials = 10,000 x_pos = zeros(n_steps,n_trials);

    for i=2:n_steps
    direction=rand;
        if direction < 0.3
           x_pos(i) = x_pos(i-1)+1;
        elseif (0.3 < direction) && (direction < 0.5)
               x_pos(i)=x_pos(i-1)-1;
        else
            x_pos(i)=x_pos(i-1);
        end
    end

Answer 1

[Akbar]

Try this. You should use two arguments (i,j) i=row number. j=column number. And one more "for" Loop, for the columns.

 n_steps = 1000;
 n_trials = 10000;
 x_pos = zeros(n_steps,n_trials);

    for i=2:n_steps
        for j = 1:n_trials
    direction=rand;
        if direction < 0.3
           x_pos(i,j) = x_pos(i-1,j)+1;
        elseif (0.3 < direction) && (direction < 0.5)
               x_pos(i,j)=x_pos(i-1,j)-1;
        else
            x_pos(i,j)=x_pos(i-1,j);
        end
        end
    end

Answer 2

[Jan]

With a vectorized approach:

n_steps  = 1000;
n_trials = 10000;
x_pos    = zeros(n_steps, n_trials);

for k = 2:n_steps
  direction = rand(1, n_trials);
  step      = zeros(1, n_trials);
  step(direction <= 0.3) = 1;
  step((0.3 < direction) && (direction < 0.5)) = -1;
  x_pos(k, :) = x_pos(k-1, :) + step;
end

I've modified one condition from < 0.3 to <= 0.3, such that direction==0.3 is caught also.