How to change a value in an array when a condition is met?

Hi, I have the following array:

Velocity = [x, y, velocityx, velocityy]
Velocity =

     2     9    -1    -1
     2     3    -1     1
    10     8     1    -1
     6     3     0    -1
     7     4    -1     0
     7     4     0    -1
     9     3     0     0  %This is the problem part
     7     3     0    -1
     7     8     1     1
    10     7     0    -1

I want to change the 0 in the last column by a 1 or a -1 when there is a 0 in the third column. I have already tried this with the code below, but the problem with this code is that than all values will be the same instead of changing between 1 and -1.

idx = Velocity(:,3) == 0
p = [1 -1]
Velocity(idx,4) = p(randperm(length(p),1))

How can I solve this problem?

Answer 1

[Stephen23]

idx = Velocity(:,3)==0;
vec = [1;-1];
Velocity(idx,4) = vec(randi(numel(vec),nnz(idx),1))

[Danny Helwegen]

Thx this works

Answer 2

[Jos (10584)]

Something like this?

idx = Velocity(:,3) == 0  % where to change
N = sum(idx)              % how many to change
p = [-1 1]                % change to one of these values
r = randi(numel(p), N, 1) % random indices into p
New4 = p(r)               % create new column
Velocity(idx,4) = New4    % replace