Array operation in Matlab tend to be much faster than other solutions. This is a situation where you can use logical indexing:
%generate some random data
data=randn(30,1)*180;data(data<-180)=-180;data(data>180)=180;
%create copy to show changes later
original=data;
ind=1:5;
tmp=data(ind);
L=tmp<0;
data(ind(L)) = data(ind(L))+360;
L=tmp>0; %NB: tmp is not updated yet, this might not be what you want
data(ind(L)) = data(ind(L))+180;
[original data]
Using a setup like this allows you to easily create the ind array inside a loop.
If performance is paramount, this is not the optimal solution.
