Modifying a Variable if a condition is met

I have a 300x1 Matrix, if a value in rows 1:75 is negative I want to add 360 to the negative value, if its positive I want to add 180 to it. There are other conditions similar to this that will be done to values in 76:150, 151:225, 226:300, but once I get started I think I can figure the rest out. I'm fairly new to Matlab so I intially tried an If statement but I couldnt get it to work. Any help would be greatly appreciated

Answer 1

[Rik]

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]

ans = 30×2

36.9248 216.9248 7.8090 187.8090 52.4707 232.4707 84.1838 264.1838 -65.3090 294.6910 -74.6702 -74.6702 -146.6197 -146.6197 164.8950 164.8950 180.0000 180.0000 -180.0000 -180.0000

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.

[Cody Kessler]

Thank You