"what's wrong on this code?"
- if does not change subsets of an array like that (you would need to use if inside of a loop).
- You also need to read the if documentation to know how it behaves with non-scalar conditions.
- MATLAB does not have ternary logical operators like 0<=x<1. What you wrote is equivalent to (0<=x)<1 which is equivalent to either 0<1 or 1<1 (but the first case will return true when you don't expect it to).
The MATLAB way is to use logical indexing, e.g.:
>> x = -5:0.01:5;
>> y = sin(x);
>> y(x<0|x>=1) = 0;
>> plot(x,y)
