What is the intention of "v(i5)=v(i5)"?
Do you need a loop?
v = w + alpha .* w;
lt0 = (v < 0);
zerocoeff = v(lt0);
v(lt0) = 0;
Please use the debugger to examine what happens in your code. Set a breakpoint and step through the program line by line. Then you will see problems like "v(i5<0)", where the value of the index is checked, and not the value of the vector v.
Note that zerocoeff = v(i5<0) overwrites zerocoeff in each iteration.
