There are two important problems with your code.
First, if you want to compare strings, such as ‘alpha’ and the strings in your if statements, you have to use the strcmp function. No other syntax will work in every instance.
Second, since you are using a variable as part of the .mat file name, you have to be certain to specify the first argument in save as specifically a .mat file. You can use square brackets [] to do the concatenation.
With those two changes, your code works:
alpha=input('input: ','s');
if strcmp(alpha,'A')
i=1;
elseif strcmp(alpha,'B')
i=4;
else
i=8;
end
x=[i+2 i];
filename = alpha;
save([filename '.mat'], 'x')
I also suggest using inputdlg rather than input, but that is my personal preference.
