MATLAB: How to solve Attempted to access (0) error.

error

In attached figure, one equation is there which i need to implement.
where,
 x[n] = a* r^n
 a = 11; r = 0.9; L = 19; 0<=m<=L;  0<=n<=L
for this i have written a script.
 clear all
 a = 11;
 r = 0.9;
 L = 19;
 for m = 0:L
    for n = 0:L
         S(m) = a*(r^n);
    end
 end
 plot(S,m)
 xlabel('Value of "S"');
 ylabel('Value of "L"');
 title('a = 11; r = 0.9; L = 19')
When i run it, it gives following error
 Attempted to access (0); index must be a positive integer or logical.
 Error in Question_6 (line 8)
        S(m) = a*(r^n);
How to solve it..?

Best Answer

Nimisha - unlike C/C++, indexing into MATLAB arrays begins at one instead of zero. Since your code is iterating (over m) from 0 to L, then the error message makes sense for
 S(m) = a*(r^n);
I don't think that you have quite gotten this equation correct. From your attached image, it looks like
 S[m] = sum(n=0...m)x[n];
which to me means that your S could be initialized as
 a = 11;
 r = 0.9;
 L = 19;
 S = zeros(L+1,1);  % size S
 S(1) = a*(r^0);    % this corresponds to S[0] in your equation
 for m = 2:L+1
    S(m) = S(m-1) + a*(r^m);
 end
Note how the above assumes that S is a recurrence relation. We can then plot it as
 plot(S,0:L)
 xlabel('Value of "S"');
 ylabel('Value of "L"');
 title('a = 11; r = 0.9; L = 19')
Try it and see what happens!
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