MATLAB: Error “Subscript indices must either be real positive integers or logicals.” At random index

forindicesloopMATLABsubscript indices

I'm having some trouble with a for loop. For some reason, it works properly until the first of the 3 loops gets to the number 4.8 (when j=4.8) where it stops the program and shows the error "Subscript indices must either be real positive integers or logicals."
This is the code:
syms s;
s=tf('s');
X=zeros(25,25,25);
for i=2:0.2:7
    for a=2:0.2:7
        for j=2:0.2:7
            L=(s+i)*(s+j)*(s+a)/s/(s+10000)^2;
            Y=step(L/(1+L),t);
            if max(Y)<1.11
                X(i*5-9,5*j-9,a*5-9)=1;
            else
                X(i*5-9,5*j-9,a*5-9)=0;
            end
        end
    end
end        
(Edited)
Anyone knows what is going on?

Best Answer

Indexing requires positive integer values just as the error message indicates. At lease one of the three indices of X is not a positive integer. 
X(i*5-9,  5*j-9,  a*5-9)
[Update]
The 2nd index (5*j-9) is producing what appears to be an integer (15) but, in fact, is not an integer. 
sprintf('%.20f',5*j-9)
ans =
    '15.00000000000000355271'
That's because "j" is not really 4.8
sprintf('%.20f',j)
ans =
    '4.80000000000000071054'
The reason is due to round-off error associated with floating point number representation when you create this sequence 2:0.2:7. A great article was written by a cofounder of MathWorks, Cleve Moler, that explains this problem.
Instead, use integers to control your loops. 
X=zeros(25,25,25);
iSeq = 2:0.2:7; 
aSeq = 2:0.2:7; 
jSeq = 2:0.2:7; 
for i=1:numel(iSeq)
    for a=1:numel(aSeq)
        for j=1:numel(jSeq)
            L=(s+iSeq(i))*(s+jSeq(j))*(s+aSeq(a))/s/(s+10000)^2;
            Y=step(L/(1+L),t);
            if max(Y)<1.11
                X(i,j,a)=1; % <-- double check that this is correct

            else
                X(i,j,a)=0; % <-- double check that this is correct
            end
        end
    end
end
Also, this will surely result in an error
 L=*(s+i)*(s+j)*(s+a)/s/(s+10000)^2;
 % ^
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