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;
% ^
