[求助]用for循环赋值矩阵出现的问题

[rain_fall]

for I=0:0.01:2,
for J=0i/100:2*pi,
result=dblquad(inline('x.*exp(i*(x.^3.*cos(y)-I.*x.*cos(y-J)-x.^2))'),0,1,0,2*pi,[],[],I,J);%求二重积分
a=I./0.01+1;b=J/(pi/100)+1;
A(a,b)=((abs(result)).^2)/(pi.^2);
end
end
A

运行时出现如下代码
??? Attempted to access A(15,1); index must be a positive integer or logical.

Error in ==> Untitled1 at 5
A(a,b)=((abs(result)).^2)/(pi.^2);
请问哪位大侠知道这是怎么回事？请赐教，不胜感激！