Britney - the error message is telling you that you are trying to access an array named int with an index that is not positive or a logical (true/1). In your case, the first two lines of your code are
a = int (-1);
b = int (0);
and you are using -1 and 0 to index the array. As neither are positive, the error message makes sense. If a and b are the endpoints for your interval, then you can initialize them as
a = int(1);
b = int(2);
Though you may want to rename this input variable as interval as int could be mistaken for integer.
Note that since you are using double precision, the line
if fm==0
is not guaranteed to ever evaluate to true. This is fine for integers, but for variables of type double, you would typically do something like
if abs(fm)<tol
where tol is some small number (could be eps).
