simple matlab interpolation help

[JXPES]

suppose I have a function f(x) which I know increases as x.

I'm trying to combine a for loop and if statement

x(1)=a;
x(2)=b;
x(3)=(a+b)/2;

for i=3:n
if f(x(i)) > A
x(i+1) = (x(i-2)+x(i))/2;
else
x(i+1) = (x(i-1)+x(i))/2
end
end

this method works for x(4), the rest come with errors! Am I missing something obvious? Thanks.

[JXPES]

never mind. worked it out- the order of the intervals had to be considered