Can anyone tell me why I am getting "Unrecognized function or variable 'a'." in this code?

g=@(x) atan(4*x);
d=@(x) 4/(16*x^2 + 1);
h=1;
for i= 0:h:10
    if (g(i)*g(i+h)<0)
        a=(i);
        b=(i+h);
        break;
    end
end

x0 = (a+b)/2;

Unrecognized function or variable 'a'.

Answer 1

[Walter Roberson]

g=@(x) atan(4*x);
d=@(x) 4/(16*x^2 + 1);
h=1;
found_it = false;
for i= 0:h:10
    if (g(i)*g(i+h)<0)
        a=(i);
        b=(i+h);
        found_it = true;
        break;
    end
end

if found_it
  x0 = (a+b)/2;
else
    fprintf('sign change not found!\n');
end

sign change not found!

fplot(g, [0 10])

g(0)

ans = 0

g is 0 at 0, but it never changes sign in the [0 10] range. You are not testing for <= 0, you are testing for < 0.

... and be careful, if a g(i) equals 0 exactly you might determine x0 incorrectly.