Avoiding -0.0000 as an output

Hello. I have the following script for differentiation and for some functions (e.g. f(x) = x) I get f'''(1) and f''''(1) equal to -0.0000. How can i avoid the minus sign when the reasult is zero ?

clear all, clc, format long e

f = @(x) x; %input('Enter f(x) = ');
x = 1; %input('Enter point: x = ');
tol = 1e-1; %input('Enter tolerance: ');


f3 = feval(f,x);
h = tol*f3;
f0 = feval(f,x-3*h);
f1 = feval(f,x-2*h);
f2 = feval(f,x-1*h);
f4 = feval(f,x+1*h);
f5 = feval(f,x+2*h);
f6 = feval(f,x+3*h);

d1f = (f1-8*f2+8*f4-f5)/12/h;
d2f = (-f1+16*f2-30*f3+16*f4-f5)/12/h^2;
d3f = (f0-8*f1+13*f2-13*f4+8*f5-f6)/8/h^3;
d4f = (-f0+12*f1-39*f2+56*f3-39*f4+12*f5-f6)/6/h^4;

p = [d1f;d2f;d3f;d4f];

fprintf('\nFirst derivative at x = %g:\tf''(%g) = %f\n',x,x,d1f);

First derivative at x = 1:	f'(1) = 1.000000

fprintf('\nSecond derivative at x = %g:\tf''''(%g) = %f\n',x,x,d2f);

Second derivative at x = 1:	f''(1) = 0.000000

fprintf('\nThird derivative at x = %g:\tf''''''(%g) = %f\n',x,x,d3f);

Third derivative at x = 1:	f'''(1) = -0.000000

fprintf('\nFourth derivative at x = %g:\tf''''''''(%g) = %f\n',x,x,d4f);

Fourth derivative at x = 1:	f''''(1) = -0.000000

Answer 1

[Torsten]

The results for d2,...,d4 aren't exactly 0 because of floating point errors in their computation. So without artificially manipulating the results, you can't get rid of the minus sign.

[Left Terry]

@Torsten That is true, i just discovered what you said by making a small change. The results are extremely small and negative.

clear all, clc, format long e

f = @(x) x; %input('Enter f(x) = ');
x = 1; %input('Enter point: x = ');
tol = 1e-1; %input('Enter tolerance: ');


f3 = feval(f,x);
h = tol*f3;
f0 = feval(f,x-3*h);
f1 = feval(f,x-2*h);
f2 = feval(f,x-1*h);
f4 = feval(f,x+1*h);
f5 = feval(f,x+2*h);
f6 = feval(f,x+3*h);

d1f = (f1-8*f2+8*f4-f5)/12/h;
d2f = (-f1+16*f2-30*f3+16*f4-f5)/12/h^2;
d3f = (f0-8*f1+13*f2-13*f4+8*f5-f6)/8/h^3;
d4f = (-f0+12*f1-39*f2+56*f3-39*f4+12*f5-f6)/6/h^4;

p = [d1f;d2f;d3f;d4f];

fprintf('\nFirst derivative at x = %g:\tf''(%g) = %g\n',x,x,d1f);

First derivative at x = 1:	f'(1) = 1

fprintf('\nSecond derivative at x = %g:\tf''''(%g) = %g\n',x,x,d2f);

Second derivative at x = 1:	f''(1) = 2.40548e-14

fprintf('\nThird derivative at x = %g:\tf''''''(%g) = %g\n',x,x,d3f);

Third derivative at x = 1:	f'''(1) = -1.38778e-13

fprintf('\nFourth derivative at x = %g:\tf''''''''(%g) = %g\n',x,x,d4f);

Fourth derivative at x = 1:	f''''(1) = -1.07322e-11