why coding this function returns un exact numerical results?

Ackley's Function (a=20,b=0.2) d=2.

function [ y ] = objfun14 (phen )
d=size(phen,2);
v=cos(2*pi*phen); x=(1/d)* sum(v,2);
t=-20*exp(-0.2*sqrt((1/d)*sum(phen.^2,2))) - exp( x ) +20+exp(1);

y=sum(t,2);
end
%when phen=[0 0] the output should be t=0 however excuting the code gives t=8.8818e-16

[Scott MacKenzie]

t=8.8818e-16 is pretty close to zero. You are dealing with floating point arithmetic, that's why the result is not exactly what you expect. See Floating-Point Numbers for further details.

Answer 1

[Kishan Dhakan]

Try using 'format long', which shows 15 decimal places. If it still doesn't work, then your value is probably smaller than what many call 'machine epsilon' and hence is getting rounded to zero.

[Walter Roberson]

The other way around, the function is returning -4*eps when the user expects 0