cospi

Compare the accuracy of cospi(X) vs. cos(X*pi).

Create a vector of values.

X = [0 1/2 1 3/2 2];

Calculate the cosine of X*pi using the normal cos function.

Y = cos(X*pi)

Y = 1×5

    1.0000    0.0000   -1.0000   -0.0000    1.0000

The results contain small numerical errors due to the fact that pi is a floating-point approximation of the true value of $\pi$. For instance, Y(2) is not exactly zero even though $\cos \left(\frac{\pi }{2}\right)=0$.

Y(2)

ans = 
6.1232e-17

Use cospi to calculate the same values. In this case, the results are exact.

Z = cospi(X)

Z = 1×5

     1     0    -1     0     1

Z(2)

ans = 
0