Y = cospi(X) computes
cos(X*pi) without explicitly computing X*pi. This
calculation is more accurate than cos(X*pi) because the floating-point
value of pi is an approximation of π. In particular:

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 $\mathrm{cos}\left(\frac{\pi}{2}\right)=0$.

Y(2)

ans = 6.1232e-17

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

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