R = rat(X) returns
the rational fraction approximation of X to
within the default tolerance, 1e-6*norm(X(:),1).
The approximation is a string containing the truncated continued fractional
expansion.

Approximate the value of π using a rational
representation of the quantity pi.

The mathematical quantity π is
not a rational number, but the quantity pi that
approximates it is a rational number since all
floating-point numbers are rational.

Find the rational representation of pi.

format rat
pi

ans =
355/113

The resulting expression is a string. You also can use rats(pi) to
get the same answer.

Use rat to see the continued fractional
expansion of pi.

R = rat(pi)

R =
3 + 1/(7 + 1/(16))

The resulting string is an approximation by continued fractional
expansion. If you consider the first two terms of the expansion, you
get the approximation $$3+\frac{1}{7}=\frac{22}{7}$$,
which only agrees with pi to 2 decimals.

However, if you consider all three terms printed by rat,
you can recover the value 355/113, which agrees
with pi to 6 decimals.

Even though all floating-point numbers are rational numbers,
it is sometimes desirable to approximate them by simple rational numbers,
which are fractions whose numerator and denominator are small integers.
Rational approximations are generated by truncating continued fraction
expansions.

The rat function approximates each element
of X by a continued fraction of the form

The Ds are obtained by repeatedly picking
off the integer part and then taking the reciprocal of the fractional
part. The accuracy of the approximation increases exponentially with
the number of terms and is worst when X = sqrt(2).
For X = sqrt(2) , the error with k terms
is about 2.68*(.173)^k, so each additional term
increases the accuracy by less than one decimal digit. It takes 21
terms to get full floating-point accuracy.