MATLAB中digits和vpa

digits:

DIGITS Set variable precision digits.
Digits determines the accuracy of variable precision numeric computations.
DIGITS, by itself, displays the current setting of Digits.
DIGITS(D) sets Digits to D for subsequent calculations. D is an
integer, or a string or sym representing an integer.
D = DIGITS returns the current setting of Digits. D is an integer.

See also vpa.

vpa:

VPA Variable precision arithmetic.
R = VPA(S) numerically evaluates each element of the double matrix
S using variable precision floating point arithmetic with D decimal
digit accuracy, where D is the current setting of DIGITS.
The resulting R is a SYM.

VPA(S,D) uses D digits, instead of the current setting of DIGITS.
D is an integer or the SYM representation of a number.

It is important to avoid the evaluation of an expression using double
precision floating point arithmetic before it is passed to VPA.
For example,
phi = vpa((1+sqrt(5))/2)
first computes a 16-digit approximation to the golden ratio, then
converts that approximation to one with d digits, where d is the current
setting of DIGITS. To get full precision, use unevaluated string or
symbolic arguments,
phi = vpa('(1+sqrt(5))/2')
or
s = sym('sqrt(5)')
phi = vpa((1+s)/2);

Additional examples:
vpa(pi,780) shows six consecutive 9's near digit 770 in the
decimal expansion of pi.

vpa(hilb(2),5) returns

[ 1., .50000]
[.50000, .33333]

See also double, digits.

Overloaded methods:
sym/vpa

　　如：若把b=vpa(0.1666)的结果转换为字符串，可以先将其转换为double类型，即double(b)，再num2str(b)，即得所需结果。