Floating-point numbers are useful for representing numbers that are
not integral. The range of floating-point numbers is
the same as the range of the C data type
double on the machine
you are using. On all computers currently supported by Emacs, this is
double-precision IEEE floating point.
The read syntax for floating-point numbers requires either a decimal point, an exponent, or both. Optional signs (‘+’ or ‘-’) precede the number and its exponent. For example, ‘1500.0’, ‘+15e2’, ‘15.0e+2’, ‘+1500000e-3’, and ‘.15e4’ are five ways of writing a floating-point number whose value is 1500. They are all equivalent. Like Common Lisp, Emacs Lisp requires at least one digit after any decimal point in a floating-point number; ‘1500.’ is an integer, not a floating-point number.
Emacs Lisp treats
-0.0 as numerically equal to ordinary zero
with respect to
=. This follows the
IEEE floating-point standard, which says
0.0 are numerically equal even though other operations can
The IEEE floating-point standard supports positive
infinity and negative infinity as floating-point values. It also
provides for a class of values called NaN or “not-a-number”;
numerical functions return such values in cases where there is no
correct answer. For example,
(/ 0.0 0.0) returns a NaN.
Although NaN values carry a sign, for practical purposes there is no other
significant difference between different NaN values in Emacs Lisp.
Here are read syntaxes for these special floating-point values:
‘1.0e+INF’ and ‘-1.0e+INF’
‘0.0e+NaN’ and ‘-0.0e+NaN’
The following functions are specialized for handling floating-point numbers:
This predicate returns
t if its floating-point argument is a NaN,
This function returns a cons cell
(s . e),
where s and e are respectively the significand and
exponent of the floating-point number x.
If x is finite, then s is a floating-point number between 0.5 (inclusive) and 1.0 (exclusive), e is an integer, and x = s * 2**e. If x is zero or infinity, then s is the same as x. If x is a NaN, then s is also a NaN. If x is zero, then e is 0.
This function returns a floating-point number corresponding to the significand sig and exponent exp.
This function copies the sign of x2 to the value of x1, and returns the result. x1 and x2 must be floating point.
This function returns the binary exponent of x. More precisely, the value is the logarithm base 2 of |x|, rounded down to an integer.
(logb 10) ⇒ 3 (logb 10.0e20) ⇒ 69