Commit cc3d7580 authored by Tom Tromey's avatar Tom Tromey
Browse files

Document bignums

* doc/lispref/numbers.texi (Numbers, Integer Basics)
(Predicates on Numbers, Comparison of Numbers)
(Arithmetic Operations, Bitwise Operations): Update for bignums.
* doc/lispref/objects.texi (Integer Type, Type Predicates):
Update for bignums.
* etc/NEWS: Update for bigums.
parent e2a78b0d
......@@ -14,9 +14,9 @@
fractional parts, such as @minus{}4.5, 0.0, and 2.71828. They can
also be expressed in exponential notation: @samp{1.5e2} is the same as
@samp{150.0}; here, @samp{e2} stands for ten to the second power, and
that is multiplied by 1.5. Integer computations are exact, though
they may overflow. Floating-point computations often involve rounding
errors, as the numbers have a fixed amount of precision.
that is multiplied by 1.5. Integer computations are exact.
Floating-point computations often involve rounding errors, as the
numbers have a fixed amount of precision.
@menu
* Integer Basics:: Representation and range of integers.
......@@ -34,7 +34,15 @@ errors, as the numbers have a fixed amount of precision.
@node Integer Basics
@section Integer Basics
The range of values for an integer depends on the machine. The
Integers in Emacs Lisp can have arbitrary precision.
Under the hood, though, there are two kinds of integers: smaller
ones, called @dfn{fixnums}, and larger ones, called @dfn{bignums}
Some functions in Emacs only accept fixnums. Also, while fixnums can
always be compared for equality with @code{eq}, bignums require the
use of @code{eql}.
The range of values for a fixnum depends on the machine. The
minimum range is @minus{}536,870,912 to 536,870,911 (30 bits; i.e.,
@ifnottex
@minus{}2**29
......@@ -49,9 +57,7 @@ to
@tex
@math{2^{29}-1}),
@end tex
but many machines provide a wider range. Many examples in this
chapter assume the minimum integer width of 30 bits.
@cindex overflow
but many machines provide a wider range.
The Lisp reader reads an integer as a nonempty sequence
of decimal digits with optional initial sign and optional
......@@ -91,14 +97,8 @@ For example:
#24r1k @result{} 44
@end example
If an integer is outside the Emacs range, the Lisp reader ordinarily
signals an overflow. However, if a too-large plain integer ends in a
period, the Lisp reader treats it as a floating-point number instead.
This lets an Emacs Lisp program specify a large integer that is
quietly approximated by a floating-point number on machines with
limited word width. For example, @samp{536870912.} is a
floating-point number if Emacs integers are only 30 bits wide and is
an integer otherwise.
An integer is read as a fixnum if it is in the correct range.
Otherwise, it will be read as a bignum.
To understand how various functions work on integers, especially the
bitwise operators (@pxref{Bitwise Operations}), it is often helpful to
......@@ -141,16 +141,6 @@ In binary, the decimal integer 4 is 100. Consequently,
0111...111111 (30 bits total)
@end example
Since the arithmetic functions do not check whether integers go
outside their range, when you add 1 to 536,870,911, the value is the
negative integer @minus{}536,870,912:
@example
(+ 1 536870911)
@result{} -536870912
@result{} 1000...000000 (30 bits total)
@end example
Many of the functions described in this chapter accept markers for
arguments in place of numbers. (@xref{Markers}.) Since the actual
arguments to such functions may be either numbers or markers, we often
......@@ -160,8 +150,8 @@ value is a marker, its position value is used and its buffer is ignored.
@cindex largest Lisp integer
@cindex maximum Lisp integer
@defvar most-positive-fixnum
The value of this variable is the largest integer that Emacs Lisp can
handle. Typical values are
The value of this variable is the largest ``small'' integer that Emacs
Lisp can handle. Typical values are
@ifnottex
2**29 @minus{} 1
@end ifnottex
......@@ -181,8 +171,8 @@ on 64-bit platforms.
@cindex smallest Lisp integer
@cindex minimum Lisp integer
@defvar most-negative-fixnum
The value of this variable is the smallest integer that Emacs Lisp can
handle. It is negative. Typical values are
The value of this variable is the smallest small integer that Emacs
Lisp can handle. It is negative. Typical values are
@ifnottex
@minus{}2**29
@end ifnottex
......@@ -315,6 +305,20 @@ use otherwise), but the @code{zerop} predicate requires a number as
its argument. See also @code{integer-or-marker-p} and
@code{number-or-marker-p}, in @ref{Predicates on Markers}.
@defun bignump object
This predicate tests whether its argument is a large integer, and
returns @code{t} if so, @code{nil} otherwise. Large integers cannot
be compared with @code{eq}, only with @code{=} or @code{eql}. Also,
large integers are only available if Emacs was compiled with the GMP
library.
@end defun
@defun fixnump object
This predicate tests whether its argument is a small integer, and
returns @code{t} if so, @code{nil} otherwise. Small integers can be
compared with @code{eq}.
@end defun
@defun floatp object
This predicate tests whether its argument is floating point
and returns @code{t} if so, @code{nil} otherwise.
......@@ -355,13 +359,13 @@ if so, @code{nil} otherwise. The argument must be a number.
To test numbers for numerical equality, you should normally use
@code{=}, not @code{eq}. There can be many distinct floating-point
objects with the same numeric value. If you use @code{eq} to
compare them, then you test whether two values are the same
@emph{object}. By contrast, @code{=} compares only the numeric values
of the objects.
and large integer objects with the same numeric value. If you use
@code{eq} to compare them, then you test whether two values are the
same @emph{object}. By contrast, @code{=} compares only the numeric
values of the objects.
In Emacs Lisp, each integer is a unique Lisp object.
Therefore, @code{eq} is equivalent to @code{=} where integers are
In Emacs Lisp, each small integer is a unique Lisp object.
Therefore, @code{eq} is equivalent to @code{=} where small integers are
concerned. It is sometimes convenient to use @code{eq} for comparing
an unknown value with an integer, because @code{eq} does not report an
error if the unknown value is not a number---it accepts arguments of
......@@ -389,15 +393,6 @@ Here's a function to do this:
fuzz-factor)))
@end example
@cindex CL note---integers vrs @code{eq}
@quotation
@b{Common Lisp note:} Comparing numbers in Common Lisp always requires
@code{=} because Common Lisp implements multi-word integers, and two
distinct integer objects can have the same numeric value. Emacs Lisp
can have just one integer object for any given value because it has a
limited range of integers.
@end quotation
@defun = number-or-marker &rest number-or-markers
This function tests whether all its arguments are numerically equal,
and returns @code{t} if so, @code{nil} otherwise.
......@@ -407,7 +402,8 @@ and returns @code{t} if so, @code{nil} otherwise.
This function acts like @code{eq} except when both arguments are
numbers. It compares numbers by type and numeric value, so that
@code{(eql 1.0 1)} returns @code{nil}, but @code{(eql 1.0 1.0)} and
@code{(eql 1 1)} both return @code{t}.
@code{(eql 1 1)} both return @code{t}. This can be used to compare
large integers as well as small ones.
@end defun
@defun /= number-or-marker1 number-or-marker2
......@@ -567,10 +563,6 @@ Except for @code{%}, each of these functions accepts both integer and
floating-point arguments, and returns a floating-point number if any
argument is floating point.
Emacs Lisp arithmetic functions do not check for integer overflow.
Thus @code{(1+ 536870911)} may evaluate to
@minus{}536870912, depending on your hardware.
@defun 1+ number-or-marker
This function returns @var{number-or-marker} plus 1.
For example,
......@@ -897,36 +889,6 @@ On the other hand, shifting one place to the right looks like this:
As the example illustrates, shifting one place to the right divides the
value of a positive integer by two, rounding downward.
The function @code{lsh}, like all Emacs Lisp arithmetic functions, does
not check for overflow, so shifting left can discard significant bits
and change the sign of the number. For example, left shifting
536,870,911 produces @minus{}2 in the 30-bit implementation:
@example
(lsh 536870911 1) ; @r{left shift}
@result{} -2
@end example
In binary, the argument looks like this:
@example
@group
;; @r{Decimal 536,870,911}
0111...111111 (30 bits total)
@end group
@end example
@noindent
which becomes the following when left shifted:
@example
@group
;; @r{Decimal @minus{}2}
1111...111110 (30 bits total)
@end group
@end example
@end defun
@defun ash integer1 count
@cindex arithmetic shift
@code{ash} (@dfn{arithmetic shift}) shifts the bits in @var{integer1}
......@@ -951,19 +913,6 @@ looks like this:
@end group
@end example
In contrast, shifting the pattern of bits one place to the right with
@code{lsh} looks like this:
@example
@group
(lsh -6 -1) @result{} 536870909
;; @r{Decimal @minus{}6 becomes decimal 536,870,909.}
1111...111010 (30 bits total)
@result{}
0111...111101 (30 bits total)
@end group
@end example
Here are other examples:
@c !!! Check if lined up in smallbook format! XDVI shows problem
......
......@@ -166,7 +166,10 @@ latter are unique to Emacs Lisp.
@node Integer Type
@subsection Integer Type
The range of values for an integer depends on the machine. The
Under the hood, there are two kinds of integers---small integers,
called @dfn{fixnums}, and large integers, called @dfn{bignums}.
The range of values for a fixnum depends on the machine. The
minimum range is @minus{}536,870,912 to 536,870,911 (30 bits; i.e.,
@ifnottex
@minus{}2**29
......@@ -182,8 +185,14 @@ to
@math{2^{29}-1})
@end tex
but many machines provide a wider range.
Emacs Lisp arithmetic functions do not check for integer overflow. Thus
@code{(1+ 536870911)} is @minus{}536,870,912 if Emacs integers are 30 bits.
Bignums can have arbitrary precision. Operations that overflow a
fixnum will return a bignum instead.
Fixnums can be compared with @code{eq}, but bignums require
@code{eql} or @code{=}. The @code{fixnump} predicate can be used to
detect such small integers, and @code{bignump} can be used to detect
large integers.
The read syntax for integers is a sequence of (base ten) digits with an
optional sign at the beginning and an optional period at the end. The
......@@ -200,11 +209,6 @@ leading @samp{+} or a final @samp{.}.
@end example
@noindent
As a special exception, if a sequence of digits specifies an integer
too large or too small to be a valid integer object, the Lisp reader
reads it as a floating-point number (@pxref{Floating-Point Type}).
For instance, if Emacs integers are 30 bits, @code{536870912} is read
as the floating-point number @code{536870912.0}.
@xref{Numbers}, for more information.
......@@ -1895,6 +1899,9 @@ with references to further information.
@item arrayp
@xref{Array Functions, arrayp}.
@item bignump
@xref{Predicates on Numbers, floatp}.
@item bool-vector-p
@xref{Bool-Vectors, bool-vector-p}.
......@@ -1928,6 +1935,9 @@ with references to further information.
@item custom-variable-p
@xref{Variable Definitions, custom-variable-p}.
@item fixnump
@xref{Predicates on Numbers, floatp}.
@item floatp
@xref{Predicates on Numbers, floatp}.
......
......@@ -24,6 +24,9 @@ When you add a new item, use the appropriate mark if you are sure it applies,
* Installation Changes in Emacs 27.1
** configure now checks for the GMP library. If not found, the
included "mini-gmp" library is used instead.
** The new configure option '--with-json' adds support for JSON using
the Jansson library. It is on by default; use 'configure
--with-json=no' to build without Jansson support. The new JSON
......@@ -644,15 +647,6 @@ as new-style, bind the new variable 'force-new-style-backquotes' to t.
integer, Emacs now signals an error if the number is too large for the
implementation to format (Bug#30408).
+++
** The Lisp reader now signals an overflow for plain decimal integers
that do not end in '.' and are outside Emacs range. Formerly the Lisp
reader silently converted them to floating-point numbers, and signaled
overflow only for integers with a radix that are outside machine range.
To get the old behavior, set the new, experimental variable
read-integer-overflow-as-float to t and please email
30408@debbugs.gnu.org if you need that. (Bug#30408).
---
** Some functions and variables obsolete since Emacs 22 have been removed:
archive-mouse-extract, assoc-ignore-case, assoc-ignore-representation,
......@@ -708,6 +702,11 @@ manual for more details.
Given a proper list as argument, this predicate returns its length;
otherwise, it returns nil.
+++
** Emacs Lisp integers can be of arbitrary precision. The new
predicates 'bignump' and 'fixnump' can be used to distinguish between
the types of integers.
** define-minor-mode automatically documents the meaning of ARG
+++
......
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment