PERLNUMBER(1) Perl Programmers Reference Guide PERLNUMBER(1)
NAME
perlnumber - semantics of numbers and numeric operations in Perl
SYNOPSIS
$n = 1234; # decimal integer
$n = 0b1110011; # binary integer
$n = 01234; # octal integer
$n = 0x1234; # hexadecimal integer
$n = 12.34e-56; # exponential notation
$n = "-12.34e56"; # number specified as a string
$n = "1234"; # number specified as a string
DESCRIPTION
This document describes how Perl internally handles numeric values.
Perl's operator overloading facility is completely ignored here. Operator overloading allows user-defined userdefined
defined behaviors for numbers, such as operations over arbitrarily large integers, floating points
numbers with arbitrary precision, operations over "exotic" numbers such as modular arithmetic or
p-adic arithmetic, and so on. See overload for details.
Storing numbers
Perl can internally represent numbers in 3 different ways: as native integers, as native floating
point numbers, and as decimal strings. Decimal strings may have an exponential notation part, as in
"12.34e-56". Native here means "a format supported by the C compiler which was used to build perl".
The term "native" does not mean quite as much when we talk about native integers, as it does when
native floating point numbers are involved. The only implication of the term "native" on integers is
that the limits for the maximal and the minimal supported true integral quantities are close to pow-ers powers
ers of 2. However, "native" floats have a most fundamental restriction: they may represent only
those numbers which have a relatively "short" representation when converted to a binary fraction.
For example, 0.9 cannot be represented by a native float, since the binary fraction for 0.9 is infi-nite: infinite:
nite:
binary0.1110011001100...
with the sequence 1100 repeating again and again. In addition to this limitation, the exponent of
the binary number is also restricted when it is represented as a floating point number. On typical
hardware, floating point values can store numbers with up to 53 binary digits, and with binary expo-nents exponents
nents between -1024 and 1024. In decimal representation this is close to 16 decimal digits and deci-mal decimal
mal exponents in the range of -304..304. The upshot of all this is that Perl cannot store a number
like 12345678901234567 as a floating point number on such architectures without loss of information.
Similarly, decimal strings can represent only those numbers which have a finite decimal expansion.
Being strings, and thus of arbitrary length, there is no practical limit for the exponent or number
of decimal digits for these numbers. (But realize that what we are discussing the rules for just the
storage of these numbers. The fact that you can store such "large" numbers does not mean that the
operations over these numbers will use all of the significant digits. See "Numeric operators and
numeric conversions" for details.)
In fact numbers stored in the native integer format may be stored either in the signed native form,
or in the unsigned native form. Thus the limits for Perl numbers stored as native integers would
typically be -2**31..2**32-1, with appropriate modifications in the case of 64-bit integers. Again,
this does not mean that Perl can do operations only over integers in this range: it is possible to
store many more integers in floating point format.
Summing up, Perl numeric values can store only those numbers which have a finite decimal expansion or
a "short" binary expansion.
Numeric operators and numeric conversions
As mentioned earlier, Perl can store a number in any one of three formats, but most operators typi-cally typically
cally understand only one of those formats. When a numeric value is passed as an argument to such an
operator, it will be converted to the format understood by the operator.
Six such conversions are possible:
native integer --> native floating point (*)
native integer --> decimal string
native floating_point --> native integer (*)
native floating_point --> decimal string (*)
decimal string --> native integer
decimal string --> native floating point (*)
These conversions are governed by the following general rules:
If the source number can be represented in the target form, that representation is used.
If the source number is outside of the limits representable in the target form, a representation
of the closest limit is used. (Loss of information)
If the source number is between two numbers representable in the target form, a representation of
one of these numbers is used. (Loss of information)
In "native floating point --> native integer" conversions the magnitude of the result is less
than or equal to the magnitude of the source. ("Rounding to zero".)
If the "decimal string --> native integer" conversion cannot be done without loss of information,
the result is compatible with the conversion sequence "decimal_string --> native_floating_point
--> native_integer". In particular, rounding is strongly biased to 0, though a number like
"0.99999999999999999999" has a chance of being rounded to 1.
RESTRICTION: The conversions marked with "(*)" above involve steps performed by the C compiler. In
particular, bugs/features of the compiler used may lead to breakage of some of the above rules.
Flavors of Perl numeric operations
Perl operations which take a numeric argument treat that argument in one of four different ways: they
may force it to one of the integer/floating/ string formats, or they may behave differently depending
on the format of the operand. Forcing a numeric value to a particular format does not change the
number stored in the value.
All the operators which need an argument in the integer format treat the argument as in modular
arithmetic, e.g., "mod 2**32" on a 32-bit architecture. "sprintf "%u", -1" therefore provides the
same result as "sprintf "%u", ~0".
Arithmetic operators
The binary operators "+" "-" "*" "/" "%" "==" "!=" ">" "<" ">=" "<=" and the unary operators "-"
"abs" and "--" will attempt to convert arguments to integers. If both conversions are possible
without loss of precision, and the operation can be performed without loss of precision then the
integer result is used. Otherwise arguments are converted to floating point format and the
floating point result is used. The caching of conversions (as described above) means that the
integer conversion does not throw away fractional parts on floating point numbers.
++ "++" behaves as the other operators above, except that if it is a string matching the format
"/^[a-zA-Z]*[0-9]*\z/" the string increment described in perlop is used.
Arithmetic operators during "use integer"
In scopes where "use integer;" is in force, nearly all the operators listed above will force
their argument(s) into integer format, and return an integer result. The exceptions, "abs", "++"
and "--", do not change their behavior with "use integer;"
Other mathematical operators
Operators such as "**", "sin" and "exp" force arguments to floating point format.
Bitwise operators
Arguments are forced into the integer format if not strings.
Bitwise operators during "use integer"
forces arguments to integer format. Also shift operations internally use signed integers rather
than the default unsigned.
Operators which expect an integer
force the argument into the integer format. This is applicable to the third and fourth arguments
of "sysread", for example.
Operators which expect a string
force the argument into the string format. For example, this is applicable to "printf "%s",
$value".
Though forcing an argument into a particular form does not change the stored number, Perl remembers
the result of such conversions. In particular, though the first such conversion may be time-consum-
ing, repeated operations will not need to redo the conversion.
AUTHOR
Ilya Zakharevich "ilya@math.ohio-state.edu"
Editorial adjustments by Gurusamy Sarathy <gsar@ActiveState.com>
Updates for 5.8.0 by Nicholas Clark <nick@ccl4.org>
SEE ALSO
overload, perlop
perl v5.8.8 2006-01-07 PERLNUMBER(1)
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