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PERLNUMBER(1)	       Perl Programmers Reference Guide 	 PERLNUMBER(1)

       perlnumber - semantics of numbers and numeric operations in Perl

	   $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

       This document describes how Perl internally handles numeric values.

       Perls operator overloading facility is completely ignored here.	Oper
       ator overloading allows user-defined behaviors for numbers, such as
       operations over arbitrarily large integers, floating points numbers
       with arbitrary precision, operations over "exotic" numbers such as mod
       ular arithmetic or p-adic arithmetic, and so on.  See overload for

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 lim
       its for the maximal and the minimal supported true integral quantities
       are close to powers of 2.  However, "native" floats have a most funda
       mental 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 infinite:


       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 exponents between -1024 and 1024.  In decimal represen
       tation this is close to 16 decimal digits and decimal 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 archi
       tectures 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 deci
       mal 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 for
       mats, but most operators typically understand only one of those for
       mats.  When a numeric value is passed as an argument to such an opera
       tor, 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 mag
	   nitude 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 per
       formed 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 inte
       ger/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 with
	   out 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 frac
	   tional 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 incre
	   ment described in perlop is used.

       Arithmetic operators during "use integer"
	   In scopes where "use integer;" is in force, nearly all the opera
	   tors 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 inter
	   nally 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 par
       ticular, though the first such conversion may be time-consuming,
       repeated operations will not need to redo the conversion.

       Ilya Zakharevich "ilya@math.ohio-state.edu"

       Editorial adjustments by Gurusamy Sarathy 

       Updates for 5.8.0 by Nicholas Clark 

       overload, perlop

perl v5.8.8			  2008-04-25			 PERLNUMBER(1)

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