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This section describes the rules that the text interpreter uses when it tries to convert a string into a number.
Let <digit> represent any character that is a legal digit in the current number base^{28}.
Let <decimal digit> represent any character in the range 0-9.
Let {a b} represent the optional presence of any of the characters in the braces (a or b or neither).
Let * represent any number of instances of the previous character (including none).
Let any other character represent itself.
Now, the conversion rules are:
By default, the number base used for integer number conversion is
given by the contents of the variable base
. Note that a lot of
confusion can result from unexpected values of base
. If you
change base
anywhere, make sure to save the old value and
restore it afterwards; better yet, use base-execute
, which does
this for you. In general I recommend keeping base
decimal, and
using the prefixes described below for the popular non-decimal bases.
dpl
– a-addr gforth “dpl”
User
variable – a-addr is the address of a cell that stores the
position of the decimal point in the most recent numeric conversion.
Initialised to -1. After the conversion of a number containing no
decimal point, dpl
is -1. After the conversion of 2.
it holds
0. After the conversion of 234123.9 it contains 1, and so forth.
base-execute
i*x xt u – j*x gforth “base-execute”
execute xt with the content of BASE
being u, and
restoring the original BASE
afterwards.
base
– a-addr core “base”
User
variable – a-addr is the address of a cell that
stores the number base used by default for number conversion during
input and output. Don’t store to base
, use
base-execute
instead.
hex
– core-ext “hex”
Set base
to &16 (hexadecimal). Don’t use hex
,
use base-execute
instead.
decimal
– core “decimal”
Set base
to &10 (decimal). Don’t use decimal
, use
base-execute
instead.
Gforth allows you to override the value of base
by using a
prefix^{29} before the first digit
of an (integer) number. The following prefixes are supported:
&
– decimal
#
– decimal
%
– binary
$
– hexadecimal
0x
– hexadecimal, if base<33.
'
– numeric value (e.g., ASCII code) of next character; an
optional '
may be present after the character.
Here are some examples, with the equivalent decimal number shown after in braces:
-$41 (-65), %1001101 (205), %1001.0001 (145 - a double-precision number), ’A (65), -’a’ (-97), &905 (905), $abc (2478), $ABC (2478).
Number conversion has a number of traps for the unwary:
base @ .
– the number base is always 10 in the current number
base. Instead, use something like base @ dec.
bin
but it does not set the number base!
It is used to specify file types.
.
of a double-precision number to be the
final character in the string. Gforth allows the .
to be
anywhere after the first digit.
base
is required to be decimal when
converting floating-point numbers. In Gforth, number conversion to
floating-point numbers always uses base &10, irrespective of the value
of base
.
You can read numbers into your programs with the words described in Line input and conversion.
For example, 0-9 when the number base is decimal or 0-9, A-F when the number base is hexadecimal.
Some Forth implementations provide a similar scheme by
implementing $
etc. as parsing words that process the subsequent
number in the input stream and push it onto the stack. For example, see
Number Conversion and Literals, by Wil Baden; Forth Dimensions
20(3) pages 26–27. In such implementations, unlike in Gforth, a space
is required between the prefix and the number.
Next: Interpret/Compile states, Previous: Input Sources, Up: The Text Interpreter [Contents][Index]