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Number Conversion

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 base¹.

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:

• A string of the form <digit><digit>* is treated as a single-precision (cell-sized) positive integer. Examples are 0 123 6784532 32343212343456 42
• A string of the form -<digit><digit>* is treated as a single-precision (cell-sized) negative integer, and is represented using 2's-complement arithmetic. Examples are -45 -5681 -0
• A string of the form <digit><digit>*.<digit>* is treated as a double-precision (double-cell-sized) positive integer. Examples are 3465. 3.465 34.65 (all three of these represent the same number).
• A string of the form -<digit><digit>*.<digit>* is treated as a double-precision (double-cell-sized) negative integer, and is represented using 2's-complement arithmetic. Examples are -3465. -3.465 -34.65 (all three of these represent the same number).
• A string of the form {+ -}<decimal digit>{.}<decimal digit>*{e E}{+ -}<decimal digit><decimal digit>* is treated as a floating-point number. Examples are 1e 1e0 1.e 1.e0 +1e+0 (which all represent the same number) +12.E-4

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. In general I recommend keeping base decimal, and using the prefixes described below for the popular non-decimal bases.

dpl       -- a-addr         gforth       ``dpl''

base       -- a-addr         core       ``base''

hex       --         core-ext       ``hex''

decimal       --         core       ``decimal''

Gforth allows you to override the value of base by using a prefix² before the first digit of an (integer) number. Four prefixes are supported:

• & - decimal
• % - binary
• $ - hexadecimal
• ' - base max-char+1

Here are some examples, with the equivalent decimal number shown after in braces:

-$41 (-65), %1001101 (205), %1001.0001 (145 - a double-precision number), 'AB (16706; ascii A is 65, ascii B is 66, number is 65*256 + 66), 'ab (24930; ascii a is 97, ascii B is 98, number is 97*256 + 98), &905 (905), $abc (2478), $ABC (2478).

Number conversion has a number of traps for the unwary:

• You cannot determine the current number base using the code sequence base @ . - the number base is always 10 in the current number base. Instead, use something like base @ dec.
• If the number base is set to a value greater than 14 (for example, hexadecimal), the number 123E4 is ambiguous; the conversion rules allow it to be intepreted as either a single-precision integer or a floating-point number (Gforth treats it as an integer). The ambiguity can be resolved by explicitly stating the sign of the mantissa and/or exponent: 123E+4 or +123E4 - if the number base is decimal, no ambiguity arises; either representation will be treated as a floating-point number.
• There is a word bin but it does not set the number base! It is used to specify file types.
• ANS Forth requires the . of a double-precision number to be the final character in the string. Gforth allows the . to be anywhere after the first digit.
• The number conversion process does not check for overflow.
• In an ANS Forth program 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 Input.

Footnotes

1. For example, 0-9 when the number base is decimal or 0-9, A-F when the number base is hexadecimal.

2. 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.