How to enter numbers in Forth

Question

Is there something like input in Basic or scanf("%d") in C in Forth?

Probably it will be something like this:

200 buffer: buf
: input ( -- n ) buf 200 accept 
  some-magic-filter
  buf swap evaluate ;

The problem in the above code, is how to define a filter that will pass only numbers, but not any words, definitions, etc?

How do you define numbers in this context? Integers (that is what the C example suggests - %d is the format specifier for decimal integer)? Real numbers ("floats")? — Peter Mortensen

ruvim ruvim · Accepted Answer · 2018-08-06T23:29:55

The standard specifies only a low level >NUMBER word to interpret integer numbers. OTOH using EVALUATE to convert strings into numbers is a quick and dirty way. Either use it without checks (in the case of trusted input) or do not use it at all. Trying to filter the string before EVALUATE is a bad idea: it has cost of >NUMBER word itself and low reusing factor.

NB: neither >NUMBER nor EVALUATE detects numeric overflow.

In any case, your word to input a single-cell integer can be defined something like:

: accept-number ( -- n )
  PAD DUP 80 ACCEPT ( addr u ) StoN ( n )
;

In the case of trusted input you can define StoN like

: StoN ( addr u -- x )
  STATE @ ABORT" This naive StoN should not be used in compilation state"
  DEPTH 2- >R
    EVALUATE
  DEPTH 1- R> <> IF -24 THROW THEN
  \ check depth to accept the single-cell numbers only
;

Otherwise (in the case of untrusted input) you have two choices: to rely on the specific words of a particular Forth system or to use some (perhaps your own) library.

I use the following lexicon to define StoN:

\ ---
\ The words from Substring Matching library
\ (where length is counted in address units)

: MATCH-HEAD ( a u a-key u-key -- a-right u-right true | a u false ) 
  2 PICK OVER U< IF  2DROP FALSE EXIT THEN 
  DUP >R
  3 PICK R@ COMPARE IF  RDROP FALSE EXIT THEN 
  SWAP R@ + SWAP R> - TRUE
; 

\ ---
\ The words from Literals interpreting library
\ (where prefix 'I-' is shortcut for Interpret)

: I-DLIT ( a u -- x x true | a u false ) 
  2DUP S" -"  MATCH-HEAD >R
  DUP 0= IF  NIP RDROP EXIT THEN 
  0 0 2SWAP >NUMBER NIP IF  RDROP 2DROP FALSE EXIT THEN 
  R> IF  DNEGATE THEN  2SWAP 2DROP TRUE
; 

: I-LIT ( a u -- x true | a u false ) 
  I-DLIT IF  D>S TRUE EXIT THEN  FALSE
;

After that StoN can be defined as:

: StoN ( a u -- x ) I-LIT IF EXIT THEN -24 THROW ;

The mentioned libraries can be found at GitHub:

Substring matching functions library
Resolvers example (for various lexemes)

How to enter numbers in Forth

3 Answers