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READINT.PL
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1991-10-31
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2KB
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48 lines
/* From the book PROLOG PROGRAMMING IN DEPTH
by Michael A. Covington, Donald Nute, and Andre Vellino.
Copyright 1988 Scott, Foresman & Co.
Non-commercial distribution of this file is permitted. */
/* READINT.PL */
/* First attempt at a general-purpose numeric input routine. */
/* Requires procedure READSTRING defined in file READSTR.PL */
:- ( clause(readstring(_),_) ; consult('readstr.pl') ).
/**********************************************************************
* readinteger(Result) *
* Accepts a string from the user and interprets it as an integer. *
* Leading, trailing, and embedded blanks are permitted. *
**********************************************************************/
readinteger(Result) :- readstring(S),
readinteger_aux(S,0,Result).
readinteger_aux([32|T],SoFar,Result) :- /* ignore blanks */
!,
readinteger_aux(T,SoFar,Result).
readinteger_aux([H|T],SoFar,Result) :- /* process a digit */
readinteger_value(H,V),
!,
NewSoFar is SoFar*10 + V,
readinteger_aux(T,NewSoFar,Result).
readinteger_aux([],Result,Result) :- !. /* string empty, */
/* return result */
readinteger_aux(_,_,Result) :- /* unrecognized character */
write('Number expected. Try again:'),
readinteger(Result).
readinteger_value(48,0). /* Table converting ASCII codes */
readinteger_value(49,1). /* of digits to numeric values */
readinteger_value(50,2).
readinteger_value(51,3).
readinteger_value(52,4).
readinteger_value(53,5).
readinteger_value(54,6).
readinteger_value(55,7).
readinteger_value(56,8).
readinteger_value(57,9).