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Arithmetic evaluation: Difference between revisions
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ReadEvalPrintLoop
end.
=={{header|Pop11}}==
<pre>
/* Scanner routines */
/* Uncomment the following to parse data from standard input
vars itemrep;
incharitem(charin) -> itemrep;
*/
;;; Current symbol
vars sym;
define get_sym();
itemrep() -> sym;
enddefine;
define expect(x);
lvars x;
if x /= sym then
printf(x, 'Error, expected %p\n');
mishap(sym, 1, 'Example parser error');
endif;
get_sym();
enddefine;
lconstant res_list = [( ) + * ];
lconstant reserved = newproperty(
maplist(res_list, procedure(x); [^x ^(true)]; endprocedure),
20, false, "perm");
/*
Parser for arithmetic expressions
*/
/*
expr: term
| expr "+" term
| expr "-" term
;
*/
define do_expr() -> result;
lvars result = do_term(), op;
while sym = "+" or sym = "-" do
sym -> op;
get_sym();
[^op ^result ^(do_term())] -> result;
endwhile;
enddefine;
/*
term: factor
| term "*" factor
| term "/" factor
;
*/
define do_term() -> result;
lvars result = do_factor(), op;
while sym = "*" or sym = "/" do
sym -> op;
get_sym();
[^op ^result ^(do_factor())] -> result;
endwhile;
enddefine;
/*
factor: word
| constant
| "(" expr ")"
;
*/
define do_factor() -> result;
if sym = "(" then
get_sym();
do_expr() -> result;
expect(")");
elseif isinteger(sym) or isbiginteger(sym) then
sym -> result;
get_sym();
else
if reserved(sym) then
printf(sym, 'unexpected symbol %p\n');
mishap(sym, 1, 'Example parser syntax error');
endif;
sym -> result;
get_sym();
endif;
enddefine;
/* Expression evaluator, returns false on error (currently only
division by 0 */
define arith_eval(expr);
lvars op, arg1, arg2;
if not(expr) then
return(expr);
endif;
if isinteger(expr) or isbiginteger(expr) then
return(expr);
endif;
expr(1) -> op;
arith_eval(expr(2)) -> arg1;
arith_eval(expr(3)) -> arg2;
if not(arg1) or not(arg2) then
return(false);
endif;
if op = "+" then
return(arg1 + arg2);
elseif op = "-" then
return(arg1 - arg2);
elseif op = "*" then
return(arg1 * arg2);
elseif op = "/" then
if arg2 = 0 then
return(false);
else
return(arg1 div arg2);
endif;
else
printf('Internal error\n');
return(false);
endif;
enddefine;
/* Given list, create item repeater. Input list is stored in a
closure are traversed when new item is requested. */
define listitemrep(lst);
procedure();
lvars item;
if lst = [] then
termin;
else
front(lst) -> item;
back(lst) -> lst;
item;
endif;
endprocedure;
enddefine;
/* Initialise scanner */
listitemrep([(3 + 50) * 7 - 100 / 10]) -> itemrep;
get_sym();
;;; Test it
arith_eval(do_expr()) =>
</pre>
=={{header|Prolog}}==
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