Operator-precedence parser: Difference between revisions

Revision as of 18:40, 24 October 2019

Revision as of 09:49, 18 December 2019

Line 17:

<syntaxhighlight lang="ebnf">

expression ::= equality-expression

equality-expression ::= additive-expression ( ( '==' | '!=' ) additive-expression ) *

additive-expression ::= multiplicative-expression ( ( '+' | '-' ) multiplicative-expression ) *

multiplicative-expression ::= primary ( ( '*' | '/' ) primary ) *

primary ::= '(' expression ')' | NUMBER | VARIABLE | '-' primary

</syntaxhighlight>

Line 34:

The pseudo-code for the algorithm is as follows. The parser starts at function ''parse_expression''. Precedence levels are greater than or equal to 0.

''parse_expression'' ()

'''return ''parse_expression_1'' parse_expression_1(''parse_primary'' (), 0)

''parse_expression_1 (lhs, min_precedence)

''lookahead'' := peek next token

'''while''' ''lookahead'' is a binary operator whose precedence is >= ''min_precedence''

''op'' := ''lookahead''

advance to next token

''rhs'' := ''parse_primary'' ()

''lookahead'' := peek next token

'''while''' ''lookahead'' is a binary operator whose precedence is greater

than ''op''<nowiki>'</nowiki>s, or a right-associative operator

whose precedence is equal to ''op'''s

Line 50:

''lookahead'' := peek next token

''lhs'' := the result of applying ''op'' with operands ''lhs'' and ''rhs''

'''return''' ''lhs''

Note that in the case of a production rule like this (where the operator can only appear once):

<syntaxhighlight lang="bnf">

equality-expression ::= additive-expression ( '==' | '!=' ) additive-expression

</syntaxhighlight>

Line 113:

#include <string.h>

int main(int argc, char *argv[]) {

int i;

printf("((((");

for (i=1;i!=argc;i++) {

if (argv[i] && !argv[i][1]) {

switch (*argv[i]) {

case '(': printf("(((("); continue;

case ')': printf("))))"); continue;