bblearn.uidaho.edu to submit a .zip archive
file containing a Flex .l source code file, with your name in a
(C-style) comment at the top.
Use the declarative lexical analysis language Flex to develop a scanner for
JSON files. JSON is described at json.org.
Your solution to this homework will be hooked together with a parser in a
subsequent homework assignment. Note that you are to develop your work for
this assignment in a single file (json.l); it will be linked to the
following makefile and main.c. The main.c will include a file json.h to
define integer codes for different categories of words/tokens. You may not
substitute your own main() procedure. Your yylex()
really must return an integer code each time it recognizes a token, it may
not just print the output itself.
| makefile |
# flex makefile. # json: main.o lex.yy.o cc -o json main.o lex.yy.o main.o: main.c json.h cc -c -g -DLEX main.c lex.yy.o: lex.yy.c json.h cc -c -g lex.yy.c lex.yy.c: json.l flex json.l |
|---|---|
| main.c |
#include "json.h"
#include <stdio.h>
#include <stdlib.h>
extern FILE *yyin;
extern char *yytext;
char *yyfilename;
int main(int argc, char *argv[])
{
int t;
if (argc < 2) { printf("usage: json file.json\n"); exit(-1); }
yyin = fopen(argv[1],"r");
if (yyin == NULL) { printf("can't open/read '%s'\n", argv[1]); exit(-1); }
yyfilename = argv[1];
while ((t=yylex()) > 0) {
if (t <= 32) {
printf("token %d text %s\n", t, yytext);
}
else {
printf("token %c\n", t);
}
}
return 0;
}
|
| json.h |
/* header file for JSON tokens, codes from json.org */
#define TRUE 1
#define FALSE 2
#define NULL 3
#define LCURLY '{'
#define RCURLY '}'
#define COMMA ','
#define COLON ':'
#define LBRACKET '['
#define RBRACKET ']'
#define STRINGLIT 4
/* #define CHARLIT 5 json does not have character literals! */
#define NUMBER 6
|
In this homework, you write out the category found for each "word" of a JSON file, one per line. If the word is from one of the lower integer categories (values 1-6), you also write out the letters that were matched for that word.
If the input file contains things that are not legal in JSON, you should
write a line containing "lexical error on line n", where n
is the line number. The easiest way to do this, as seen in class, is to use
%option yylineno in your .l file, so you are required to do that.
Lexical errors do not return integer categories from yylex(), they are treated
as whitespace.
{
"name":"Clint",
"age":54,
"cars": {
"car1":"Ford",
"car2":"Pontiac",
"car3":"Pontiac"
}
}
You don't have to worry yet about the syntax or semantics of this file,
your job is to break it into lexical tokens: adjacent sequences of characters
that form indivisible values or entities.
Your program (linked with the main.c above) would print out
token {
token 4 text "name"
token :
token 4 text "Clint"
token ,
token 4 text "age"
token :
token 6 text 54
token ,
token 4 text "cars"
token :
token {
token 4 text "car1"
token :
token 4 text "Ford"
token ,
token 4 text "car2"
token :
token 4 text "Pontiac"
token ,
token 4 text "car3",
token :
token 4 text "Pontiac"
token }
token }