How to resolve the algorithm S-expressions step by step in the C programming language

Published on 7 June 2024 03:52 AM
#C

How to resolve the algorithm S-expressions step by step in the C programming language

Table of Contents

Problem Statement

S-Expressions   are one convenient way to parse and store data.

Write a simple reader and writer for S-Expressions that handles quoted and unquoted strings, integers and floats. The reader should read a single but nested S-Expression from a string and store it in a suitable datastructure (list, array, etc). Newlines and other whitespace may be ignored unless contained within a quoted string. “()”   inside quoted strings are not interpreted, but treated as part of the string. Handling escaped quotes inside a string is optional;   thus “(foo"bar)” maybe treated as a string “foo"bar”, or as an error. For this, the reader need not recognize “\” for escaping, but should, in addition, recognize numbers if the language has appropriate datatypes. Languages that support it may treat unquoted strings as symbols. Note that with the exception of “()"” (“\” if escaping is supported) and whitespace there are no special characters. Anything else is allowed without quotes. The reader should be able to read the following input and turn it into a native datastructure. (see the Pike, Python and Ruby implementations for examples of native data structures.) The writer should be able to take the produced list and turn it into a new S-Expression. Strings that don't contain whitespace or parentheses () don't need to be quoted in the resulting S-Expression, but as a simplification, any string may be quoted.

Let the writer produce pretty printed output with indenting and line-breaks.

Let's start with the solution:

Step by Step solution about How to resolve the algorithm S-expressions step by step in the C programming language

The code provided is a C program that implements a basic S-expression parser. S-expressions are a type of data structure used in the Lisp programming language and are composed of lists of symbols, strings, and other S-expressions.

The code defines an expr data type and a number of functions for parsing and printing S-expressions. The main function reads an input string and parses it into an expr data structure, then prints the parsed expression to the standard output.

Here is a more detailed description of the code:

  • The s_expr structure defines the expr data type, which stores the type, length, and buffer of an S-expression. The type can be one of the following:
    • S_NONE: A null expression
    • S_LIST: A list of expressions
    • S_STRING: A string
    • S_SYMBOL: A symbol
  • The whine() function prints an error message to the standard error output and exits the program. It is used to report parse errors.
  • The parse_string() function parses a string from an input string and returns an expr data structure representing the string. It handles escape sequences and quotes.
  • The parse_symbol() function parses a symbol from an input string and returns an expr data structure representing the symbol. It handles escape sequences, but does not allow quotes.
  • The append() function adds an expr to the end of a list expr.
  • The parse_list() function parses a list of expressions from an input string and returns an expr data structure representing the list. It handles nested lists and strings.
  • The parse_term() function parses a single expression from an input string and returns an expr data structure representing the expression. It can parse lists, strings, and symbols.
  • The print_expr() function prints an expr data structure to the standard output in a human-readable format.
  • The main() function reads an input string from the standard input and parses it into an expr data structure. It then prints the parsed expression to the standard output.

The example input string in the code is a valid S-expression that represents a list of two lists. The first list contains the symbols data, da\(\)ta, the string "quot\\ed data", the number 123, and the floating-point number 4.5. The second list contains the string "data", the symbol !@#, a nested list with the number 4.5, the string "(mo\"re", and the string "data".

The output of the program shows the parsed S-expression in a human-readable format.

Source code in the c programming language

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>

enum { S_NONE, S_LIST, S_STRING, S_SYMBOL };

typedef struct {
	int type;
	size_t len;
	void *buf;
} s_expr, *expr;

void whine(const char *s)
{
	fprintf(stderr, "parse error before ==>%.10s\n", s);
}

expr parse_string(const char *s, char **e)
{
	expr ex = calloc(sizeof(s_expr), 1);
	char buf[256] = {0};
	int i = 0;

	while (*s) {
		if (i >= 256) {
			fprintf(stderr, "string too long:\n");
			whine(s);
			goto fail;
		}
		switch (*s) {
		case '\\':
			switch (*++s) {
			case '\\':
			case '"':	buf[i++] = *s++;
					continue;

			default:	whine(s);
					goto fail;
			}
		case '"':	goto success;
		default:	buf[i++] = *s++;
		}
	}
fail:
	free(ex);
	return 0;

success:
	*(const char **)e = s + 1;
	ex->type = S_STRING;
	ex->buf = strdup(buf);
	ex->len = strlen(buf);
	return ex;
}

expr parse_symbol(const char *s, char **e)
{
	expr ex = calloc(sizeof(s_expr), 1);
	char buf[256] = {0};
	int i = 0;

	while (*s) {
		if (i >= 256) {
			fprintf(stderr, "symbol too long:\n");
			whine(s);
			goto fail;
		}
		if (isspace(*s)) goto success;
		if (*s == ')' || *s == '(') {
			s--;
			goto success;
		}

		switch (*s) {
		case '\\':
			switch (*++s) {
			case '\\': case '"': case '(': case ')':
					buf[i++] = *s++;
					continue;
			default:	whine(s);
					goto fail;
			}
		case '"':	whine(s);
				goto success;
		default:	buf[i++] = *s++;
		}
	}
fail:
	free(ex);
	return 0;

success:
	*(const char **)e = s + 1;
	ex->type = S_SYMBOL;
	ex->buf = strdup(buf);
	ex->len = strlen(buf);
	return ex;
}

void append(expr list, expr ele)
{
	list->buf = realloc(list->buf, sizeof(expr) * ++list->len);
	((expr*)(list->buf))[list->len - 1] = ele;
}

expr parse_list(const char *s, char **e)
{
	expr ex = calloc(sizeof(s_expr), 1), chld;
	char *next;

	ex->len = 0;

	while (*s) {
		if (isspace(*s)) {
			s++;
			continue;
		}

		switch (*s) {
		case '"':
			chld = parse_string(s+1, &next);
			if (!chld) goto fail;
			append(ex, chld);
			s = next;
			continue;
		case '(':
			chld = parse_list(s+1, &next);
			if (!chld) goto fail;
			append(ex, chld);
			s = next;
			continue;
		case ')':
			goto success;

		default:
			chld = parse_symbol(s, &next);
			if (!chld) goto fail;
			append(ex, chld);
			s = next;
			continue;
		}
	}

fail:
	whine(s);
	free(ex);
	return 0;

success:
	*(const char **)e = s+1;
	ex->type = S_LIST;
	return ex;
}

expr parse_term(const char *s, char **e)
{
	while (*s) {
		if (isspace(*s)) {
			s++;
			continue;
		}
		switch(*s) {
		case '(':
			return parse_list(s+1, e);
		case '"':
			return parse_string(s+1, e);
		default:
			return parse_symbol(s+1, e);
		}
	}
	return 0;
}

void print_expr(expr e, int depth)
{
#define sep() for(i = 0; i < depth; i++) printf("    ")
	int i;
	if (!e) return;


	switch(e->type) {
	case S_LIST:
		sep();
		puts("(");
		for (i = 0; i < e->len; i++)
			print_expr(((expr*)e->buf)[i], depth + 1);
		sep();
		puts(")");
		return;
	case S_SYMBOL:
	case S_STRING:
		sep();
		if (e->type == S_STRING) putchar('"');
		for (i = 0; i < e->len; i++) {
			switch(((char*)e->buf)[i]) {
			case '"':
			case '\\':
				putchar('\\');
				break;
			case ')': case '(':
				if (e->type == S_SYMBOL)
					putchar('\\');
			}

			putchar(((char*)e->buf)[i]);
		}
		if (e->type == S_STRING) putchar('"');
		putchar('\n');
		return;
	}
}

int main()
{
	char *next;
	const char *in = "((data da\\(\\)ta \"quot\\\\ed data\" 123 4.5)\n"
			" (\"data\" (!@# (4.5) \"(mo\\\"re\" \"data)\")))";

	expr x = parse_term(in, &next);

	printf("input is:\n%s\n", in);
	printf("parsed as:\n");
	print_expr(x, 0);
	return 0;
}


input is:
((data da\(\)ta "quot\\ed data" 123 4.5)
 ("data" (!@# (4.5) "(mo\"re" "data)")))
parsed as:
(
    (
        data
        da\(\)ta
        "quot\\ed data"
        123
        4.5
    )
    (
        "data"
        (
            !@#
            (
                4.5
            )
            "(mo\"re"
            "data)"
        )
    )
)

You may also check:How to resolve the algorithm First power of 2 that has leading decimal digits of 12 step by step in the C# programming language
You may also check:How to resolve the algorithm Sorting algorithms/Cocktail sort step by step in the Objeck programming language
You may also check:How to resolve the algorithm Ludic numbers step by step in the VBScript programming language
You may also check:How to resolve the algorithm Read entire file step by step in the KAP programming language
You may also check:How to resolve the algorithm Bioinformatics/Sequence mutation step by step in the Ruby programming language