111 lines
3.5 KiB
C
111 lines
3.5 KiB
C
// 简单逻辑表达式求值：支持 true/false 与 not/and/or，带错误检测
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// 规则：
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// - 仅包含单词 true, false, not, and, or 与空格；全部小写
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// - 优先级：not > and > or；同级从左到右（左结合）
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// - 输入为一行，输出为 true / false；如果语法或词法错误，输出 error
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#include <stdio.h>
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#include <string.h>
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#include <ctype.h>
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typedef enum { TK_TRUE, TK_FALSE, TK_AND, TK_OR, TK_NOT } Tok;
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// 读取一行到缓冲区，并去除尾部换行符
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static int read_line(char *buf, size_t sz) {
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if (!fgets(buf, (int)sz, stdin)) return 0;
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size_t n = strlen(buf);
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while (n && (buf[n-1] == '\n' || buf[n-1] == '\r')) buf[--n] = '\0';
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return 1;
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}
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// 词法：按字母分组，提取单词，映射到令牌；出错返回 -1
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static int tokenize(const char *s, Tok *out, int maxn) {
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int nt = 0;
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size_t i = 0, n = strlen(s);
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while (i < n) {
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while (i < n && isspace((unsigned char)s[i])) ++i;
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if (i >= n) break;
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if (!isalpha((unsigned char)s[i])) return -1; // 非法字符
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size_t j = i;
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while (j < n && isalpha((unsigned char)s[j])) ++j;
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size_t wlen = j - i;
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if (wlen == 4 && strncmp(s + i, "true", 4) == 0) {
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if (nt >= maxn) return -1;
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out[nt++] = TK_TRUE;
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} else if (wlen == 5 && strncmp(s + i, "false", 5) == 0) {
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if (nt >= maxn) return -1;
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out[nt++] = TK_FALSE;
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} else if (wlen == 3 && strncmp(s + i, "and", 3) == 0) {
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if (nt >= maxn) return -1;
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out[nt++] = TK_AND;
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} else if (wlen == 2 && strncmp(s + i, "or", 2) == 0) {
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if (nt >= maxn) return -1;
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out[nt++] = TK_OR;
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} else if (wlen == 3 && strncmp(s + i, "not", 3) == 0) {
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if (nt >= maxn) return -1;
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out[nt++] = TK_NOT;
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} else {
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return -1; // 未知单词
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}
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i = j;
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}
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return nt;
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}
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// 递归下降（迭代实现）语法：
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// unary := { not }* ( true | false )
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// and := unary { and unary }*
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// or := and { or and }*
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static int parse_unary(const Tok *toks, int nt, int *idx, int *ok) {
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int neg = 0;
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while (*idx < nt && toks[*idx] == TK_NOT) { ++neg; ++*idx; }
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if (*idx >= nt) { *ok = 0; return 0; }
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int val;
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if (toks[*idx] == TK_TRUE) { val = 1; }
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else if (toks[*idx] == TK_FALSE) { val = 0; }
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else { *ok = 0; return 0; }
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++*idx;
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if (neg & 1) val = !val;
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return val;
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}
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static int parse_and(const Tok *toks, int nt, int *idx, int *ok) {
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int lhs = parse_unary(toks, nt, idx, ok);
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if (!*ok) return 0;
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while (*idx < nt && toks[*idx] == TK_AND) {
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++*idx;
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int rhs = parse_unary(toks, nt, idx, ok);
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if (!*ok) return 0;
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lhs = (lhs && rhs);
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}
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return lhs;
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}
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static int parse_or(const Tok *toks, int nt, int *idx, int *ok) {
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int lhs = parse_and(toks, nt, idx, ok);
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if (!*ok) return 0;
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while (*idx < nt && toks[*idx] == TK_OR) {
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++*idx;
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int rhs = parse_and(toks, nt, idx, ok);
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if (!*ok) return 0;
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lhs = (lhs || rhs);
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}
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return lhs;
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}
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int main(void) {
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char line[512];
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if (!read_line(line, sizeof(line))) return 0;
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Tok toks[300];
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int nt = tokenize(line, toks, 300);
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if (nt <= 0) { printf("error\n"); return 0; }
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int idx = 0, ok = 1;
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int result = parse_or(toks, nt, &idx, &ok);
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if (!ok || idx != nt) { printf("error\n"); return 0; }
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printf(result ? "true\n" : "false\n");
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return 0;
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} |