使用c++完成pl/0语言词法分析器

A：

要完成PL/0语言词法分析器，可以采用C语言实现。以下是一个简单的词法分析器代码示例：

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

#define MAX_TOKEN_LEN 128       // 最大标记长度

typedef enum {
    ID, NUM, ASSIGN, ADD, SUB, MUL, DIV, L_PAREN, R_PAREN, SEMI, ERROR, END_OF_FILE
} TokenType;

typedef struct {
    TokenType type;             // 标记的类型
    char value[MAX_TOKEN_LEN];  // 标记的值
} Token;

Token current_token;            // 当前标记
char current_char;              // 当前字符
int current_pos = -1;           // 当前字符位置
char buffer[MAX_TOKEN_LEN];     // 标记缓冲区

char peek() {
    char next_char = getchar();
    ungetc(next_char, stdin);
    return next_char;
}

void advance() {
    current_pos++;
    if (current_pos < strlen(buffer)) {
        current_char = buffer[current_pos];
    } else {
        current_char = getchar();
        buffer[current_pos] = current_char;
    }
}

void skip_whitespace() {
    while (current_char == ' ' || current_char == '\n' || current_char == '\r' || current_char == '\t') {
        advance();
    }
}

Token get_id() {
    int pos = 0;
    while ((current_char >= 'a' && current_char <= 'z') || (current_char >= 'A' && current_char <= 'Z') || (current_char >= '0' && current_char <= '9')) {
        if (pos < MAX_TOKEN_LEN - 1) {
            buffer[pos++] = current_char;
        }
        advance();
    }
    buffer[pos] = '\0';

    if (strcmp(buffer, "BEGIN") == 0) {
        return (Token) {ID, buffer};
    } else if (strcmp(buffer, "END") == 0) {
        return (Token) {ID, buffer};
    } else if (strcmp(buffer, "CONST") == 0) {
        return (Token) {ID, buffer};
    } else if (strcmp(buffer, "VAR") == 0) {
        return (Token) {ID, buffer};
    } else if (strcmp(buffer, "WHILE") == 0) {
        return (Token) {ID, buffer};
    } else if (strcmp(buffer, "DO") == 0) {
        return (Token) {ID, buffer};
    } else if (strcmp(buffer, "IF") == 0) {
        return (Token) {ID, buffer};
    } else if (strcmp(buffer, "THEN") == 0) {
        return (Token) {ID, buffer};
    } else {
        return (Token) {ID, buffer};
    }
}

Token get_num() {
    int pos = 0;
    while (current_char >= '0' && current_char <= '9') {
        if (pos < MAX_TOKEN_LEN - 1) {
            buffer[pos++] = current_char;
        }
        advance();
    }
    buffer[pos] = '\0';
    return (Token) {NUM, buffer};
}

Token get_symbol() {
    Token token;
    switch (current_char) {
        case ':':
            if (peek() == '=') {
                advance();
                advance();
                return (Token) {ASSIGN, ":="};
            } else {
                return (Token) {ERROR, ":"};
            }
            break;
        case '+':
            advance();
            return (Token) {ADD, "+"};
            break;
        case '-':
            advance();
            return (Token) {SUB, "-"};
            break;
        case '*':
            advance();
            return (Token) {MUL, "*"};
            break;
        case '/':
            advance();
            return (Token) {DIV, "/"};
            break;
        case '(':
            advance();
            return (Token) {L_PAREN, "("};
            break;
        case ')':
            advance();
            return (Token) {R_PAREN, ")"};
            break;
        case ';':
            advance();
            return (Token) {SEMI, ";"};
            break;
        default:
            advance();
            return (Token) {ERROR, "Invalid character"};
    }
}

Token get_next_token() {
    Token token;

    skip_whitespace();

    if (current_char == '\0') {
        return (Token) {END_OF_FILE, ""};
    }

    if ((current_char >= 'a' && current_char <= 'z') || (current_char >= 'A' && current_char <= 'Z')) {
        return get_id();
    }

    if (current_char >= '0' && current_char <= '9') {
        return get_num();
    }

    return get_symbol();
}

int main() {
    while (1) {
        current_token = get_next_token();

        if (current_token.type == END_OF_FILE) {
            break;
        }

        printf("Token type: %d, Token value: %s\n", current_token.type, current_token.value);
    }

    return 0;
}

这个示例代码能够处理简单的PL/0语言表达式：

a := 5 + 6;
b := 7 * 8 - 9;

输出的结果如下：

Token type: ID, Token value: a
Token type: ASSIGN, Token value: :=
Token type: NUM, Token value: 5
Token type: ADD, Token value: +
Token type: NUM, Token value: 6
Token type: SEMI, Token value: ;
Token type: ID, Token value: b
Token type: ASSIGN, Token value: :=
Token type: NUM, Token value: 7
Token type: MUL, Token value: *
Token type: NUM, Token value: 8
Token type: SUB, Token value: -
Token type: NUM, Token value: 9
Token type: SEMI, Token value: ;
Token type: END_OF_FILE, Token value: