Flex：使用文本文件作为输入流

使用文本文件作为输入流是一种灵活且方便的方式，可以帮助我们在程序中处理大量的文本数据。在本文中，我们将探讨如何使用Flex来读取文本文件并将其作为输入流进行处理。

c
%{
#include 
%}
%%
. { printf("%s\n", yytext); }
%%
int main(int argc, char *argv[]) {
    if (argc < 2) {
        printf("Usage: %s \n", argv[0]);
        return 1;
    }
    
    FILE *input_file = fopen(argv[1], "r");
    if (!input_file) {
        printf("Failed to open input file: %s\n", argv[1]);
        return 1;
    }
    
    yyin = input_file;
    yylex();
    
    fclose(input_file);
    return 0;
}

c
%{
#include 
#include 
#include 
#define MAX_WORD_LENGTH 100
typedef struct {
    char word[MAX_WORD_LENGTH];
    int count;
} WordCount;
WordCount *word_counts = NULL;
int num_words = 0;
int compare_word_counts(const void *a, const void *b) {
    WordCount *wc1 = (WordCount *)a;
    WordCount *wc2 = (WordCount *)b;
    return wc2->count - wc1->count;
}
void add_word_count(const char *word) {
    for (int i = 0; i < num_words; i++) {
        if (strcmp(word, word_counts[i].word) == 0) {
            word_counts[i].count++;
            return;
        }
    }
    
    word_counts = realloc(word_counts, (num_words + 1) * sizeof(WordCount));
    strcpy(word_counts[num_words].word, word);
    word_counts[num_words].count = 1;
    num_words++;
}
void print_top_words(int n) {
    qsort(word_counts, num_words, sizeof(WordCount), compare_word_counts);
    
    for (int i = 0; i < n && i < num_words; i++) {
        printf("%s: %d\n", word_counts[i].word, word_counts[i].count);
    }
}
%}
letter [a-zA-Z]
word {letter}({letter}|[0-9])*
%%
{word} { add_word_count(yytext); }
.|\n
%%
int main(int argc, char *argv[]) {
    if (argc < 3) {
        printf("Usage: %s  \n", argv[0]);
        return 1;
    }
    
    FILE *input_file = fopen(argv[1], "r");
    if (!input_file) {
        printf("Failed to open input file: %s\n", argv[1]);
        return 1;
    }
    
    yyin = input_file;
    yylex();
    
    int num_top_words = atoi(argv[2]);
    print_top_words(num_top_words);
    
    fclose(input_file);
    return 0;
}