如何计算一个字母在字符串中出现了多少次？C程序设计

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

int count_letter(const char *str, char letter) {
    int count = 0;
    for (int i = 0; i < strlen(str); i++) {
        if (str[i] == letter) {
            count++;
        }
    }
    return count;
}

int main() {
    const char *str = "Hello, World!";
    char letter = 'l';
    int count = count_letter(str, letter);
    printf("The letter '%c' appears %d times in the string.\n", letter, count);
    return 0;
}

#include <ctype.h>

int count_letter_case_insensitive(const char *str, char letter) {
    int count = 0;
    char lower_letter = tolower(letter);
    for (int i = 0; i < strlen(str); i++) {
        if (tolower(str[i]) == lower_letter) {
            count++;
        }
    }
    return count;
}

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

int count_letter_case_insensitive(const char *str, char letter) {
    int count = 0;
    char lower_letter = tolower(letter);
    for (int i = 0; i < strlen(str); i++) {
        if (tolower(str[i]) == lower_letter) {
            count++;
        }
    }
    return count;
}

int main() {
    const char *str = "Hello, World!";
    char letter = 'L';
    int count = count_letter_case_insensitive(str, letter);
    printf("The letter '%c' appears %d times in the string (case insensitive).\n", letter, count);
    return 0;
}