【C语言 字符函数和字符串函数】—— 文本数据的奇幻加工坊，代码世界的魔法编织者

int isalpha ( int c );

int main() {
    char ch = 'A';
    if (isalpha(ch)) {
        printf("%c is a letter.\n", ch);
    }
    else {
        printf("%c is not a letter.\n", ch);
    }
    return 0;
}

A is a letter.

int isdigit ( int c );

int main() {
    char ch = '5';
    if (isdigit(ch)) {
        printf("%c is a digit.\n", ch);
    }
    else {
        printf("%c is not a digit.\n", ch);
    }
    return 0;
}

5 is a digit.

int islower ( int c );

int main() {
    char ch = 'b';
    if (islower(ch)) {
        printf("%c is a lower case letter.\n", ch);
    }
    else {
        printf("%c is not a lower case letter.\n", ch);
    }
    return 0;
}

b is a lower case letter.

int islower ( int c );

int main() {
    char ch = 'F';
    if (isupper(ch)) {
        printf("%c is an upper case letter.\n", ch);
    }
    else {
        printf("%c is not an upper case letter.\n", ch);
    }
    return 0;
}

F is an upper case letter.

int tolower ( int c );

int main() {
    char ch = 'G';
    char lowerCh = tolower(ch);
    printf("The lower case of %c is %c.\n", ch, lowerCh);
    return 0;
}

The lower case of G is g.

int toupper ( int c );

int main() {
    char ch = 'h';
    char upperCh = toupper(ch);
    printf("The upper case of %c is %c.\n", ch, upperCh);
    return 0;
}

The upper case of h is H.

int strcmp ( const char * str1, const char * str2 );

int main() {
    char s1[] = "apple";
    char s2[] = "banana";
    int result = strcmp(s1, s2);
    if (result < 0) {
        printf("%s is less than %s.\n", s1, s2);
    }
    else if (result == 0) {
        printf("%s is equal to %s.\n", s1, s2);
    }
    else {
        printf("%s is greater than %s.\n", s1, s2);
    }
    return 0;
}

apple is less than banana.

int my_strcmp(const char* str1, const char* str2)
{
    int ret = 0;
    assert(str1 != NULL);
    assert(str2 != NULL);
    while (*str1 == *str2)
    {
        if (*str1 == '\0')
            return 0;
        str1++;
        str2++;
    }
    return *str1 - *str2;
}

int strncmp ( const char * str1, const char * str2, size_t num );

int main() {
    char s1[] = "apple";
    char s2[] = "applet";
    int result = strncmp(s1, s2, 5);
    if (result < 0) {
        printf("%s is less than %s in the first 5 characters.\n", s1, s2);
    }
    else if (result == 0) {
        printf("%s is equal to %s in the first 5 characters.\n", s1, s2);
    }
    else {
        printf("%s is greater than %s in the first 5 characters.\n", s1, s2);
    }
    return 0;
}

apple is equal to applet in the first 5 characters.

char * strcpy ( char * destination, const char * source );

int main() {
    char source[] = "Hello, World!";
    char destination[20];
    strcpy(destination, source);
    printf("Copied string: %s\n", destination);
    return 0;
}

Copied string: Hello, World!

char* my_strcpy(char* dest, const char* src)
{
    char* ret = dest;
    assert(dest != NULL);
    assert(src != NULL);
    while ((*dest++ = *src++))
    {
        ;
    }
    return ret;
}

char * strncpy ( char * destination, const char * source, size_t num );

int main() {
    char source[] = "Hello, World!";
    char destination[10];
    strncpy(destination, source, 5);
    destination[5] = '\0';  // 确保目标字符串以'\0'结尾
    printf("Copied string: %s\n", destination);
    return 0;
}

Copied string: Hello

char * strcat ( char * destination, const char * source );

int main() {
    char destination[20] = "Hello, ";
    char source[] = "World!";
    strcat(destination, source);
    printf("Concatenated string: %s\n", destination);
    return 0;
}

Concatenated string: Hello, World!

char* my_strcat(char* dest, const char* src)
{
	char* ret = dest; 
	assert(dest != NULL);
	assert(src != NULL);
	while (*dest)
	{
		dest++;
	}
	while ((*dest++ = *src++))
	{
		;
	}
	return ret;
}

char * strncat ( char * destination, const char * source, size_t num );

int main() {
    char destination[20] = "Hello, ";
    char source[] = "World!";
    strcat(destination, source);
    printf("Concatenated string: %s\n", destination);
    return 0;
}

Concatenated string: Hello, World!

size_t strlen ( const char * str );

int main() {
    char s[] = "Hello, World!";
    size_t length = strlen(s);
    printf("The length of the string is %zu.\n", length);
    return 0;
}

The length of the string is 13.

int my_strlen(const char* str)
{
	int count = 0;
	assert(str);
	while (*str)
	{
		count++;
		str++;
	}
	return count;
}

int my_strlen(const char* str)
{
	assert(str);
	if (*str == '\0')
		return 0;
	else
		return 1 + my_strlen(str + 1);
}

int my_strlen(char* str)
{
	assert(str);
	char* p = str;
	while (*p != '\0')
		p++;
	return p - str;
}

const char * strchr ( const char * str, int character );  
char * strchr (char * str, int character );

int main() {
    char str[] = "Hello, World!";
    char* result = strchr(str, 'l');
    if (result != NULL) {
        printf("find 'l'，position is：%ld\n", result - str);
    }
    else {
        printf("not find 'l'\n");
    }
    return 0;
}

find 'l'，position is：2

const char * strrchr ( const char * str, int character );    
char * strrchr ( char * str, int character );

int main() {
    char str[] = "Hello, World!";
    char* result = strrchr(str, 'o');
    if (result != NULL) {
        printf("find 'o'，the position is ：%ld\n", result - str);
    }
    else {
        printf("not find 'o'\n");
    }
    return 0;
}

find 'o'，the position is ：8

const char * strstr ( const char * str1, const char * str2 );
char * strstr (char * str1, const char * str2 );

int main() {
    char str1[] = "I love programming in C.";
    char str2[] = "programming";
    char* result = strstr(str1, str2);
    if (result != NULL) {
        printf("find str2，the position is ：%ld\n", result - str1);
    }
    else {
        printf("not find str2\n");
    }
    return 0;
}

find str2，the position is ：7

char* strstr(const char* str1, const char* str2)
{
	char* cp = (char*)str1;
	char* s1, * s2;
	if (!*str2)
		return((char*)str1);
	while (*cp)
	{
		s1 = cp;
		s2 = (char*)str2;
		while (*s1 && *s2 && !(*s1 - *s2))
			s1++, s2++;
		if (!*s2)
			return(cp);
		cp++;
	}
	return(NULL);
}

char * strtok ( char * str, const char * delimiters );

int main() {
    char str[] = "Hello,World,This,is,C";
    const char delim[] = ","; // 分隔符
    char* token;
    // 第一次调用，传入待分割的字符串
    token = strtok(str, delim);
    while (token != NULL) {
        printf("tokens: %s\n", token);
        token = strtok(NULL, delim); // 继续分割剩余字符串
    }
    return 0;
}

tokens: Hello
tokens: World
tokens: This
tokens: is
tokens: C

int main() {
    char str[] = "Hello;World,This:is-C";
    const char delim[] = ";,:-"; // 多个分隔符
    char* token;
    token = strtok(str, delim);
    while (token != NULL) {
        printf("tokens: %s\n", token);
        token = strtok(NULL, delim);
    }
    return 0;
}

tokens: Hello
tokens: World
tokens: This
tokens: is
tokens: C

char * strerror ( int errnum );

int main() {
    // 尝试打开一个不存在的文件
    FILE* fp = fopen("nonexistent.txt", "r");  
    if (fp == NULL) {
        // 打印错误代码和错误描述
        printf("错误代码: %d\n", errno);
        printf("错误信息: %s\n", strerror(errno));
    }
    return 0;
}

错误代码: 2
错误信息: No such file or directory

int main() {
    // 遍历 0-133 范围内的错误代码
    for (int i = 0; i < 134; i++) {
        printf("错误码 %d: %s\n", i, strerror(i));
    }
    return 0;
}

错误码 0: No error
错误码 1: Operation not permitted
错误码 2: No such file or directory
错误码 3: No such process
错误码 4: Interrupted function call
错误码 5: Input/output error
错误码 6: No such device or address
错误码 7: Arg list too long
错误码 8: Exec format error
错误码 9: Bad file descriptor
错误码 10: No child processes
错误码 11: Resource temporarily unavailable
错误码 12: Not enough space
错误码 13: Permission denied
错误码 14: Bad address
错误码 15: Unknown error
错误码 16: Resource device
错误码 17: File exists
错误码 18: Improper link
错误码 19: No such device
错误码 20: Not a directory
错误码 21: Is a directory
错误码 22: Invalid argument
错误码 23: Too many open files in system
错误码 24: Too many open files
错误码 25: Inappropriate I/O control operation
错误码 26: Unknown error
错误码 27: File too large
错误码 28: No space left on device
错误码 29: Invalid seek
错误码 30: Read-only file system
错误码 31: Too many links
错误码 32: Broken pipe
错误码 33: Domain error
错误码 34: Result too large
错误码 35: Unknown error
错误码 36: Resource deadlock avoided
错误码 37: Unknown error
错误码 38: Filename too long
错误码 39: No locks available
错误码 40: Function not implemented
错误码 41: Directory not empty
错误码 42: Illegal byte sequence
错误码 43: Unknown error
错误码 44: Unknown error
错误码 45: Unknown error
错误码 46: Unknown error
错误码 47: Unknown error
错误码 48: Unknown error
错误码 49: Unknown error
错误码 50: Unknown error
错误码 51: Unknown error
错误码 52: Unknown error
错误码 53: Unknown error
错误码 54: Unknown error
错误码 55: Unknown error
错误码 56: Unknown error
错误码 57: Unknown error
错误码 58: Unknown error
错误码 59: Unknown error
错误码 60: Unknown error
错误码 61: Unknown error
错误码 62: Unknown error
错误码 63: Unknown error
错误码 64: Unknown error
错误码 65: Unknown error
错误码 66: Unknown error
错误码 67: Unknown error
错误码 68: Unknown error
错误码 69: Unknown error
错误码 70: Unknown error
错误码 71: Unknown error
错误码 72: Unknown error
错误码 73: Unknown error
错误码 74: Unknown error
错误码 75: Unknown error
错误码 76: Unknown error
错误码 77: Unknown error
错误码 78: Unknown error
错误码 79: Unknown error
错误码 80: Unknown error
错误码 81: Unknown error
错误码 82: Unknown error
错误码 83: Unknown error
错误码 84: Unknown error
错误码 85: Unknown error
错误码 86: Unknown error
错误码 87: Unknown error
错误码 88: Unknown error
错误码 89: Unknown error
错误码 90: Unknown error
错误码 91: Unknown error
错误码 92: Unknown error
错误码 93: Unknown error
错误码 94: Unknown error
错误码 95: Unknown error
错误码 96: Unknown error
错误码 97: Unknown error
错误码 98: Unknown error
错误码 99: Unknown error
错误码 100: address in use
错误码 101: address not available
错误码 102: address family not supported
错误码 103: connection already in progress
错误码 104: bad message
错误码 105: operation canceled
错误码 106: connection aborted
错误码 107: connection refused
错误码 108: connection reset
错误码 109: destination address required
错误码 110: host unreachable
错误码 111: identifier removed
错误码 112: operation in progress
错误码 113: already connected
错误码 114: too many symbolic link levels
错误码 115: message size
错误码 116: network down
错误码 117: network reset
错误码 118: network unreachable
错误码 119: no buffer space
错误码 120: no message available
错误码 121: no link
错误码 122: no message
错误码 123: no protocol option
错误码 124: no stream resources
错误码 125: not a stream
错误码 126: not connected
错误码 127: state not recoverable
错误码 128: not a socket
错误码 129: not supported
错误码 130: operation not supported
错误码 131: Unknown error
错误码 132: value too large
错误码 133: owner dead

void perror ( const char * str );

int main() {
    FILE* file = fopen("nonexistent.txt", "r");
    if (file == NULL) {
        perror("文件打开失败"); // 输出错误信息
    }
    return 0;
}

文件打开失败: No such file or directory

int main() {   
    // 第一次错误：打开不存在的文件
    FILE* file = fopen("nonexistent.txt", "r");
    if (file == NULL) {
        perror("第一次错误");
    }// 第二次错误：写入只读文件
    file = fopen("/etc/passwd", "r"); // 只读文件
    if (file != NULL) {
        if (fputc('A', file) == EOF) {
            perror("第二次错误");
        }
        fclose(file);
    }
    return 0;
}

第一次错误: No such file or directory
第二次错误: Bad file descriptor

int main() {
    FILE* file = fopen("nonexistent.txt", "r");
    if (file == NULL) {
        // 使用 perror
        perror("perror 输出");
        // 使用 strerror
        printf("strerror 输出: %s\n", strerror(errno));
    }
    return 0;
}

perror 输出: No such file or directory
strerror 输出: No such file or directory