经典编程900例（c语言）（第二十九篇）

例353：字符串比较函数strncmp()

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

int main(int argc, char const *argv[])
{
    // int strncmp(const char *str1, const char *str2, size_t n)
    // strncmp比较两个字符串的前n个字符
    // 如果返回值 < 0，则表示 str1 小于 str2。
    // 如果返回值 > 0，则表示 str2 小于 str1。
    // 如果返回值 = 0，则表示 str1 等于 str2。
    printf("Comparing 3 letters Abc with Abc %d\n", strncmp("Abc", "Abc", 3));  //Comparing 3 letters Abc with Abc 0
    printf("Comparing 3 letters abc with Abc %d\n", strncmp("abc", "Abc", 3));  // Comparing 3 letters abc with Abc 1
    printf("Comparing 3 letters abcd with abc %d\n", strncmp("abcd", "abc", 3));// Comparing 3 letters abcd with abc 0
    printf("Comparing 5 letters Abc with Abcd %d\n", strncmp("Abc", "Abcd", 5));// Comparing 5 letters Abc with Abcd -1

    return 0;
}

例354：自定义函数用来查找字符串中字符的位置

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

/**
 * 返回字符串中指定字符的指针
 */
char *strrchr(const char *string, int letter)
{
    char *ptr = NULL;

    while (*string)
    {
        if (*string == letter)
        ptr = string;
        string++;
    }
    return(ptr);
}

int main(int argc, char const *argv[])
{
    char title[64] = "1001 C/C++ Tips!";
    char *ptr;

    if (ptr = strrchr(title, 'C'))
        printf("Rightmost occurrence of C is at offset %d\n", ptr - title);
    else
        printf("Character not found\n");
    return 0;
}

例355：自定义函数用来反转字符串

#include <stdio.h>

/**
 * 反转字符串, 返回字符串首地址
 */
char *strrev(char *string)
{
    char *original = string;
    char *forward = string;
    char temp;

    while (*string)
        string++;

    while (forward < string) 
    {
        temp = *(--string);
        *string = *forward; 
        *forward++ = temp;
    }
    return(original);
}

int main(int argc, char const *argv[])
{
    // 想想这样调用能输出吗？
    printf(strrev("A"));
    printf("\n");

    printf(strrev(""));
    printf("\n");

    printf(strrev("ABC"));
    printf("\n");

    printf(strrev("AABBCCDDEE"));
    return 0;
}

例356：自定义函数用来查找字符串字符的下标（最后一次出现的）

#include <stdio.h>

/**
 * 返回字符串中指定字符的下标
 */
int strr_index(char *string, int letter)
{
    char *original = string;
    char *ptr = NULL;

    while (*string)
    {
        if (*string == letter)
            ptr = string;

        string++;
    }

    return((ptr) ? ptr-original : string-original);
}

int main(int argc, char const *argv[])
{
    printf("Location of C is %d\n", strr_index("1001 C/C++", 'C'));
    printf("Location of x is %d\n", strr_index("1001 C/C++", 'x'));
    return 0;
}

例357：自定义函数全部用单个字符给字符串赋值

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

/**
 * 将字符串用字符letter赋值
 */
char *strset(char *string, int letter)
{
    char *original = string;

    while (*string)
        *string++ = letter;

    return(original);
}

int main(int argc, char const *argv[])
{
    char name[] = "22222";

    printf(strset(name, '1'));
    return 0;
}

例358：字符串查找下标函数strspn()

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

int main(int argc, char const *argv[])
{
    // size_t strspn(const char *str1, const char *str2)
    // 检索字符串 str1 中第一个不在字符串 str2 中出现的字符下标
    printf("Searching for Abc in AbcDef %d\n", strspn("AbcDef", "Abc"));// Searching for Abc in AbcDef 3
    printf("Searching for cbA in AbcDef %d\n", strspn("AbcDef", "cbA"));// Searching for cbA in AbcDef 3
    printf("Searching for Def in AbcAbc %d\n", strspn("AbcAbc", "Def"));// Searching for Def in AbcAbc 0
    return 0;
}

例359：查找子串函数strstr()

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

int main(int argc, char const *argv[])
{
    // char *strstr(const char *haystack, const char *needle) 
    // 在字符串haystack中查找第一次出现字符串needle的位置，不包含终止符'\0'
    printf("Looking for Abc in AbcDef %s\n", (strstr("AbcDef", "Abc")) ? "Found" : "Not found");// Looking for Abc in AbcDef Found
    printf("Looking for Abc in abcDef %s\n", (strstr("abcDef", "Abc")) ? "Found" : "Not found");// Looking for Abc in abcDef Not found
    printf("Looking for Abc in AbcAbc %s\n", (strstr("AbcAbc", "Abc")) ? "Found" : "Not found");// Looking for Abc in AbcAbc Found
    return 0;
}

例360：字符串转换函数strxfrm

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

int main(int argc, char const *argv[])
{
    char buffer[64] = "Jamsa's 1001 C/C++ Tips";
    char target[64];
    int length;

    // size_t strxfrm(char *dest, const char *src, size_t n) 
    // 转换字符串src的前n个字符，并把它们放置在字符串dest中
    length = strxfrm(target, buffer, sizeof(buffer));

    printf("Length %d Target %s Buffer %s\n", length, target, buffer);// Length 23 Target Jamsa's 1001 C/C++ Tips Buffer Jamsa's 1001 C/C++ Tips
    return 0;
}

例361：大写字母转换小写字母函数tolower()

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

int main(int argc, char const *argv[])
{
    char string[] = "Jamsa's 1001 C & C++ Tips";
    int i;

    // 把字符转换成小写字母,非字母字符不做出处理
    for (i = 0; string[i]; i++)
        putchar(tolower(string[i]));
    putchar('\n');

    for (i = 0; string[i]; i++)
        putchar(_tolower(string[i]));
    putchar('\n');
    
    return 0;
}

例362：小写字母转换大写字母函数toupper()

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

int main(int argc, char const *argv[])
{
    char string[] = "Jamsa's 1001 C & C++ Tips";
    int i;

    // 把字符转换成大写字母,非字母字符不做出处理
    for (i = 0; string[i]; i++)
        putchar(toupper(string[i]));
    putchar('\n');

    for (i = 0; string[i]; i++)
        putchar(_toupper(string[i]));
    putchar('\n');
    
    return 0;
}

例363：判断文件读写权限的函数access

#include <stdio.h>
#include <io.h>

/**
 * int access(const char *filenpath, int mode)
 * 确定文件或文件夹的访问权限。即，检查某个文件的存取方式，比如说是只读方式、只写方式等。
 * 如果指定的存取方式有效，则函数返回0，否则函数返回-1
 * 00 只存在
 * 02 写权限
 * 04 读权限
 * 06 读和写权限
 */

int main(int argc, char const *argv[])
{
    int access_mode;

    
    access_mode = access(argv[1], 0);

    if (access_mode)
        printf("File %s does not exist\n");
    else
    {
    	// 判断第1个程序参数指定的文件是否可写
        access_mode = access(argv[1], 2);
        if (access_mode)
            printf("File cannot be written\n");
        else
            printf("File can be written\n");

		// 判断第1个程序参数指定的文件是否可读
        access_mode = access(argv[1], 4);
        if (access_mode)
            printf("File cannot be read\n");
        else
            printf("File can be read\n");
            
		// 判断第1个程序参数指定的文件是否可读写
        access_mode = access(argv[1], 6);
        if (access_mode)
            printf("File cannot be read/write\n");
        else
            printf("File can be read/write\n");
    }
    return 0;
}