string的标准库练习

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#include <stdio.h>#include <assert.h>size_t strlen(const char *s);void *memset(void *src, int n, size_t count);char *strstr(const char *s1, const char *s2);char *strcat(char *des, const char *src);char *strncat(char *des, const char *src, size_t n);int strcmp(const char *s1, const char *s2);int strncmp(const char *s1, const char *s2, size_t n);char *mystrcpy(char *des, const char *src);char *mystrncpy(char *des, const char *src ,size_t n);size_t strlen(const char *s)//计算字符串长度{if( s == NULL)return 0;size_t length = 0;while(*s){length ++;//长度加一s ++;//地址加一}return length;}void *memset(void *src, int c, size_t count)//第二参数为整形，原因为大多开发人员填入为0，转换成字符就是'\0'，也就是终结符，如果是字符参数，不可以要用户填写'\0'吧！！！{assert(src != NULL);//assert是出错提醒，在BUGED有效，在relseas无效，当有两个条件时，无法识别哪个出错char *tmp = (char*)src;//取得首地址while(count--){*tmp = (char)c; tmp ++;}return tmp;}char *strstr(const char *s1, const char *s2)//从字符串找存在指定的小字符串{int i = 0;assert(s1 != NULL);if (s2 != NULL){while( *s1 ){for(i = 0; *(s1+i) == *(s2+i); i++){if( !*(s2+i+1) )return (char*)s1;//找到符合条件，字符串1返回找到值以后的全部字符}s1++;}return NULL;//搜索完未找到符合的返回NULL；}else{return (char*)s1;}}char *strcat(char *des, const char *src) //合并字符串{char *address = des;assert( des != NULL );assert( src != NULL );while(*des)//取得第一个字符串最后一位地址{des++;}while(*des++ = *src++)//将第二个字符串存储区内容赋给第一个字符串后面的//以上这句可以理解为 do {*des++ = *src; src++; while( *des != '\0' ) }{NULL;}return address;}char *strncat(char *des, const char *src, size_t n){size_t des_len = strlen(des);size_t i;for (i = 0; i < n && src[i] != '\0'; i++){des[des_len + i] = src[i];}des[des_len + i] = '\0';return des;}int strcmp(const char *s1, const char *s2)//比较字符串大小{while( *s1 == *s2 ){if( *s1 == '\0' )return 0;s1 ++;s2 ++;}return *s1 - *s2;}int strncmp(const char *s1, const char *s2, size_t n){while( *s1 == *s2 && n > 0){if( *s1 == '\0' )return 0;s1 ++;s2 ++;n --;}return *s1 - *s2;}char *mystrcpy(char *des, const char *src)//注意这里可能和系统中strcpy冲突，最好换个函数名称。{assert(des != NULL);assert(src != NULL);char *addr = des;while(*src){*des = *src;des++;src++;}*des = '\0';return addr;}char *mystrncpy(char *des, const char *src ,size_t n)//这里和strcpy存在相同问题{assert(des != NULL);assert(src != NULL);char *addr = des;int i = 0;for(i = 0; i < n && *src != '\0'; i++){*des = *src;des++;src++;}*des = '\0';return addr;}int main(){char x[20]; char y[20]={"abcxyz"};char *p_x = y;memset(x, 0, sizeof(x));char *x1=strstr(y,"xy");printf("%s\n",x1);int i = 0;for(i = 0; i<5; i++)x[i] = 'a' + i;printf("x[] = %s\n",x);printf("strcat = %s\n",strcat(x, y));printf("strncat = %s\n",strncat(x, y,1));printf("strcmp = %d\n",strcmp("abd", "abd"));printf("strncmp = %d\n",strncmp("abc", "abd", 3));printf("x[] = %s\n",x);printf("mystrcpy = %s\n",mystrcpy(x,"desdd"));printf("mystrncpy = %s\n",mystrncpy(x,"desss",2));return 0;}
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