strlen,strcpy,strcat,strcmp,strchr 的模拟实现

c语言中对于字符串的处理是非常不方便的。因为C语言中字符串本质上是一个字符数组，一个常量指针。

模拟实现一下字符串的这些常用函数，加深对数组及常量指针的理解：

//// Created by cat on 2017/10/22.//#include <stdio.h>/*对字符串函数的模拟实现*/int strlen(const char *__s);int strcmp(const char *__s1, const char *__s2);char *strcat(char *dst, const char *src);char *strcpy(char *dst, const char *src);char *strchr(const char *__s, int __c);char *strstr(const char *__big, const char *__little);int main(int argc, char const *argv[]){    printf("strlen(Hello) = %d\n", strlen("Hello"));    printf("strcmp(abc,abc) = %d\n", strcmp("abc", "abc"));    char a[15] = "hello";    char b[] = "world";    printf("strcat = %s\n",strcat(a,b) );    char c[] = "Hello";    char d[] = "world";    printf("strcpy = %s\n", strcpy(c,d));    char s[] = "python,hello";    char p = 't';    printf("strchr = %s\n", strchr(s,p));    return 0;}char *strstr(const char *__big, const char *__little){    // todo    return NULL;}char *strchr(const char *s1, int c){    int i=0;    while(1){        // printf("s1 = %c\n", *(s1+i));        if (*(s1+i)==c ){            return (char *)(s1+i); // 这里必须强制转换一下，否则运行失败        }        if(*(s1+i)=='\0'){            break;        }        i +=1;    }    return NULL;}char *strcpy(char *s1, const char *s2){    int i=0;    while(1){        char t  = s2[i];        if('\0' == t){            break;        }        s1[i] = s2[i];        i++;    }    return s1;}char *strcat(char *s1, const char *s2){    int s1len = strlen(s1);    int i=0;    while(1){        char t  = s2[i];        if('\0' == t){            break;        }        s1[s1len + i] = s2[i];        i++;    }    return s1;}int strcmp(const char *s1, const char *s2){    int totals1 = 0;    int i=0;    while(1){        char t  = s1[i];        if('\0' == t){            break;        }        totals1 += (int)t;        i++;    }    int totals2 = 0;    i = 0;    while(1){        char t = s2[i];        if('\0' == t){            break;        }        totals2 += (int)t;        i++;    }    return totals1 - totals2;}int strlen(const char *str){    int len = 0;    int i = 0;    while(1){        char t = str[i];        if( '\0'==t ){            break;        }        // printf("t = %c ", t );        i++;        len  = i;    }    return len;}/*console:strlen(Hello) = 5strcmp(abc,abc) = 0strcat = helloworldstrcpy = worldstrchr = thon,hello[Finished in 0.4s]*/