2019 1-22

#include <stdio.h>
static int string_length(const char*p);
//void strcpy(char *p1, const char *p2);
int strcmp(char *m1, char *m2);
char *strcat(char *n1, char *n2, char *n3);



int main()
{
    char a[]="hello";
    char b[]="helll";
    char s[]="";
    char *p2=a;
    char *p=b;
    int c;
    long int length;
    length=string_length(p2);
    printf("length:%ld\n", length);
    
    strcpy(p, p2);
    printf("%s\n", p);
    c=strcmp(a,  b);
    printf("%d\n", c);
    strcat(a, b, s);
    printf("%s\n", s);
    return 0;
}

//输出字符串的字长
static int string_length(const char*p)
{
    int length=0;
    while(*p++)
        length++;

    return length;

}


//复制
 void strcpy(char *p1, const char *p2)
{
    while((*p1=*p2)!='\0'){
    
        *p2++;
        *p1++;
    }
}
    

//strcmp:
int strcmp(char *m1, char *m2)
{
    while(*m1==*m2){
        if(*m1=='\0')
            return 0;
        else {
            *m1++;
            *m2++;
        }

    }
    return (*m1-*m2);
    
}

//strcat
char *strcat(char *n1, char *n2, char *n3)
{    
    static int i=0;
    while(*n1!='\0'){
        *(n3+i)=*n1;
        i++;
        *n1++;
    }
    do {
        *(n3+i)=*n2;
        i++;
        *n2++;
    
    }while(*n2!='\0');
}