C language implements StrCmp, strstr, strcat, strcpy

> strtok— Cut string
> strcmp— Compare string length
> strstr— Look for substrings in the string source string
> strcat— Append string
> strcpy— String copy
> strtok— Cut string

The following are the codes and points needing attention

1. strcmp

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
#include <assert.h>
//assert Make sure the pointer is meaningful , Because you need to dereference 
// String comparison function -- Compare string length 
// imitation strcmp function , Three return values ,>0,<0,=0;
//int my_strcmp(const char* str1, const char* str2)
//{
    
// assert(str1 && str2);
// while ( *str1==*str2 )
// {
    
// if (*str1 == '\0')
// {
    
// return 0;
// }
// str1++;
// str2++;
// }
// return (*str1 - *str2);
//}
//int main()
//{
    
// char a[] = "abcdef";
// char b[] = "abcde";
// printf("%d",my_strcmp(a, b));
// return 0;
//}

2. strstr

// 
// Find substring function -----------strstr
// function ： Find the substring in the string 
// principle ： Find the substring by comparing the corresponding characters of the string 
// Return value ： Return the substring address after finding , No return found NULL
// Consider two situations ： One match （ The pointer goes through the source string ） And multiple matches （ The pointer walks multiple times in the source string , The pointer of the substring also returns to the starting point many times ）
//char* my_strstr(const char* str1, const char* str2)
//{
    
// assert(str1 && str2);
// const char* s1 = str1;
// const char* s2 = str2;
// const char* p = str1;
//
// while (*p)
// {
    
// s1 = p;// When matching multiple times p The position of the source string will go all the way to the right , until \0
// s2 = str2;// When matching many times str2 To return to the original position 
// while (*s1 != '\0' && *s2 != '\0' && *s1 == *s2)//bbbc( The source string )
// { //bbc（ Substring ） The situation of 
// s1++;
// s2++;
// }
// if (*s2 == '\0')// It means that the substring has been found , Exists in the source string 
// {
    
// return (char*)p;
// }
// p++;//-- It means that the substring is not finished , It is because the third judgment condition does not meet the jump cycle , here 
// //- hold p Point to the next character , You can retrieve multiple matches 
// }
// return NULL;// The source string has gone , No substring found yet 
//}
//
//int main()
//{
    
// char email[] = "aaaabc";
// char substr[] = "abc";
// char* ret = my_strstr(email, substr);
// //char arr1[] = "abcdef";
// //char arr2[] = "def";
// //char* ret = my_strstr(arr1, arr2);
//
// if (ret == NULL)
// {
    
// printf(" Substring does not exist \n");
// }
// else
// {
    
// printf("%s\n", ret);
// }
// return 0;
//}

3. strcat

//------------ String append function strcat
// 1. The newly appended string should have \0
// 2.dest The array must be large enough , And can be modified 
// 3. Find the end of the target space first \0, Copy again 
//char* my_strcat(char* dest, const char* src)
//{
    
// assert(dest && src);
// char* ret = dest;
// // seek 
// while (*dest != '\0')
// {
    
// dest++;// When the loop ends, it points to \0 Next position of 
// }
// // Copy 
// while (*dest++ = *src++)
// {
    
// ;
// }
// return ret;
//}
//int main()
//{
    
// char str[20] = "hello ";// Mark the space size 
// my_strcat(str, "world");
// printf("%s\n", str);
// // When you want to add yourself , Due to the \0 Be overwritten , It's going to create a dead cycle 
// //my_strcat(str,str);----error
// return 0;
//}

4. strcpy

//------------ String copy function my_strcpy
// The source string must be in '\0' end 
// In the source string '\0' Copy to target space 
// The target space has to be large enough , To ensure that the source string can be stored 
// The target space has to be variable 
// 
//char* my_strcpy(char* dest, const char* src)
//{
    
// assert(dest && src);
// char* ret = dest;
// while (*dest++ = *src++)//src The value of is \0 Assign a value to dest, The cycle stops 
// {
    
// ;
// }
// return ret;
//}
//int main()
//{
    
// char a[20] = { 0 };  Mark the space size 
// char b[] = "abcdef";
// my_strcpy(a,b);
// printf("%s", a);
// return 0;
//}

5. strtok

//strtok Cut string 
// The place starting from the separator will be changed to \0
//int main()
//{
    
// char str[] = "- This, a sample string.";
// char* pch;
// printf("Splitting string \"%s\" into tokens:\n", str);
// pch = strtok(str, " ,.-");
// while (pch != NULL)
// {
    
// printf("%s\n", pch);
// pch = strtok(NULL, " .-");//' ' and '.' and '-' Separator 
// }
// return 0;
//}
// Print the results ：
//This
//a
//sample
//string

I hope you can point out the shortcomings of the code .