C secret script Chapter 3 (detailed explanation of string function) (Section 2)

In the last section, I sent it out because I didn't finish writing it , I'm sorry for you , In the last section, we introduced the usage and Simulation Implementation of two string functions , Next, we will continue to discuss .

Add ： Last time strlen The return value of the function is size_t(unsigned int) Unsigned shaping , This is used by library functions typedef Keyword defines a type , So this function should pay attention to the pit .

The following code ：

#include<stdio.h>
int main()
{
if(strlen("abc")-strlen("abcdef")>0)
{
    printf("hehe\n");
}
else
{
    printf("haha\n");
}
return 0;
}

        The result of the above code is ：hehe, the reason being that strlen Function is an unsigned integer , Unsigned integer has no negative number , So even abc Do than abcdef Small subtraction or >0 Of .

3.strcat function

        explain ： This function has something similar to strcpy function , however strcpy The function copies a string to the element starting with the first character , and strcat The function appends a string to the destination string from \0 Starting position . Parameters 1: Destination string , Parameters 2: Original string

        Ideas ： Since it is from the destination string \0 Start appending the position of , Then we have to find the destination string first \0 The location of , Then start adding .

The implementation code is as follows ：

#include<stdio.h>
#include<assert.h>
char* my_strcat(char* str1,const char* str2)
{
    assert(str1!=NULL && str2!=NULL);    // Determine the validity of the two pointers 
    char* ret=str1;    // Because the back str1 The element address of will change , Therefore, initializing the first element address is convenient for the following parameters to return and print .
    while(*str1 !='\0')    // find \0 Location 
    {
        str1++;        // We can't put autoincrement here while In the condition of , Because we are looking for \0 Location , Before the last judgment, he will increase himself , After it comes out, it is not \0 Location but \0 Back address .
    }
    while(*str1++=*str2++)        // hold str2 The element of is appended to str1
    {
        ;
    }
    return ret;    // return str1 Initial address .
}
int main()
{
   char arr1[30]="hello";    // Note that the size of the array space must be given here , The default is just the size it is now storing , If you do not specify the size, you will not be able to fit it .
   char arr2[]="world";
   my_strcat(arr1,arr2);
   printf("%s\n",arr1);
   return 0;
}

4.strcpy function

        function ： This function is similar to the previous strcat function , It's a little simpler , Don't look for \0 Copy directly from the first element , Parameters 1: Destination string , Parameters 2: String to copy .

        Ideas ： Copy directly from the first element .

The implementation code is as follows ：

#include<stdio.h>
#include<assert.h>
char* my_strcpy(char* str1,const char* str2)
{
assert(str1!=NULL && str2!=NULL)
char* ret=str1;
while(*str1++=*str2++;)
{
    ;
}
return ret;
}
int main()
{
char arr1[15]="abc";        // Follow strcat Function to initialize the array size , Make sure you can fit two strings .
char arr2[]="def";
my_strcpy(arr1,arr2);
printf("%s\n",arr1);
return 0;
}

Key points ：5.strstr function

        explain ： This function is a search substring function , Parameters 1: String to find , Parameters 2: Substring to find . If you find the return address , If it cannot be found, it returns a null pointer .

        Ideas ： This kind of function is more complicated , We should discuss it according to the situation , The first is arr1 Than arr2 Small, that means arr2 Absolutely not arr1 Substring of , In another case, their two strings are equal and both are \0 That means they arr2 yes arr1 Substring of , Note that the initial search address should be kept during the search process , For the next search .

        The specific code is as follows ：

#include<stdio.h>
#include<assert.h>
char* my_strstr(const char* str1,const char* str2)
{
    assert(str1!=NULL && str2!=NULL);
    char* s1=NULL;
    char* s2=NULL;
    char* cor=(char*)str1;
if(*str2=='\0')
{
return str1;        // Each string has \0,str2 It must be str1 Substring of .
}
while(cor)        // if cor（str1） Smaller than substring ,str2 It must not be str1 Substring of 
{
    s1=cor;
    s2=(char*)str2;
    while((*s1 != '\0' && *s2 != '\0') && (*s1==*s2))    // if s1 and s2 None of them is equal to \0 And equal 
    {
    s1++;
    s2++;
    }
    if(*s2=='\0')
    {
    return cor;        // If substring has reached \0 explain str2 yes str1 Substring of 
    }
    else if(*s1=='\0')
    {
    return NULL;    // if str1 Than str2 Arrive ahead of time \0 that str2 It must not be str1 Substring of 
    }
cor++;
}
return NULL;
}
int main()
{
char arr1[]="abbbcdef";
char arr2[]="bbc";
char* ret=my_strstr(arr1,arr2);
if(ret==NULL)
{
    printf(" String does not exist \n");
}
else
{
    printf("%s\n",ret);
}
return 0;
}

        OK, that's all for this section , If you have any mistakes, please tell me , I must accept , Take leave ！