C language learning record -- use and analysis of string function (2)

strcmp

int strcmp(const char* string1, const char* string2)

1 The string is greater than 2 character string , Return is greater than the 0 The number of

1 The string is less than 2 character string , Back to less than 0 The number of

Equal return 0

#include <stdio.h>

int main()
{
    char* p1 = "abcdef";
    char* p2 = "qwert";
    int ret = strcmp(p1, p2);
    printf("%d\n", ret);
    return 0;
}

The method of comparison is not the ratio of the number of elements to the size , But the corresponding element ratio ANSCII Code value , such as a and q Than ,a Less than q, So back -1.

But this is vs What's going on inside , Different compilation environments return different values , But the scope is the same , Greater than 0, Less than 0, be equal to 0.

Simulation Implementation

#include <stdio.h>

int my_strcmp(const char* str1, const char* str2)
{
    assert(str1 && str2);
    while (*str1 = *str2)
    {
        if (*str1 == '\0')
        {
            return 0;
        }
        str1++;
        str2++;
    }
    if (*str1 > *str2)
        return 1;
    else(*str1 < *str2)
        return -1;
}
int main()
{
    char* p1 = "abcdef";
    char* p2 = "abqwe";
    int ret = my_strcmp(p1, p2);
    printf("ret = %d\n", ret);
    return 0;
}

If abc and abcd Compare , Then cycle to \0 and d Compare , Dissimilarity , Just come to the following if else,0 Less than d, return -1.

perhaps if else Change to return (*str1 - *str2） Can also be .

Functions with unlimited length ：strcpy,strcat,strcmp. Read \0 Just stop , But it's not safe , such as strcpy function , If the destination is not enough, prevent the source string , But it was copied . So it's not safe . There are also length limited functions ：strncpy,strncat,strncmp.

strncpy

char* strncpy(char* strDest, const char* strSource, size_t count)

count That is to copy a few bytes , Unit is byte . such as

int main()
{
    char arr1[10] = "abcdef";
    char arr2[] = "hello bit";
    strncpy(arr1, arr2, 4);
    return 0;
}

Copy 4 Bytes ,\0 It won't be copied . When the specified number is greater than the size of the source string , The rest will be made up \0, Until the specified number of bytes

About this function , That's how it's defined

#include <stdio.h>

char* _cdecl strncpy(char* dest, const char* source, size_t count)
{
    char* start = dest;
    while (count && (*dest++ = *source++))      /* copy string*/
        count--;
    if (count)                                  /* pad out with zeroes*/
        while (--count)
            *dest++ = '\0';
    return(start)
}

alike , The other two are also based on the original plus the number of bytes

Append function strncat, After addition , It will automatically add one last \0, Because it will eventually be appended to a string . If count Greater than the number of strings appended in the past , Then after the addition, put a \0, The addition is over , There won't be several in the back \0

Definition

char* _cdecl strncat(char* front, const char* back, size_t count)
{
    char* start = front;
    while (*front++)
        front--;
    while (count--)
        if (!(*front++ = *back++))
            return(start);
    *front = '\0';
    return(start);
}

strncmp

That is to compare several characters , The value is different .

strstr

char* strstr(const char*, const char*)

int main()
{
    char* p1 = "abcdef";
    char* p2 = "def";
    char* ret = strstr(p1, p2);
    if (ret == NULL)
    {
        printf(" Substring does not exist \n");
    }
    else
    {
        printf("%s\n", ret);
    }
    return 0;
}

If the function cannot be found, it will return a null pointer , If found, it will return d The address of , Using pointer variables ret After receiving ,hiu from d Start printing back , If at this time p1 yes abcdefgh, Then the result will come out abcdefgh. If p1 yes abcdefabcdef, There are two def, Then the result will start from the first .

Simulation Implementation

#include <stdio.h>
#include <assert.h>

char* my_strstr(const char* p1, const char* p2)
{
    assert(p1 != NULL);
    assert(p2 != NULL);
    char* s1 = p1;
    char* s2 = p2;
    char* cur = (char*)p1;
    if (*p2 = '\0')
    {
        return p1;
    }
    while (*cur)
    {
        s1 = cur;
        s2 = (char*)p2;
        while ( (*s1 != '\0') && (*s2 != '\0') && (*s1 == *s2) )
        {
            s1++;
            s2++;
        }
        if (*s2 == '\0')
        {
            return cur;
        }
        cur++;
    }
    return NULL;
}
int main()
{
    char* p1 = "abcdefgh";
    char* p2 = "def";
    char* ret = strstr(p1, p2);
    if (ret == NULL)
    {
        printf(" Substring does not exist \n");
    }
    else
    {
        printf("%s\n", ret);
    }
    return 0;
}

You'd better draw a picture to understand . Library functions are defined as

char* my_strstr(const char* str1, const char* str2)
{
    char* cp = str1;
    char* s1, * s2;
    if (!*str2)
        return((char*)str1);
    while (*cp)
    {
        s1 = cp;
        s2 = (char*)str2;
        while (s1 && s2 && !(*s1 - *s2))
            s1++, s2++;
        if (!*s2)
            return(cp);
        cp++;
    }
    return (NULL);
}

In fact, it can also be optimized based on the first one . stay s2 Of if Write this after the statement

if (*s1 == '\0')
        {
            return NULL;
        }

end .