1. strcpy function

Syntax: strcpy(str1,str2)

Function: Copy the string str2 to the string str1, and override the STR1 original string, which can be used to assign values to string variables.

Return: str1

Be careful:

(1). The string str2 will cover all the characters in str 1;

(2) The length of the string STR 2 cannot exceed str1;

(3) Copy principle: Start with the first element and end with\0.

int main()
{
    char *str1 = "hello world";
    char *str2;
    // Copy the contents of str1 to str2, parameter is character array pointer
    strcpy(str2, str1);
    printf("str2 = %s\n", str2);
    // str2 = hello world

    char str3[15] = "hello\0 world";
    char str4[15];
    // The contents of str4 are copied to str3 and end with\0
    strcpy(str4, str3);
    printf("str4 = %s\n", str4);
    // str4 = hello

    return 0;
}

2. strncpy function

Syntax: strncpy(str1,str2,n)

Function: Copy the first n characters of string str2 into the first n characters of string str1.

Return: str1

Be careful:

(1). Not all strings in str 1 will be cleared, only the first n strings will be changed;

(2). n cannot be greater than the length of strings str1 and str 2;

(3). But if strncpy_is used S will clear all strings in str1.

int main()
{
    char str1[] = "day day up";
    char str2[] = "you are";
    strncpy(str1, str2, strlen(str2));
    printf("%s\n", str1); 
    // you are up
    return 0;
}

3. strcat function

Syntax: strcat(str1,str2)

Function: Add the string str2 to the end of the string str1, that is, stitch two strings together.

Return: str1

int main()
{
    char str1[] = "hello";
    char str2[] = "world";
    strcat(str1, str2); 

    printf("%s\n", str1);
    // hello world
    return 0;
}

4. strncat function

Syntax: strncat(str1,str2,n)

Function: Add the first n characters of the string str2 to the end of the string str1.

Return: str1

Note: The length after stitching cannot exceed the length of the string array str1.

int main()
{
    char str1[] = "hello ";
    char str2[] = "world";
    strncat(str1, str2, 2); 
    printf("%s\n", str1);
    // hello wo
    return 0;
}

5. strlen function

Syntax: strlen(str1)

Function: Calculates the length of the string str1.

Return: int value

Note: The length of the string does not include the character'\0'.

int main()
{
    char *str1 = "hello world";
    char *str2 = "hello,\0 kite";
    int l1 = strlen(str1);
    int l2 = strlen(str2);
    printf("l1 = %d\n", l1); 
    // 11
    printf("l2 = %d\n", l2); 
    // 6
    return 0;
}

6. strcmp function

(1). Syntax: strcmp(str1,str2)

Function: Compare two strings.

A. Returns 0 if the two strings are equal;

B. If str1 is greater than str2, return a positive number, which is not necessarily 1;

C. If str1 is less than str2, a negative number is returned, which is not necessarily -1;

(2). Syntax: strncmp(str1,str2,n)

Function: Compare the first n characters of two strings.

(3). Syntax: stricmp(str1,str2) (stricmp in Windows and strcasecmp in Linux)

Function: Ignore case comparison strings in two strings, that is, insensitive to case.

Note: If stricmp is used directly in VS2017, the following error will be prompted.

  Processing available _ stricmp instead.

Return: 0 or a positive or negative number.

int main()
{
    char str1[] = "Wearecsdn!";
    char str2[] = "Wearecsdn!";
    char str3[] = "Wearea!";
    char str4[] = "Wearef!";
    char str5[] = "Weare";
    char str6[] = "weAreCsdn!";
 
    int cmp1 = strcmp(str1, str2);        //cmp1=0
    int cmp2 = strcmp(str1, str3);        //cmp2=1
    int cmp3 = strcmp(str1, str4);        //cmp3=-1
    int cmp4 = strcmp(str1, str5);        //cmp4=1
 
    int cmp5 = strncmp(str1, str2, 5);    //cmp5=0
    int cmp6 = strncmp(str1, str3, 5);    //cmp6=0
    int cmp7 = strncmp(str1, str4, 5);    //cmp7=0
    int cmp8 = strncmp(str1, str5, 5);    //cmp8=0
 
    int cmp9 = _stricmp(str1, str6);      //cmp9=0

    return 0;
}

7. strchr function

(1). Syntax: strchr(str,c)

Function: Start with the first address of the string, and find the position of the first character c in the str string.

(2). Syntax: strrchr(str,c)

Function: Searches for the first occurrence of the character c from the back to the front in the string str.

(3). Syntax: strstr(str1,str2)

Function: Searches for the position of the string str2 in the string str1, returns a pointer to the position of the first character of str2 in STR1 if found, and returns NULL if not found.

Return: A pointer to the position of the character c, or a null pointer NULL if the character c is not found.

int main()
{
    // strchr query character 
    char *str = "no one can help you";
    // Query the character h in the string if there is an address that returns H
    // If no return NULL exists
    char *s = strchr(str, 'h');
    if(s){
        printf("Existing characters:%c\n", *s);
    } else {
        printf("No characters exist\n");
    }

    // strstr query string function
    char *st = strstr(str, "help");
    // Query the character help in the string, if there is a first address to return help
    // If no return NULL exists
    if(st){
        printf("There is a string:%p\n", *st); 
    } else {
        printf("Not in string\n");
    }
    return 0;
}

8. strpbrk function

Syntax: strpbrk(str1,str2)

Function: Checks the characters in the string str1 in turn, stops the check and returns the character position when the character being checked is also included in the string str2.

Return: A pointer to the position of the first two characters in str1.

int main()
{
    char str1[] = "We12are34csdn!";
    char str2[] = "32";
 
    char* ret1;
    ret1 = strpbrk(str1, str2);   
 
    int r1 = ret1 - str1;        
 
    printf("%c\n", *ret1); 
    // *ret1 = 2
    printf("%d\n", r1);   
    // r1 = 3
    
    return 0;

}

9. String to Number atoi, atof, atol Functions

(1). Syntax: atoi(str)

Function: Convert string to int integer

(2). Syntax: atof(str)    

Function: Convert string to double floating point number

(3). Syntax: atol(str)   

Function: Convert string to long integer