One 、strcspn

1、 Function definition

size_t strcspn ( const char * str1, const char * str2 );

2、 Parameters

str1 – To be retrieved C character string .
str2 – The string contains the string to be used in str1 A list of characters to match in .

3、 Return value

This function returns str1 There is no string at the beginning str2 Number of characters in .

The length of the initial part of str1 not containing any of the characters that are part of str2.
This is the length of str1 if none of the characters in str2 are found in str1.
size_t is an unsigned integral type.

4、demo

/* strcspn example */
#include <stdio.h>
#include <string.h>

int main()
{
    
	char str[] = "fcba73";
	char keys[] = "1234567890";
	int i;
	i = strcspn(str, keys);
	printf("The first number in str is at position %d.\n", i + 1);
	return 0;
}

Output

The first number in str is at position 5.

Reference resources ：
link : https://cplusplus.com/reference/cstring/strcspn/
link : https://www.runoob.com/cprogramming/c-function-strcspn.html

Two 、strchr

1、 Function definition

const char * strchr ( const char * str, int character );
char * strchr ( char * str, int character );

2、 Parameters

str1 – To be retrieved C character string .
str2 – The string contains the string to be used in str1 A list of characters to match in .

3、 Return value

This function returns str1 There is no string at the beginning str2 Number of characters in .

The length of the initial part of str1 not containing any of the characters that are part of str2.
This is the length of str1 if none of the characters in str2 are found in str1.
size_t is an unsigned integral type.

4、demo

#include <stdio.h>
#include <string.h>

int main(void)
{
    
    char str[] = "I welcome any ideas from readers, of course.";
    char* lc = strchr(str, 'o');
    printf("strchr： %s\n", lc);
    char* rc = strrchr(str, 'o');
    printf("strrchr： %s\n", rc);
    return 0;
}

Output

strchr： ome any ideas from readers, of course.
strrchr： ourse.

5、 Why is the function “character ” Parameter is int type ？

• Last but not least , Why is the function “c” Parameter is int type , instead of “char” Type? ？

• The reason is simple , Here we use character ASCII code （ Because each character corresponds to a ASCII code ）, In this way, when transferring values, you can transfer “char” Type value , You can pass it on again “int” Type value （0~127）.

Reference resources ：
link : http://c.biancheng.net/view/401.html
link : https://cplusplus.com/reference/cstring/strchr/

3、 ... and 、 Special character check

1. demand

• 1, No special characters in English “,./;[]<>?:{}| [email protected]#$%^&*()_+=-'”\`~"
• 2. The first character cannot be a number

2、demo

#include <iostream>
static const char INVALD_CHARS[] = ",./;[]\<>?:{}|[email protected]#$%^&*()_+=-'\"\\`~";
using namespace std;
void check(char * input)
{
    
    int len = strlen(input);
    if (strcspn(input, INVALD_CHARS) < len)
    {
    
        cout << " Contains at least one special character "<<endl;
    }
    if (strchr("0123456789", input[0]) != nullptr)
    {
    
        cout << " The first character contains 0123456789"<<endl<<endl;
    }
}
int main()
{
    
    cout << "begin check input1"<<endl;
    char input1[] = "woaihecaicha";
    check(input1);

    cout << "begin check input2"<<endl;
    char input2[] = "woaihecaicha,";
    check(input2);

    cout << "begin check input3"<<endl;
    char input3[] = "9woaihecaicha";
    check(input3);    
}

3. Output

begin check input1
begin check input2
Contains at least one special character
begin check input3
The first character contains 0123456789