Reverse Find pointer to nth occurrence of a character in cstring

Question

How to Reverse Find pointer to nth occurrence of a character in cstring/BSTR?

char * RFindNthOccurrence(char* src, char t, int n)
{
   //for i/p string src = "HI,There,you,All"
   // and t =','
   // n =2
   //returned pointer should be at ",you,All" in same unchanged string
}

I've found first and last occurrence searching, but without modifying string reverse finding nth occurrence is problem.

Answer 1

How about this?

#include <iostream>

const char * RFindNthOccurrence( const char *s, char c, size_t n )
{
    if ( !n ) return NULL;

    while ( ( n -= *s == c ) && *s ) ++s;

    return n == 0 ? s : NULL;
}

char * RFindNthOccurrence( char *s, char c, size_t n )
{
    if ( !n ) return NULL;

    while ( ( n -= *s == c ) && *s ) ++s;

    return n == 0 ? s : NULL;
}

int main() 
{
    const char *s1 = "HI,There,you,All";

    std::cout << RFindNthOccurrence( s1, ',', 2 ) << std::endl;

    char s2[] = "HI,There,you,All";

    std::cout << RFindNthOccurrence( s2, ',', 2 ) << std::endl;

    return 0;
}

The program output is

,you,All
,you,All

The function behaves the same way as standard C function strchr that is it finds the terminating zero character but only in case when n = 1.

Another example

#include <iostream>

const char * RFindNthOccurrence( const char *s, char c, size_t n )
{
    if ( !n ) return NULL;

    while ( ( n -= *s == c ) && *s ) ++s;

    return n == 0 ? s : NULL;
}

char * RFindNthOccurrence( char *s, char c, size_t n )
{
    if ( !n ) return NULL;

    while ( ( n -= *s == c ) && *s ) ++s;

    return n == 0 ? s : NULL;
}

int main() 
{
    const char *s = "HI,There,you,All";
    const char *p = s;

    for ( size_t i = 1; p = RFindNthOccurrence( s, ',', i ); ++i )
    {
        std::cout << i << ": " << p << std::endl;
    }

    return 0;
}

The program output is

1: ,There,you,All
2: ,you,All
3: ,All

The same you could do using standard C function strchr without writing a special function. For example

#include <iostream>
#include <cstring>

int main() 
{
    const char *s = "HI,There,you,All";
    const char *p = s;
    size_t n = 2;

    while ( ( p = std::strchr( p, ',' ) ) && --n ) ++p;

    if ( n == 0 ) std::cout << p << std::endl;

    return 0;
}

The program output is

,you,All

If you need indeed the reverse search then the function can look like in this demonstrative program

#include <iostream>
#include <cstring>

const char * RFindNthOccurrence( const char *s, char c, size_t n )
{
    if ( !n ) return NULL;

    const char *p = s + std::strlen( s );

    while ( ( n -= *p == c ) && p != s ) --p;

    return n == 0 ? p : NULL;
}

int main() 
{
    const char *s = "HI,There,you,All";
    const char *p = s;

    for ( size_t i = 1; p = RFindNthOccurrence( s, ',', i ); ++i )
    {
        std::cout << i << ": " << p << std::endl;
    }

    return 0;
}

In this case the program output is

1: ,All
2: ,you,All
3: ,There,you,All