c++ - Pointer Arithmetic, Pass by Reference -

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i got following question past exam paper:

consider following source code:

using namespace std;  int main() {    char dest[20];    printf( "%s\n", altcopy( dest, "demo" ) );    printf( "%s\n", altcopy( dest, "demo2" ) );    printf( "%s\n", altcopy( dest, "longer" ) );    printf( "%s\n", altcopy( dest, "even longer" ) );    printf( "%s\n", altcopy( dest, "and long string" ) ); }

provide implementation function called altcopy() uses pointer arithmetic copy alternate characters of c-type string destination (i.e. first, third, fifth etc character). answer must not use [] operator access array index. above code output following:

dm dm2 lne ee ogr adaral ogsrn

and have attempted follows:

using namespace std;  char* altcopy (char* dest, const char* str) {   char* p = dest;   const char* q = str;    ( int n=0; n<=20; n++ )   {     *p = *q;     p++;     q++;     q++;   }   return dest; }  int main() {    char dest[20];    printf( "%s\n", altcopy( dest, "demo" ) );    printf( "%s\n", altcopy( dest, "demo2" ) );    printf( "%s\n", altcopy( dest, "longer" ) );    printf( "%s\n", altcopy( dest, "even longer" ) );    printf( "%s\n", altcopy( dest, "and long string" ) ); }

and results are:

dm dm2lne lne ee ogradaral ogsrn adaral ogsrn

i'm not sure why happened have duplicate of next statement result on output instead of performing question asked for. here?

your function invalid @ least because uses magic number 20. function should similar standard function strcpy has copy source string until terminating 0 encountered.

here simple function realization

#include <iostream>  char * altcopy( char *dest, const char *str ) {     char *p = dest;      while ( *p++ = *str++ )     {         if ( *str ) ++str;     }      return dest; }  int main() {     char dest[20];      std::cout << altcopy( dest, "demo" ) << std::endl;     std::cout << altcopy( dest, "demo2" ) << std::endl;     std::cout << altcopy( dest, "longer" ) << std::endl;     std::cout << altcopy( dest, "even longer" ) << std::endl;     std::cout << altcopy( dest, "and long string" ) << std::endl;      return 0; }

the output is

dm dm2 lne ee ogr adaral ogsrn

enjoy!:)


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