程序代码：

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

void pick( const char* s, char* sub, size_t beg, size_t end, size_t step )
{
    size_t n = strlen(s);
    end = end<n ? end : n;

    for( size_t i=beg; i<end; i+=step ) // 这里有个bug --- i+=step 可能溢出，明天再改
        *sub++ = s[i];
    *sub = '\0';
}

int main( void )
{
    char sub[100];
    pick( "Stringhandling", sub, 2, 8, 2 );
    puts( sub );
}