#include <iostream>
 
void do_math( const int (&a)[3] )
{
    std::cout << "do_math -" ;
    for( int v : a ) std::cout << ' ' << v ;
    std::cout << '\n' ;
}
 
int main()
{
    constexpr int constants[][3] = { {}, {0,1,2}, {4,5,6}, {7,8,9}, {10,11,12} } ;
    constexpr std::size_t n = sizeof(constants) / sizeof(*constants) ;
 
    std::size_t ans ; std::cout << "please enter number: " && std::cin >> ans ;
    if( ans>0 && ans<n ) do_math( constants[ans] ) ;
}
 

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