Question

This is really easy in some languages, not so easy in others. I can’t quite get it for C++. I’m reading in a file with thousands of lines of the format in the question title, and reading the file over and over again thousands of times.

Basically, I have some integers in mind in my program, say “100, 4, 1“, and I want to look up in a text file what the last digit in the sequence is, so the line might be “100_4_1 - 15“, i want to get the 15. How can this be done?

Answer 1

some integers in mind in my program, say “100, 4, 1”,
“100_4_1 – 15”, i want to get the 15

struct Key
{
    int a_, b_, c_;
    Key(int a, int b, int c) : a_(a), b_(b), c_(c) { }
    bool operator<(const Key& rhs) const
    {
        return a_ < rhs.a_ ||
               a_ == rhs.a_ && b_ < rhs.b_ ||
               a_ == rhs.a_ && b_ == rhs.b_ && c_ < rhs.c_;
    }
};

typedef std::map<Key, int> Lookup_Table;
Lookup lookup_table;

if (std::ifstream if("filename.txt"))
{
    int a, b, c;
    char ignore_char;
    int number;
    while (if >> a >> ignore_char >> b >> ignore_char >> c >> ignore_char >> number)
        lookup_table[Key(a, b, c)] = number;
}
else
{
    std::cerr << "can't read input file...\n";
    exit(1);
}

// to find something:
Lookup_Table::const_iterator i = lookup_table.find(Key(x, y, z));
if (i != lookup_table.end())
{
    // found... value accessible using *i...
}