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CS253 Pair And Tuple

pair

• A pair (defined in <utility>):
  • is a templated class
  • has two public data members: .first and .second
  • can be compared to other pairs lexicographically.

pair<string, double> teacher("Jack", 3.42);
cout << teacher.first << " makes $" << teacher.second << "/hour\n";

Jack makes $3.42/hour

• *map::begin() returns a pair of key, value.
• [0] and [1] do not work.

pair definition

pair is not much more than this:

template<typename T1, typename T2>
struct pair {
    T1 first;
    T2 second;
};

• I consider it to be a cheap struct for very local use.
• For wider use, make a struct with mnemonic names.

pair comparison

vector<pair<string, string>> ff = {
    {"Storm", "Johnny"},
    {"Storm", "Sue"},
    {"Grimm", "Ben"},
    {"Richards", "Reed"},
};
sort(ff.begin(), ff.end());
for (auto &p : ff)
    cout << p.second << ' ' << p.first << '\n';

Ben Grimm
Reed Richards
Johnny Storm
Sue Storm

• For sort() to work, the pairs had to be less-than comparable.
• Primary sort key: .first
• Secondary sort key: .second
• Did you notice the valid trailing comma?

pair & multimap

multimap<string, string> ff = {
    {"Storm", "Johnny"},
    {"Storm", "Sue"},
    {"Grimm", "Ben"},
    {"Richards", "Reed"},
};
for (auto &p : ff)
    cout << p.second << ' ' << p.first << '\n';

Ben Grimm
Reed Richards
Johnny Storm
Sue Storm

• A map is not much more than a set of pairs.
• I needed a multimap because there are two Storm siblings.
• Two-level sorting: first by .first, second by .second.

tuple

• A tuple:
  • rhymes with “scruple”
  • is defined in <tuple>
  • sort of an extension of pair
  • has as many fields as you like
  • does not have .first, .second, .third, etc.
  • has non-methods get<0>(tuple), get<1>(tuple), get<2>‍(tuple), etc.
    • These are compile-time indices. [0], [1], … do not work.
  • has non-method get<type>(tuple)
  • has all comparison methods (<, >, <=, >=, ==, !=), lexicographically defined

tuple

using person = tuple<string, double, int, bool>;
person orange("DJT", 75, 239, false);
cout << boolalpha
     << "Name:    " << get<0>(orange)      << "\n"
     << "Height:  " << get<double>(orange) << "″\n"
     << "Weight:  " << get<2>(orange)      << "#\n"
     << "Popular: " << get<bool>(orange)   << "\n";

Name:    DJT
Height:  75″
Weight:  239#
Popular: false

Output

You can’t just << a pair or a tuple. What would go between the elements?

pair<int,int> p(1,2);
cout << p.first << '/' << p.second;

1/2

pair<int,int> p(3,4);
cout << p;

c.cc:2: error: no match for 'operator<<' in 'std::cout << p' (operand types are 
   'std::ostream' {aka 'std::basic_ostream<char>'} and 'std::pair<int, int>')