Class map. Methods that provide information from the container

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Contents

• 1. Method at(). Get the value by key
• 2. Methods begin(), end().Set iterator to start, end of array
• 3. Method count().Get number of duplicates by key
• 4. Method empty(). Determine if an array is empty
• 5. Method find(). Get iterator to given key
• 6. Method max_size(). Maximum container size
• 7. Method size(). Get the current number of elements in the array
• 8. Operator function operator[](). Get value by given key
• Related topics

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1. Method at(). Get the value by key

The at() method allows you to get a value based on a given key. The general form of a method declaration is as follows

inline const valueType& at(const keyType& key) const;
inline valueType& at(const keyType& key);

here

• keyType – key type;
• valueType – value type;
• key – key value.

Example.

#include <iostream>
#include <map>
using namespace std;

void main()
{
  // Method clear() - clear the array
  // 1. Form an associative array
  map<string, int> Days;
  Days.insert(make_pair("January", 31));
  Days.insert(make_pair("February", 28));
  Days.insert(make_pair("March", 31));
  Days.insert(make_pair("April", 30));

  // 2. Print the number of days in January - the at() method
  cout << Days.at("January") << endl; // 31

  // 3. Get the number of days in April
  int daysApril = Days.at("April");
  cout << daysApril << endl; // 30

  // 4. Set a non-existent key - you need to process it with a try-catch block
  try
  {
    // the key "ABCD" is not in the container
    int t = Days.at("ABCD");
  }
  catch (std::out_of_range e)
  {
    cout << e.what() << endl;
  }
}

Result

31
30
invalid map<K, T> key

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2. Methods begin(), end().Set iterator to start, end of array

The begin() and end() methods allow you to get an iterator to the beginning and end of an associative array. The end of an associative array is defined as the element following the last element of the array.

The general form of a method declaration is as follows:

iterator begin();
const iterator begin() const;
iterator end();
const iterator end() const;

Figure 1 shows an example that shows the positions returned by the begin() and end() methods.

Figure 1. Returning a result from the begin() and end() methods

Example.

In the example, an associative container is formed, shown in Figure 1. Then, using the begin() and end() methods, the boundaries of the container are determined and the container is displayed on the screen using an iterator.

#include <iostream>
#include <map>
using namespace std;

void main()
{
  // Methods begin() and end()
  // 1. Create an associative container
  map<int, string> m1;
  m1.insert(pair<int, string>(1, "Winter"));
  m1.insert(pair<int, string>(2, "Spring"));
  m1.insert(pair<int, string>(3, "Summer"));
  m1.insert(pair<int, string>(4, "Autumn"));

  // 2. Get an iterator to the beginning and end of a container
  map<int, string>::iterator it, itEnd;
  it = m1.begin();
  itEnd = m1.end();

  // 3. Display the contents of the container on the screen
  cout << "m1:" << endl;
  while (it!=itEnd)
  {
    cout << it->first << " : " << it->second << endl;
    it++;
  }
}

Result

m1:
1 : Winter
2 : Spring
3 : Summer
4 : Autumn

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3. Method count().Get number of duplicates by key

The count() method returns the number of duplicates of an element with a key value. This method is relevant for the multimap class, in which several values can correspond to one key. The general form of a method declaration is as follows:

inline size_t count(const keyType &keyVal) const;

here

• keyType – key type;
• keyVal – value type.

Example.

#include <iostream>
#include <map>
using namespace std;

void main()
{
  // Method count() - get the number of duplicates for a given key
  // 1. Form an associative array
  map<char, string> Colors;
  Colors.insert(make_pair('R', "Red"));
  Colors.insert(make_pair('G', "Green"));
  Colors.insert(make_pair('B', "Blue"));

  // 2. Attempt to create another duplicate of key 'B'
  Colors.insert(make_pair('B', "Black")); 

  // 3. Print number of duplicates for key 'B'
  cout << Colors.count('B') << endl; // 1

  // 4. Print number of duplicates for key 'R'
  cout << Colors.count('R') << endl; // 1

  // 5. Attempting to access a non-existent key
  try
  {
    cout << Colors.count('F') << endl; // 0
  }
  catch (out_of_range e)
  {
    cout << e.what() << endl;
  }
}

Result

1
1
0

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4. Method empty(). Determine if an array is empty

The empty() method allows you to determine if the associative container contains elements. The specification of the method is

inline bool empty() const;

The method returns true if the container has at least one element. Otherwise, the method returns false.

Example.

#include <iostream>
#include <map>
using namespace std;

void main()
{
  // The empty() method - determine if an array is empty
  // 1. Create an empty container
  map<int, char> m1;

  // 2. Call the empty() function
  if (m1.empty())
    cout << "m1 is empty" << endl;
  else
    cout << "m1 is not empty" << endl;

  // 3. Add multiple elements to a container
  m1.insert(make_pair(1, 'A'));
  m1.insert(make_pair(2, 'B'));
  m1.insert(make_pair(3, 'C'));

  // 4. Re-call empty()
  if (m1.empty())
    cout << "m1 is empty" << endl;
  else
    cout << "m1 is not empty" << endl;
}

Result

m1 is empty
m1 is not empty

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5. Method find(). Get iterator to given key

The find() method returns an iterator that is set to the given key. The syntax for declaring the most common method implementations is:

inline iterator find(const keyType &keyVal) const;
inline iterator find(const keyType &keyVal);

here

• keyType – the type of key;
• keyVal – the type of value.

Example.

#include <iostream>
#include <map>
using namespace std;

void main()
{
  // Method find() - find element by key
  // 1. Form an associative array
  map<bool, string> m1;
  m1.insert(make_pair(true, "True"));
  m1.insert(make_pair(false, "False"));

  // 2. Find key true and print it
  map<bool, string>::iterator it;
  it = m1.find(true);
  cout << it->first << " : " << it->second << endl;

  // 3. Find key false and print it
  it = m1.find(false);
  cout << it->first << " : " << it->second << endl;
}

Result

1 : True
0 : False

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6. Method max_size(). Maximum container size

The max_size() method returns the maximum number of elements in the container. This number depends on the type of key and value in the pair, the size of free memory, etc.

Syntax of method declaration

inline size_t max_size() const;

Example.

#include <iostream>
#include <map>
using namespace std;

void main()
{
  // Method max_size() - get the maximum possible container size
  // 1. Declare an empty container
  map<string, string> code;

  // 2. Add element to container
  code.insert(make_pair("USA", "1"));

  // 3. Display max_size()
  cout << code.max_size() << endl; // 164703072086692425

  // 4. Add another element
  code.insert(make_pair("Ukraine", "380"));

  // 5. Display max_size()
  cout << code.max_size() << endl; // 164703072086692425
}

Result

164703072086692425
164703072086692425

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7. Method size(). Get the current number of elements in the array

The size() method returns the number of elements in the associative container. When creating an empty container, the method will return 0.

Syntax of method declaration

inline size_t size() const;

Example.

#include <iostream>
#include <map>
using namespace std;

void main()
{
  // Method size() - get the size of the container
  // 1. Declare an empty container
  map<string, string> code;

  // 2. Get the size of the container
  int size = code.size();
  cout << size << endl; // 0

  // 3. Add element to container
  code.insert(make_pair("USA", "1"));

  // 4. Display the size()
  cout << code.size() << endl; // 1

  // 5. Add another element
  code.insert(make_pair("Ukraine", "380"));

  // 6. Display size()
  cout << code.size() << endl; // 2
}

Result

0
1
2

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8. Operator function operator[](). Get value by given key

Using the operator function operator[](), you can access the values of an array by its keys in a natural way using square brackets []. The syntax for declaring a function in the map class is as follows

inline vType& map<kType, vType>::operator[](kType &&_KeyVal);

here

• kType – the type of key;
• vType – the type of value;
• _KeyVal – the value of a key.

The method can be used both in an expression and on the left side of the assignment operator =. If the _KeyVal key is not present in the container, and the [] operator is used on the left side of the assignment statement, a key is created with the corresponding value.

Example.

#include <iostream>
#include <map>
using namespace std;

void main()
{
  // Operator function operator[] - redefinition of operator []
  // 1. Create container via initialization list
  initializer_list<pair<const string, int>> lst = {
    make_pair("Jan", 31),
    make_pair("Feb", 28),
    make_pair("Mar", 31),
    make_pair("Apr", 30),
    make_pair("May", 31),
    make_pair("Jun", 30),
    make_pair("Jul", 31),
    make_pair("Aug", 31),
    make_pair("Sep", 30),
    make_pair("Oct", 31),
    make_pair("Nov", 30),
    make_pair("Dec", 31)
  };
  map<string, int> m1;
  m1.insert(lst);

  // 2. Get the number of days in March
  int d = m1["Mar"];
  cout << "d = " << d << endl;

  // 3. Change the number of days in February
  m1["Feb"] = 29;

  // 4. Checking if the number of days in February has changed
  d = m1["Feb"]; // d = 29
  cout << "d = " << d << endl;

  // 5. Create new pair "ABCD":333
  m1["ABCD"] = 333;

  // 6. Display the array
  map<string, int>::iterator it = m1.begin();
  cout << "m1:" << endl;
  while (it != m1.end())
  {
    cout << it->first << ":" << it->second << endl;
    it++;
  }
}

Result

d = 31
d = 29
m1:
ABCD:333
Apr:30
Aug:31
Dec:31
Feb:29
Jan:31
Jul:31
Jun:30
Mar:31
May:31
Nov:30
Oct:31
Sep:30

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Related topics

• Creating a container. Constructors. Creating pair. Overview of class functions and operators
• Methods that change data in the container: clear(), erase(), insert(), swap(), operator=()

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