C++ Maps are sorted associative containers that contain unique key/value pairs. For example, you could create a map that associates a string with an integer, and then use that map to associate the number of days in each month with the name of each month.
Map Operators
- begin - returns an iterator to the beginning of the map
- clear - removes all elements from the map
- count - returns the number of elements matching a certain key
- empty - true if the map has no elements
- end - returns an iterator just past the last element of a map
- equal_range - returns iterators to the first and just past the last elements matching a specific key
- erase - removes elements from teh map
- find - returns an iterator to specific elements
- insert - insert items into a map
- key_comp - returns the function that compares keys
- lower_bound - returns an iterator to the first element greater than or equal to a certain value
- max_size - returns the maximum number of elements the map can hold
- rbegin - returns a reverse_iterator to the end of the map
- rend - returns a reverse_iterator to the beginning of the map
- size - returns the number of items in the map
- swap - swap the contests of this map with another
- upper_bound - returns an iterator to the first element greater than a certain value
- value_comp - returns the functions that compares values
Multimap Operators
- begin - returns an iterator to the beginning of the multimap
- clear - removes all elements from the multimap
- count - returns the number of elements matching a certain key
- empty - true if the multimap has no elements
- end - returns an iterator just past the last element of a multimap
- equal_range - returns iterators to the first and just past the last elements matching a specific key
- erase - removes elements from a multimap
- find - returns an iterator to specific elements
- insert - inserts items into a multimap
- key_comp - returns the function that compares keys
- lower_bound - returns an iterator to the first element greater than or equal to a certain value
- max_size - returns the maximum number of elements that the multimap can hold
- rbegin - returns a reverse_iterator to the end of the multimap
- rend - returns a reverse_iterator to the beginning of the multimap
- size - returns the number of items in the multimap
- swap - swap the contents of the multimap with another
- upper_bound - returns an iterator to the first element greater than a certain value
- value_comp - returns the function that compares values
Below is the implementation of the Map Operators. Let us start by creating a map object first...
begin()
- The function begin() returns an iterator to the first element of the map. begin() should run in constant time.
end()
- The end() function returns an iterator just past the end of the map.
- Note that before you can access the last element of the map using an iterator that you get from a call to end(), you'll have to decrement the iterator first.
#include <iostream>
#include <map>
using namespace std;
int main() {
// Creating maps with String mapped to Number.
// Here we will be mapping day
// to day number.
map<string, int> day;
// To add elements to the map we can use [] Operator.
day["Monday"] = 1;
day["Tuesday"] = 2;
day["Wednesday"] = 3;
day["Thrusday"] = 4;
day["Friday"] = 5;
day["Saturday"] = 6;
day["Sunday"] = 7;
// Printing all the element from the map.
// Maps can't be iterated like we iterate in contiguous
// array.
map<string, int>::iterator iter;
for (iter = day.begin(); iter != day.end(); iter++) {
cout << iter->first << " " << iter->second << "\n";
}
return 0;
}
Output:
Friday 5
Monday 1
Saturday 6
Sunday 7
Thrusday 4
Tuesday 2
Wednesday 3
Now we will be looking into clear() and empty()
clear()
- The function clear() deletes all of the elements in the map. clear() runs in linear time.
empty()
- The empty() function returns (1)true if the map has no elements, (0)false otherwise.
#include <iostream>
#include <map>
using namespace std;
int main() {
map<string, int> day;
day["Monday"] = 1; // adds one element.
cout << "Is map is empty: " << day.empty() << "\n"; // 0-false/1-true
// clearing all the element from the map.
day.clear();
cout << "Is map is empty: " << day.empty() << "\n";
return 0;
}
Output:
Is map is empty: 0
Is map is empty: 1
Now let us use count(), size(), and max_size().
count()
- The function count() returns the number of occurrences of key in the map. count() runs in logarithmic time.
size()
- The size() function returns the number of elements in the current map.
max_size()
- The max_size() function returns the maximum number of elements that the map can hold before more memory will have to be allocated, respectively.
#include <iostream>
#include <map>
using namespace std;
int main() {
// Creating maps with String mapped to Number. Here we will be mapping day
// to day number.
map<string, int> day;
day["Monday"] = 1;
day["Tuesday"] = 2;
day["Wednesday"] = 3;
day["Thrusday"] = 4;
day["Friday"] = 5;
day["Saturday"] = 6;
day["Sunday"] = 7;
// Counting the number of items present in the map.
cout << "Number of element present in the map: " << day.size() << "\n";
cout << "Number of element the this map can hold: " << day.max_size()
<< "\n"; // Depends in your system memory availability.
cout << "Number of Monday present in Map: " << day.count("Monday")
<< "\n"; // duplicate key can't be present so it will
// return 0-if key is present/ 1-if key is not present.
return 0;
}
Output:
Number of element present in the map: 7
Number of element the this map can hold: 288230376151711743
Number of Monday present in Map: 1
Now let us learn about some more interesting things in maps erase(), find() and insert():
insert()
- inserts pair<key,val>, but only if no element with the key key already exists. The return value is an iterator to the element inserted (or an existing pair with key key), and a boolean which is true if an insertion took place.
find()
- The find() function returns an iterator to key, or an iterator to the end of the map if key is not found. find() runs in logarithmic time.
erase()
- The erase function() either erases the element at pos, erases the elements between start and end, or erases all elements that have the value of key.
#include <iostream>
#include <map>
using namespace std;
int main() {
map<string, int> day;
// To add element into the map we can also use insert().
// Data are Lexicographically Sorted by default.
day.insert(make_pair("Monday", 1));
day.insert(make_pair("Tuesday", 2));
day.insert(make_pair("Wednesday", 3));
day.insert(make_pair("Thrusday", 4));
day.insert(make_pair("Friday", 5));
// Now we will be printing all the data present in the map "day" using
// iterator.
map<string, int>::iterator iter;
for (iter = day.begin(); iter != day.end(); iter++) {
cout << iter->first << " has day number: " << iter->second << "\n";
}
// The find() function returns an iterator to key, or an iterator to the end
// of the map if key is not found.
iter = day.find("Friday");
cout << "Found '" << iter->first << "' having Value:" << iter->second
<< "\n";
// Erasing the whole data from the map
map<string, int>::iterator niter;
while (!day.empty()) {
niter = day.begin();
cout << "Erasing :" << niter->first << " " << niter->second << "\n";
day.erase(niter);
}
// Checking the presence of element in the Map "day"
cout << "Is map 'day' is empty: " << day.empty() << "\n";
return 0;
}
Output:
Friday has day number: 5
Monday has day number: 1
Thrusday has day number: 4
Tuesday has day number: 2
Wednesday has day number: 3
Found 'Friday' having Value:5
Erasing :Friday 5
Erasing :Monday 1
Erasing :Thrusday 4
Erasing :Tuesday 2
Erasing :Wednesday 3
Is map 'day' is empty: 1
Now let us talk about lower_bound() and upper_bound()
lower_bound()
- The lower_bound() function returns an iterator to the first element which has a value greater than or equal to key. lower_bound() runs in logarithmic time.
upper_bound()
- The function upper_bound() returns an iterator to the first element in the map with a key greater than key.
#include <iostream>
#include <map>
using namespace std;
int main() {
map<string, int> day;
// Inserting element into the map 'day'.
day["Monday"] = 1;
day["Tuesday"] = 2;
day["Wednesday"] = 3;
day["Thrusday"] = 4;
day["Friday"] = 5;
day["Saturday"] = 6;
day["Sunday"] = 7;
map<string, int>::iterator iter;
// lower_bound() function returns an iterator to the first element which has
// a value greater than or equal to key.
iter = day.lower_bound("Monday");
cout << "Lower bound for Monday: " << iter->first << " " << iter->second
<< "\n";
iter = day.lower_bound("Moon"); // find lower bound from lexicographically
// arranged map same for upper_bound().
cout << "Lower bound for Moon: " << iter->first << " " << iter->second
<< "\n\n";
// upper_bound() returns an iterator to the first element in the map with a
// key greater than key.
iter = day.upper_bound("Monday");
cout << "Upper bound for Monday: " << iter->first << " " << iter->second
<< "\n";
iter = day.upper_bound("Moon");
cout << "Upper bound for Moon: " << iter->first << " " << iter->second
<< "\n";
return 0;
}
Output:
Lower bound for Monday: Monday 1
Lower bound for Moon: Saturday 6
Upper bound for Monday: Saturday 6
Upper bound for Moon: Saturday 6
The related articles which you should read after this article.
- Introduction to Hash Table
- Collision handling and Hash Table Implementation in C++.
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Tags:
C++ STL