8.3 KiB
8.3 KiB
Lecture 15 --- Associative Containers (Maps), Part 1
Associative Containers (STL Maps)
- STL Maps: associative containers for fast insert, access and remove
- Example: Counting word occurrences
- STL Pairs
- Map iterators
- Map member functions: operator[], find, insert, erase.
- Efficiency
- STL maps vs. STL vectors vs. STL lists
15.1 STL Maps: Associative Containers
- STL maps store pairs of “associated” values. We will see several examples today, in Lab 9, and in Lecture 16:
- An association between a string, representing a word, and an int representing the number of times that word has been seen in an input file.
- An association between a string, representing a word, and a vector that stores the line numbers from a text file on which that string occurs (next lecture).
- An association between a phone number and the name of the person with that number (Lab 9).
- An association between a class object representing a student name and the student’s info (next lecture). A particular instance of a map is defined (declared) with the syntax: std::map<key_type, value_type> var_name In our first two examples above, key type is a string. In the first example, the value type is an int and in the second it is a std::vector<int>.
- Entries in maps are pairs: std::pair<const key_type, value_type>
- Map iterators refer to pairs.
- Map search, insert and erase are all very fast: O(log n) time, where n is the number of pairs stored in the map. Note: The STL map type has similarities to the Python dictionary, Java HashMap, or a Perl hash, but the data structures are not the same. The organization, implementation, and performance is different. In a couple weeks we’ll see an STL data structure that is even more similar to the Python dictionary.
- Map search, insert and erase are O(log n). First, let’s see how some of this works with a program to count the occurrences of each word in a file. We’ll look at more details and more examples later.
15.2 Counting Word Occurrences
Here’s a simple and elegant solution to this problem using a map:
#include <iostream>
#include <map>
#include <string>
int main() {
std::string s;
std::map<std::string, int> counters; // store each word and an associated counter
// read the input, keeping track of each word and how often we see it
while (std::cin >> s){
++counters[s];
}
// write the words and associated counts
std::map<std::string, int>::const_iterator it;
for (it = counters.begin(); it != counters.end(); ++it) {
std::cout << it->first << "\t" << it->second << std::endl;
}
return 0;
}
15.3 Maps: Uniqueness and Ordering
- Maps are ordered by increasing value of the key. Therefore, there must be an operator< defined for the key.
- Once a key and its value are entered in the map, the key can’t be changed. It can only be erased (together with the associated value).
- Duplicate keys can not be in the map.
15.4 STL Pairs
The mechanics of using std::pairs are relatively straightforward:
- std::pairs are a templated struct with just two members, called first and second. Reminder: a struct is basically a wimpy class and in this course you aren’t allowed to create new structs. You should use classes instead.
- To work with pairs, you must #include <utility>. Note that the header file for maps (#include <map>) itself includes utility, so you don’t have to include utility explicitly when you use pairs with maps.
- Here are simple examples of manipulating pairs:
std::pair<int, double> p1(5, 7.5);
std::pair<int, double> p2 = std::make_pair(8, 9.5);
p1.first = p2.first;
p2.second = 13.3;
std::cout << p1.first << " " << p1.second << std::endl;
std::cout << p2.first << " " << p2.second << std::endl;
p1 = p2;
std::pair<const std::string, double> p3 = std::make_pair(std::string("hello"), 3.5);
p3.second = -1.5;
// p3.first = std::string("illegal"); // (a)
// p1 = p3; // (b)
- The function std::make pair creates a pair object from the given values. It is really just a simplified constructor, and as the example shows there are other ways of constructing pairs.
- Most of the statements in the above code show accessing and changing values in pairs. The two statements at the end are commented out because they cause syntax errors:
- In (a), the first entry of p3 is const, which means it can’t be changed.
- In (b), the two pairs are different types! Make sure you understand this.
- Returning to maps, each entry in the map is a pair object of type: std::pair<const key_type, value_type>
- The const is needed to ensure that the keys aren’t changed! This is crucial because maps are sorted by keys!
15.5 Maps: operator[]
- We’ve used the [] operator on vectors, which is conceptually very simple because vectors are just resizable arrays. Arrays and vectors are efficient random access data structures.
- But operator[] is actually a function call, so it can do things that aren’t so simple too, for example:
++counters[s];
-
For maps, the [] operator searches the map for the pair containing the key (string) s.
– If such a pair containing the key is not there, the operator:
- creates a pair containing the key and a default initialized value,
- inserts the pair into the map in the appropriate position, and
- returns a reference to the value stored in this new pair (the second component of the pair).
This second component may then be changed using operator++.
– If a pair containing the key is there, the operator simply returns a reference to the value in that pair.
-
In this particular example, the result in either case is that the ++ operator increments the value associated with string s (to 1 if the string wasn’t already it a pair in the map).
-
For the user of the map, operator[] makes the map feel like a vector, except that indexing is based on a string (or any other key) instead of an int.
-
Note that the result of using [] is that the key is ALWAYS in the map afterwards.
15.6 Map Iterators
- Iterators may be used to access the map contents sequentially. Maps provide begin() and end() functions for accessing the bounding iterators. Map iterators have ++ and -- operators.
- Each iterator refers to a pair stored in the map. Thus, given map iterator it, it->first is a const string and it->second is an int. Notice the use of it-> , and remember it is just shorthand for (*it).
15.7 Map Find
- One of the problems with operator[] is that it always places a key / value pair in the map. Sometimes we don’t want this and instead we just want to check if a key is there.
- The find member function of the map class does this for us. For example: m.find(key); where m is the map object and key is the search key. It returns a map iterator: If the key is in one of the pairs stored in the map, find returns an iterator referring to this pair. If the key is not in one of the pairs stored in the map, find returns m.end().
15.8 Map Insert
- The prototype for the map insert member function is: m.insert(std::make_pair(key, value)); insert returns a pair, but not the pair we might expect. Instead it is pair of a map iterator and a bool: std::pair<map<key_type, value_type>::iterator, bool>
- The insert function checks to see if the key being inserted is already in the map.
– If so, it does not change the value, and returns a (new) pair containing an iterator referring to the existing
pair in the map and the bool value false.
– If not, it enters the pair in the map, and returns a (new) pair containing an iterator referring to the newly added pair in the map and the bool value true.
15.9 Map Erase
- Maps provide three different versions of the erase member function:
- void erase(iterator p) — erase the pair referred to by iterator p.
- void erase(iterator first, iterator last) — erase all pairs from the map starting at first and going up to, but not including, last.
- size_type erase(const key_type& k) — erase the pair containing key k, returning either 0 or 1, depending on whether or not the key was in a pair in the map.
15.10 Leetcode Exercises
- Leetcode problem 1: Two Sum. Solution: p1_twosum.cpp.
- Leetcode problem 290: Word Pattern. Solution: p290_word_pattern.cpp.