CSCI-1200/lectures/16_maps_II/README.md

Lecture 16 --- Associative Containers (Maps), Part 2

16.1 More Complicated Values

• Let’s look at the example:

map<string, vector<int> > m;
map<string, vector<int> >::iterator p;

Note that the space between the > > is required (by many compiler parsers). Otherwise, >> is treated as an operator

• Here’s the syntax for entering the number 5 in the vector associated with the string "hello":

m[string("hello")].push_back(5);

• Here’s the syntax for accessing the size of the vector stored in the map pair referred to by map iterator p:

p = m.find(string("hello"));
p->second.size()

Now, if you want to access (and change) the ith entry in this vector you can either using subscripting:

(p->second)[i] = 15;

(the parentheses are needed because of precedence) or you can use vector iterators:

vector<int>::iterator q = p->second.begin() + i;
*q = 15;

Both of these, of course, assume that at least i+1 integers have been stored in the vector (either through the use of push back or through construction of the vector).

• We can figure out the correct syntax for all of these by drawing pictures to visualize the contents of the map and the pairs stored in the map. We will do this during lecture, and you should do so all the time in practice.

16.2 Typedefs

• One of the painful aspects of using maps is the syntax. For example, consider a constant iterator in a map associating strings and vectors of ints:

map < string, vector<int> > :: const_iterator p;

• Typedefs are a syntactic means of shortening this. For example, if you place the line:

typedef map < string, vector<int> > map_vect;

before your main function (and any function prototypes), then anywhere you want the map you can just use the identifier map_vect:

map_vect :: const_iterator p;

The compiler makes the substitution for you.

16.10 Leetcode Exercises

• Leetcode problem 49: Group Anagrams. Solution: p49_group_anagrams.cpp.
• Leetcode problem 290: Word Pattern. Solution: p290_word_pattern.cpp.