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Chapter 11. Associative Containers

Exercise 11.1:

Describe the differences between a map and a vector.

A map is a collection of key-value pairs. we can get a value lookup by key efficiently.

A vector is a collection of objects, and every object has an associated index, which gives access to that object.

Exercise 11.2:

Give an example of when each of list, vector, deque, map, and set might be most useful.

  • list : a to-do list. always need insert or delete the elements anywhere.
  • vector : save some important associated data, always need query elements by index.
  • deque : message handle. FIFO.
  • map : dictionary.
  • set : bad_checks.

Exercise 11.3 and 11.4

Exercise 11.5:

Explain the difference between a map and a set. When might you use one or the other?

  • set : the element type is the key type.
  • map : we should use a key-value pair, such as {key, value} to indicate that the items together from one element in the map.

I use set when i just need to store the key, In other hand, I　would like use map when i need to store a key-value pair.

Exercise 11.6:

Explain the difference between a set and a list. When might you use one or the other?

set is unique and order, but list is neither. using which one depend on whether the elements are unique and order to store.

Exercise 11.7

Exercise 11.8

Exercise 11.9 and 11.10

Exercise 11.11

Exercise 11.12 and 11.13

Exercise 11.14

Exercise 11.15:

What are the mapped_type, key_type, and value_type of a map from int to vector<int>?

  • mapped_type : vector<int>
  • key_type : int
  • value_type : std::pair<const int, vector<int>>

Exercise 11.16:

Using a map iterator write an expression that assigns a value to an element.

std::map<int, std::string> map;
map[25] = "Alan";
std::map<int, std::string>::iterator it = map.begin();
it->second = "Wang";

Exercise 11.17:

Assuming c is a multiset of strings and v is a vector of strings, explain the following calls. Indicate whether each call is legal:

copy(v.begin(), v.end(), inserter(c, c.end())); // legal
copy(v.begin(), v.end(), back_inserter(c)); // illegal, no `push_back` in `set`.
copy(c.begin(), c.end(), inserter(v, v.end())); // legal.
copy(c.begin(), c.end(), back_inserter(v)); // legal.

Exercise 11.18

Exercise 11.19:

Define a variable that you initialize by calling begin() on the multiset named bookstore from 11.2.2 (p. 425). Write the variable’s type without using auto or decltype.

using compareType = bool (*)(const Sales_data &lhs, const Sales_data &rhs);
std::multiset<Sales_data, compareType> bookstore(compareIsbn);
std::multiset<Sales_data, compareType>::iterator c_it = bookstore.begin();

Exercise 11.20

Exercise 11.21:

Assuming word_count is a map from string to size_t and word is a string, explain the following loop:

while (cin >> word)
    ++word_count.insert({word, 0}).first->second;
++ (word_count.insert({word, 0}).first->second)

Exercise 11.22:

Given a map<string, vector>, write the types used as an argument and as the return value for the version of insert that inserts one element.

std::pair<std::string, std::vector<int>>    // argument
std::pair<std::map<std::string, std::vector<int>>::iterator, bool> // return

Exercise 11.23

Exercise 11.24 ~ 11.26

Exercise 11.27 ~ 11.30

Exercise 11.31

Exercise 11.32

Exercise 11.33

Exercise 11.34:

What would happen if we used the subscript operator instead of find in the transform function?

In gcc 4.8.3, will report error:

error: passing ‘const std::map<std::basic_string<char>, std::basic_string<char> >’ as ‘this’ argument of ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = std::basic_string<char>; _Tp = std::basic_string<char>; _Compare = std::less<std::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::basic_string<char>, std::basic_string<char> > >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = std::basic_string<char>; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::basic_string<char>]’ discards qualifiers [-fpermissive]
     auto key = m[s];
                   ^

Because std::map's operator is not declared as const,but m is declared as a reference to std::map with const.If insert new pair,it will cause error.

Exercise 11.35:

In buildMap, what effect, if any, would there be from rewriting trans_map[key] = value.substr(1); as trans_map.insert({key, value.substr(1)})?

  • use subscript operator: if a word does appear multiple times, our loops will put the last corresponding phrase into trans_map
  • use insert: if a word does appear multiple times, our loops will put the first corresponding phrase into trans_map

Exercise 11.36:

Our program does no checking on the validity of either input file. In particular, it assumes that the rules in the transformation file are all sensible. What would happen if a line in that file has a key, one space, and then the end of the line? Predict the behavior and then check it against your version of the program.

we added a file that name "word_transformation_bad.txt" to folder data. the file only has a key, one space.

the program of 11.33 don't influenced by that.

Exercise 11.37:

What are the advantages of an unordered container as compared to the ordered version of that container? What are the advantages of the ordered version?

  • the advantages of an unordered container:
    • useful when we have a key type for which there is no obvious ordering relationship among the elements
    • useful for applications in which the cost of maintaining the elements in order is prohibitive
  • the advantages of the ordered version:
    • Iterators for the ordered containers access elements in order by key
    • we can directly define an ordered container that uses a our own class types for its key type.

Exercise 11.38