STL Maps

Notes:

Intro

In class we discussed the Standard Template Library, and all the great things it provides for us. Specifically you were introduced to the set and vector ADT's, and the sort algorithm. Today you'll add another tool to your container toolkit: the map. In addition to the overview provided in this lab, you should do some research of your own: http://www.cplusplus.com/reference/map/map/

The map is one of the most powerful container classes in the standard library. A map stores pairs of objects called key-value pairs. You search for a key in the map and, if it's found, the value paired with that key is returned. Usually it's the value you were interested in. The map class makes use of another templated class named pair, which is defined in the utility library. A pair has data members first and second, which may be of different types. Here is an example of a simple program using just about all you need to know about the class pair:

sample.cpp

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

int main() {
  pair<char,int> A, B;

  cout << "Enter a letter and a number: ";
  cin >> A.first >> A.second;

  cout << "Enter a letter and a number: ";
  cin >> B.first >> B.second;

  if (A.first > B.first)
    cout << "The larger letter is from the first" << endl;
  else
    cout << "The larger letter is from the second" << endl;

  if (A.second > B.second)
    cout << "The larger number is from the first" << endl;
  else
    cout << "The larger number is from the second" << endl;

  return 0;
}

Compiling and Running

$ g++ -o sample sample.cpp
$ ./sample
Enter a letter and a number: D5
Enter a letter and a number: X4
The larger letter is from the second
The larger number is from the first
$

Part 1 (20 pts)

Do some research of the map class. Download part1.txt and answer the following questions in the file:

  1. Are the elements in a map container ordered? If so, how?
  2. Is it possible for two elements in a map to have the same key?
  3. Is it possible for two elements in a map to have the same value?
  4. By default, what comparison operator does the map class use?
  5. What member function of the map class would you call to find out the maximum number of elements a map container can hold as content?

Part 2 (40 pts)

If you type "c++ pair" into google you'll get more information on the pair class. When you declare an object of type map, you specify types for the key and value of the map. For example if you say

map<string,double> M;

that says that the map stores values of type double with keys of type string. So you lookup double values using a string keys. Key-value pairs are added to the map with the insert function. Example:

map<string,double> M;
pair<string,double> PG("gold",14.21);
pair<string,double> PS("silver",8.00);
pair<string,double> PB("bronze",2.25);

M.insert(PG);
M.insert(PS);
M.insert(PB);

Now, given a key (a string in this case), you want to find the matching key-value pair. The find function takes a key as an argument and returns an iterator to a key-value pair with matching key. If no matching key is found, the iterator returned is equal to the M.end() for map M. Example:

map<string,double> M;
pair<string,double> PG("gold",14.21);
pair<string,double> PS("silver",8.00);
pair<string,double> PB("bronze",2.25);

M.insert(PG);
M.insert(PS);
M.insert(PB);

// The following should print out "2.25"
map<string,double>::iterator itr = M.find("bronze");
if (itr != M.end())
  cout << (*itr).second << endl;
else
  cout << "No match found!" << endl;

Your task:

  1. Create a file named map1.cpp
  2. In your new file, place the above code in a main function and get it to compile and run.
  3. Then modify it so that the user enters a string and the program either prints out the price per ounce for that metal or prints out "No match found!".

Part 3 (40 pts)

In this part we'll really use the power of maps. The file rates.txt contains exhange rates for March 25th, 2008. In particular, it gives a currency name, the worth of $1.00 in that currency, and a shorthand name for the currency.

Your task is to write a program called map2 that reads in those currency names (ignore the shorthand names) and exchange rates and uses the information to do currency conversion for the user. Here's an example run of the program:

$ ./map2
Enter <amt> <curr1> <curr2>: 4 Euro JapaneseYen
4 Euro is 620.501 JapaneseYen
Enter <amt> <curr1> <curr2>: 3 AustrianSchilling UnitedStatesDollar
Currency not found!
Enter <amt> <curr1> <curr2>: 7 UnitedStatesDollar Euro
7 UnitedStatesDollar is 4.54723 Euro
Enter <amt> <curr1> <curr2>:

To help you along with this, your first real map programming experience, use the below template. You need to complete the three sections as indicated by the comment blocks:

map2.cpp

#include <iostream>
#include <fstream>
#include <string>
#include <map>
#include <algorithm>
using namespace std;

int main() {
  // Read & store currency names and exchange rates
  ifstream IN("rates.txt");

  /* 1. create new map object named Val that uses a string key and double value */

  string currency, shorthand;

  double valOfDollar;
  while(IN >> currency >> valOfDollar >> shorthand){
    pair<string,double> KV(currency,valOfDollar);

    /* 2. add the new pair to your map */

  }

  // Read amount, currency1 & currency2 and convert
  string curr1, curr2;
  double amt;
  while(cout << "Enter <amt> <curr1> <curr2>: ", cin >> amt >> curr1 >> curr2) {
    map<string,double>::iterator itr1 = Val.find(curr1);
    map<string,double>::iterator itr2 = Val.find(curr2);

    /* 3. What do we do now? Add code to determine whether both currencies were
     *    found and, if so, what the value of "amt" in "curr1" is in "curr2".
     *    Print results!
     */

  }

  return 0;
}

Deliverables

Due: 2359, Monday 12 Nov 2018

Submit your source code via the the submisison website: submit.cs.usna.edu.

Specifically:

Do NOT submit any executable, .o, or provided/unaltered files (i.e. rates.txt)! 

~/bin/submit -c=SI221 -p=Lab09 part1.txt map1.cpp map2.cpp