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+# Lab 9 — Maps
+
+This lab gives you practice initial practice in working with the STL associative container, maps. No downloads
+are needed until Checkpoint 3.
+
+## Checkpoint 1
+*estimate: 10-20 minutes*
+
+Write a program from scratch that uses a map to find all the modes in an input sequence of integers.
+Remember, a mode is an integer that occurs at least as many times in the sequence as any other integer.
+
+Thus, in the sequence
+
+```console
+19 83 -12 83 65 19 45 -12 45 19 45
+```
+
+the two modes are 19 and 45. Include one command-line argument to provide an input file. Use operator[] for maps when inserting values.
+
+**To complete this checkpoint**: show a TA your debugged implementation and how it runs correctly on several interesting test cases.
+
+## Checkpoint 2
+*estimate: 10-20 minutes*
+
+Rewrite your program from checkpoint 1 to use find or insert or both instead of operator[].
+
+**To complete this checkpoint**: show a TA your revised and tested program.
+
+## Checkpoint 3
+*estimate: 20-40 minutes*
+
+Please download the [phonebook.cpp](phonebook.cpp) needed for the final checkpoint:
+
+This code implements a simple caller ID program. This program could be used by a university or company
+to perform a “reverse lookup” — given the 4 digit phone number extension, return the name of the caller. A
+vector of strings stores the complete database of names assigned to phone number extensions. Compile and
+run this program. Add your own test cases to the main function.
+
+Part 1: Analyze the computational cost of this program using the big ‘O’ notation in terms of n the number
+of assigned phone numbers in the phonebook, and N the largest possible phone number (number of different
+possible phone numbers). What is the running time of constructing the phonebook and of the add and
+identify functions? How much memory is used by this program? Express this using order notation. What
+would happen if you extended this to a 7- or a 10-digit number? Would it work on a cell phone?
+
+Part 2: Rewrite this program to use maps, storing only the numbers that are assigned. Analyze the cost of
+creating the map, and of the add and the identify functions. Test it on 7 digit numbers. In what ways is the
+vector version better and in what ways is the map version better?
+
+To complete this checkpoint: Present your analysis from Part 1 to a TA, demonstrate the new version
+of your program, and be prepared to discuss your analysis of Part 2.
diff --git a/labs/09_maps/phonebook.cpp b/labs/09_maps/phonebook.cpp
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+// A simple "caller ID" program
+
+#include <iostream>
+#include <vector>
+#include <string>
+using namespace std;
+
+// add a number, name pair to the phonebook
+void add(vector<string> &phonebook, int number, string const& name) {
+  phonebook[number] = name;
+}
+
+// given a phone number, determine who is calling
+void identify(const vector<string> & phonebook, int number) {
+  if (phonebook[number] == "UNASSIGNED") 
+    cout << "unknown caller!" << endl;
+  else 
+    cout << phonebook[number] << " is calling!" << endl;
+}
+
+
+int main() {
+  // create the phonebook; initially all numbers are unassigned
+  vector<string> phonebook(10000, "UNASSIGNED");
+
+  // add several names to the phonebook
+  add(phonebook, 1111, "fred");
+  add(phonebook, 2222, "sally");
+  add(phonebook, 3333, "george");
+
+  // test the phonebook
+  identify(phonebook, 2222);
+  identify(phonebook, 4444);
+}