Collection types

CS101 2012.1

Chakrabarti

Motivation Thus far the only collection types we have

used are vector<T> and matrix<T> Problem #1: given an input stream of

numbers between 0 and 9, maintain a histogram of counts of each number• ++hist[num]

Problem #2: given a series of (string) words from a long text, maintain a histogram of counts of each word• ++hist[word] — how to write?

Chakrabarti

unordered_map<K,V> K is the type of the key (for us, string) V is the type of the value (for us, int) Declaration looks likeunordered_map<string, int> hist;

Initially empty Can now write int hn = hist[“hello”]; If key “hello” does not exist, it will be created

and value initialized to zero Can also write hist[“world”] = 5; And ++hist[“world”];

Chakrabarti

Iterating over an unordered map Provides begin() and end() functions like

other collections Which have type unordered_map<K,V>::iterator

Code looks likefor (unordered_map<K,V>::iterator hx = hist.begin(); hx != hist.end(); ++hx) { … }

Inside the loop, access key as hx->first And value as hx->second

Chakrabarti

Testing if a key exists hist.find(“peace”) returns an unordered_map<string, int>::iterator

If key “peace” exists in the map then we get a valid iterator from which we can access first and second

Otherwise, the returned value equals hist.end()

Will not modify hist if key not foundif (hist.find(“peace”)!=hist.end()) { // do stuff with key and value}

Chakrabarti

Printing histogram sorted by words Two ways Extract all keys into a vector<K>, sort it,

iterate over it, while extracting values from hist

Use an ordered map<K,V> Feels very similar to unordered_map

except iterator runs in increasing key order Can also do key range traversal

• find(key), lower_bound(lowKey), upper_bound(highKey)

Chakrabarti

Extracting the minimum element map<K,V> myMap; If myMap is non-empty, myMap.begin()

points to the entry with the smallest key After recording the key and value, can

remove it using myMap.erase(myMap.begin());

Chakrabarti

multimap<K, V> unordered_map<K,V> and map<K,V> allow

at most one entry for each distinct key This is not a loss of generality, because you

could always declaremap<string, vector<int>> myMultiMap;

But for convenience, C++ also provides multimap<K,V>

Does not provide hist[word] because it is not clear which entry you mean

Access via iterator and pair<K,V> only

Chakrabarti

list<T>

Like a train but more flexible in some ways Unlike vector<T>, no access by index Only access by iterator Can delete and insert item at iterator in

constant time front, push_front, pop_front back, push_back, pop_back

a b c e

begin endd

Chakrabarti

A queue simulation problem Customers arrive at a queue If many queues, join the shortest one Arrival time between successive customers

follows some distribution Service time at the queue follows some

distribution Exponential distribution is often used

How many toll lanes? Typical delay?

Chakrabarti

Event simulation Events happen at discrete points in time 0th customer C0 arrives at time 0 Triggers two events in future

• 0th customer C0 leaves queue after service• 1th customer C1 arrives

multimap<Time,??> registers events

0

C0 arrives

C0’s service time

C0 leaves

Inter-arrival time

C1 arrives

C1 waits

C1’s service time

C1 leaves

Chakrabarti

Event simulation loop Remove next event from event map If event is customer arrival

• Assign customer ID, record arrival time• Push customer to back of queue• Generate random next customer arrival time• Record next customer arrival time in event map

If event is customer service completion• Pop customer from front of queue• Collect statistics• Generate random completion time for next

customer and record in event mapIf this customer is at the front of queue, can start

service immediately

Chakrabarti

Event simulation data structures

Customer at front ofqueue, being served

pop_front

list<Customer> queue

multimap<Time, Action> events

Next event What will happen

For each customer:• Customer ticket number• Arrival time, start service time, end service time

struct

push_back