L07 - Part 2 - Hashing and Maps.pdf

L07 - Part 2 - Hashing and Maps.pdf - Hashing Maps Maps A...

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Hashing
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Maps
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© Goodrich and Tamassia Maps } A map models a searchable collection of key-value entries } The main operations of a map are for } searching, } inserting, and } deleting items } Multiple entries with the same key are not allowed } Applications: } address book } student-record database 3
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© Goodrich and Tamassia The Map ADT } get (k): if the map M has an entry with key k, return its associated value; else, return null } put (k, v): insert entry (k, v) into the map M; if key k is not already in M, then return null; else, return old value associated with k } remove (k): if the map M has an entry with key k, remove it from M and return its associated value; else, return null } size (), isEmpty () } entrySet (): return an iterable collection of the entries in M } keySet (): return an iterable collection of the keys in M } values (): return an iterator of the values in M 4
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© Goodrich and Tamassia Example Operation Output Map isEmpty() true Ø put(5 ,A ) null (5 ,A ) put(7 ,B ) null (5 ,A ) , (7 ,B ) put(2 ,C ) null (5 ,A ) , (7 ,B ) , (2 ,C ) put(8 ,D ) null (5 ,A ) , (7 ,B ) , (2 ,C ) , (8 ,D ) put(2 ,E ) C (5 ,A ) , (7 ,B ) , (2 ,E ) , (8 ,D ) get(7) B (5 ,A ) , (7 ,B ) , (2 ,E ) , (8 ,D ) get(4) null (5 ,A ) , (7 ,B ) , (2 ,E ) , (8 ,D ) get(2) E (5 ,A ) , (7 ,B ) , (2 ,E ) , (8 ,D ) size() 4 (5 ,A ) , (7 ,B ) , (2 ,E ) , (8 ,D ) remove(5) A (7 ,B ) , (2 ,E ) , (8 ,D ) remove(2) E (7 ,B ) , (8 ,D ) get(2) null (7 ,B ) , (8 ,D ) isEmpty() false (7 ,B ) , (8 ,D ) 5
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© Goodrich and Tamassia A Simple List-Based Map } We can efficiently implement a map using an unsorted list } We store the items of the map in a list S (based on a doubly-linked list), in arbitrary order trailer header nodes/positions entries 9 c 6 c 5 c 8 c 6
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© Goodrich and Tamassia The get(k) Algorithm Algorithm get(k): B = S.positions() {B is an iterator of the positions in S} while B.hasNext() do p = B.next() { the next position in B } if p.element().getKey() == k then return p.element().getValue() return null {there is no entry with key equal to k} 7 trailer header nodes/positions entries 9 c 6 c 5 c 8 c
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© Goodrich and Tamassia The put(k,v) Algorithm Algorithm put(k,v): B = S.positions() while B.hasNext() do p = B.next() if p.element().getKey() == k then t = p.element().getValue() S.set(p,(k,v)) return t {return the old value} S.addLast((k,v)) n = n + 1 {increment variable storing number of entries} return null { there was no entry with key equal to k } 8
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© Goodrich and Tamassia The remove(k) Algorithm Algorithm remove(k): B =S.positions() while B.hasNext() do p = B.next() if p.element().getKey() = k then t = p.element().getValue() S.remove(p) n = n – 1 {decrement number of entries} return t {return the removed value} return null {there is no entry with key equal to k} 9 Doubly linked list!
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© Goodrich and Tamassia Performance of a List-Based Map } Performance: } put takes O (1) time since we can insert the new item at the beginning or at the end of the sequence } get and remove take O ( n ) time since in the worst case (the item is not found) we traverse the entire sequence to look for an item with the given key } The unsorted list implementation is effective only for maps of small size or for maps in which puts are the most common operations, while searches and removals are rarely performed (e.g., historical record of logins to a workstation) 10
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© Goodrich and Tamassia Performance of a List-Based Map }
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