Data Structures
HashMap
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A hash table (also known as associative array, lookup table, dictionary, hash)
is a way of storing values against keys. Often the keys are strings, in this
example the values are numbers.
Typical operations required:
- add or put or set
- insert a new (key,value) pairs
- lookup or get
- retrieve the value for a given key
- iterate
- run over every key
hash
| key | value |
|---|
| andrew | 2753 |
| sally | 2742 |
| gordon | 2754 |
This structure is so useful that we find it in most programming languages.
Often there are many different options that have subtly different
characteristics. Usually we want fast retrieval and we may or may not care
about: fast insert, efficient use of memory, the order of the keys.
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We can put values into a HashMap then get
them out.
The HashMap allows us to index a list of items using a string (or other
objects). This we can treat the HashMap as a lookup table.
The HashMap is fabulously useful. It is fast for both inserts and
lookups.
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See also
- TreeMap
- The
TreeMap is slower, but it keeps keys in sort order
- Hashtable
- The
Hashtable is slower than HashMap but it is safe to use
if more than one thread is reading/writing to the structure.
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1. [ C++ ] Telephone book
1. [ Java ] Telephone bookWhen we create the structure we specify the type of the
key and the type of the value.
We can lookup the phone number for "Sally" using the get method.
test text
1. [ C# ] Telephone bookWhen we create the structure we specify the type of the
key and the type of the value.
We can lookup the phone number for "Sally" using the get method.
test text
2. [ Java ] Missing itemsIf we look for an item that isn't there the null value is returned.
In this example we try to retrieve an entry that isn't in the list.
We also use containsKey to test if the item exists.
test text
2. [ C# ] Missing itemsYou don't want to risk trying to retrieve a value that is not in the
list. It will cause an exceptions.
Instead we can test using ContainsKey
test text
3. [ C++ ] Getting all values back
3. [ Java ] Getting all values backCommonly we want to loop over all of the keys in the HashMap.
The method keySet() permits this. We can also use
values() to get
a list of values or entrySet() to get both keys and values.
test text
3. [ C# ] Getting all values backThe property Keys permits this. We can also use
Values to get
a list of values.
test text
4. [ C++ ] Getting keys back in the right orderIf you want the keys in order it is best to use a structure that
maintains order in the first place.
test text
4. [ Java ] Getting keys back in the right orderIf you want the keys in order it is best to use a structure that
maintains order in the first place.
The keys come back as a Set - this cannot be sorted.
However we can copy them into a ArrayList this can be sorted.
If we want to maintain key order we use a TreeMap
test text
4. [ C# ] Getting keys back in the right orderIf you want the keys in order it is best to use a structure that
maintains order in the first place.
For C# use a SortedDictionary test text
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