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-- status: rewritten September 2018
-- author: Lily Silverstein
doc ///
Key
merge
(merge, HashTable, HashTable, Function)
Headline
merge two hash tables
Usage
z = merge(x, y, f)
Inputs
x:HashTable
y:HashTable
f:Function
of two variables specifying how to handle duplicate keys
Outputs
z:HashTable
a new hash table whose keys are the union of
the keys of {\tt x} and the keys of {\tt y}
Description
Text
Key-value pairs whose keys appear in only one of the input tables
appear unchanged in the output table. If the key {\tt k} appears in
both {\tt x} and {\tt y}, then {\tt f(x#k, y#k)} is the value
associated to {\tt k} in the output hash table.
For example, we could take the max of the two values, their average,
or make a list containing both.
Example
x = new HashTable from {1 => 203, 2 => 21, 3 => 5, 4 => 130}
y = new HashTable from {2 => 37, 3 => 5, 4 => 56, 5 => 1}
merge(x, y, max)
merge(x, y, (i,j) -> (i+j)/2)
merge(x, y, (i,j) -> {i,j})
Text
If the function {\tt f(x#k, y#k)} returns @TO "continue"@, then the key {\tt k} is
omitted from the merged table.
Example
merge(x, y, (i,j) -> if i==j then i else continue)
Text
Here is a simple implementation of the free abelian group on
four letters, where each element is represented as
a hash table that associates coefficients to strings.
Example
Free = new Type of HashTable
p = new Free from { "x" => 2, "y" => 3, "z" => 5 }
q = new Free from { "x" => 100, "y" => 200, "w" => 7 }
Free + Free := (p,q) -> merge(p, q, plus);
p+q
Text
If {\tt x} and {\tt y} have the same @TO class@
and have the same @TO parent@, as in the previous example,
then so will {\tt z}.
Example
x = new MutableHashTable from {"alice" => 53709, "bob" => 6549};
y = new MutableHashTable from {"bob" => 86, "charlie" => 23};
mutable merge(x, y, plus)
Text
The function @TO combine@ allows much greater control when combining
two hash tables: you can give functions for how to handle every key and
value of the input tables, not just the duplicates. The function
@TO mergePairs@ is similar to {\tt merge}, but works on lists of pairs
rather than hash tables.
SeeAlso
applyKeys
applyPairs
applyValues
combine
keys
mergePairs
pairs
scanKeys
scanPairs
scanValues
values
"hash tables"
///
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