mapping over hash tables -- apply a function to each element of a hash table

mapping over hash tables -- apply a function to each element of a hash table

Each entry in a hash table consists of a key and a value. We provide three ways to map functions over a hash table, depending on whether the function is to receive a value and return a new value, to receive a key and return a new key, or to receive a key-value pair and return a new key-value pair. The corresponding functions, applyValues, applyKeys, and applyPairs are illustrated in the next example.

i1 : x = new HashTable from {a=>1, b=>2}



o1 = HashTable{a => 1}

               b => 2



o1 : HashTable

i2 : applyValues(x, value -> 1000*value)



o2 = HashTable{a => 1000}

               b => 2000



o2 : HashTable

i3 : applyKeys(x, key -> {key})



o3 = HashTable{{a} => 1}

               {b} => 2



o3 : HashTable

i4 : applyPairs(x, (key,value) -> (value,key))



o4 = HashTable{1 => a}

               2 => b



o4 : HashTable

The functions, scanValues, scanKeys, and scanPairs are similar, but the values returned are discarded instead of being assembled into a new hash table.

i5 : x = new HashTable from {a=>1, b=>2}



o5 = HashTable{a => 1}

               b => 2



o5 : HashTable

i6 : scanValues(x, print)

1

2

i7 : scanKeys(x, print)

a

b

i8 : scanPairs(x, print)

(a, 1)

(b, 2)

The function merge can be used to merge two hash tables. The result is a hash table whose keys are those occuring in one of the two incoming hash tables. We must provide a function of two arguments that is used to combine the values when a key occurs in both hash tables.

i9 : y = new HashTable from {b=>200, c=>300}



o9 = HashTable{b => 200}

               c => 300



o9 : HashTable

i10 : merge(x, y, plus)



o10 = HashTable{a => 1  }

                b => 202

                c => 300



o10 : HashTable

The function combine can be used to combine two hash tables x and y into a new hash table. Three functions must be provided. The first one produces new keys from a key of x and a key of y. The second one produces a new values from a value of x and a value of y. The third one is used to combine values when two new keys turn out to be the same.

i11 : combine(x,y,identity,times,plus)



o11 = HashTable{(a, b) => 200}

                (a, c) => 300

                (b, b) => 400

                (b, c) => 600



o11 : HashTable