Multisets
A Set
is an unordered collection of things in which an item may appear at most once.
A Multiset
is an unordered collection of things with repetition permitted.
New in Version 0.4.x
 A
Multiset
is now a subtype ofAbstractSet
.  Set operations between a
Multiset
and aSet
(orBitSet
) are now supported.  The operators
&
and
have been removed; use∩
and∪
instead.  The operators
<
,<=
,>
, and>=
have been removed. Use⊆
and⊇
instead, or use the functionissubset
.
Creating a Multiset
julia> using Multisets
julia> M = Multiset{Type}()
where Type
is the type of elements to be held in M
(e.g., Int
or String
).
If Type
is omitted, this defaults to Any
.
Given a collection list
of elements (such as a Vector
or Set
)
invoking Multiset(list)
creates a new Multiset
in which the elements
of list
appear with the appropriate multiplicity. For example,
Multiset(ones(Int,3))
creates the multiset {1,1,1}
.
julia> M = Multiset([1,1,2,3,5])
{1,1,2,3,5}
julia> M = Multiset(5,3,2,1,1)
{1,1,2,3,5}
julia> eltype(M)
Int64
Adding/Deleting Elements
push!(M,x)
increases the multiplicity ofx
inM
by 1. Ifx
is not already inM
, then it is added toM
.push!(M,x,incr)
increases the multiplicity ofx
inM
byincr
. We allowincr
to be negative to decrease the multiplicity ofx
(but not below 0).M[x]=m
explicitly sets the multiplicty ofx
tom
.delete!(M,x)
removesx
fromM
.M[x]=0
has the same effect.
Keys/Values/Pairs
keys(M)
returns an iterator for the elements ofM
(that have multiplicyt at least one).values(M)
returns an iterator for the multiplicities of the elements ofM
.pairs(M)
returns an iterator overelement => multiplicity
pairs forM
.
julia> M = Multiset("alpha", "beta", "beta", "gamma", "gamma", "gamma")
{alpha,beta,beta,gamma,gamma,gamma}
julia> collect(keys(M))
3element Array{String,1}:
"alpha"
"gamma"
"beta"
julia> collect(values(M))
3element Array{Int64,1}:
1
3
2
julia> pairs(M)
Dict{String,Int64} with 3 entries:
"alpha" => 1
"gamma" => 3
"beta" => 2
Access
To determine the multiplicity of x
in M
use M[x]
. This returns 0
if x
was never added to M
.
To get a list of all the elements in M
, use collect
:
julia> collect(M)
6element Array{Int64,1}:
1
1
2
2
3
4
Notice that elements are repeated per their multiplicity.
To get a list of the elements in which elements appear
only once each use collect(keys(M))
.
To convert M
into a Julia Set
(effectively, set all multiplicities to 1)
use Set(M)
:
julia> Set(M)
Set([4,2,3,1])
Printing
The result of println(M)
can be controlled by the following functions.
Suppose a multiset is created as follows:
julia> M = Multiset{String}();
julia> push!(M,"alpha");
julia> push!(M,"beta", 2);
set_braces_show()
causes multisets to be printed as a list enclosed in curly braces:{alpha,beta,beta}
. This is the default. If the multiset is empty,∅
is printed.set_short_show()
causes multisets to be printed in an abbreviated format like this:Multiset{String} with 3 elements
.set_julia_show()
causes multisets to be printed in a form that would be a proper Julia definition of that multiset:Multiset(String["alpha","beta","beta"])
.
Operations
Union/Intersection
The functions union
and intersect
compute the union and intersection
of multisets. For example:
julia> A = Multiset(1,2,2,3)
{1,2,2,3}
julia> B = Multiset(1,1,1,2,4)
{1,1,1,2,4}
julia> union(A,B)
{1,1,1,2,2,3,4}
julia> intersect(A,B)
{1,2}
The multiplicity of x
in union(A,B)
is max(A[x],B[x])
and
the multiplicity in intersect(A,B)
is min(A[x],B[x])
.
See +
below (disjoint union) which behaves differently.
Product/Sum/Difference

The Cartesian product of multisets
A
andB
is computed withA*B
. Ifa
is an element ofA
andb
is an element ofB
then the multiplicity of(a,b)
inA*B
isA[x]*B[x]
. 
For a nonnegative integer
n
and a multisetA
the result ofn*A
is a new multiset in which the multiplicy ofx
isn*A[x]
. 
The disjoint union of
A
andB
is computed withA+B
. Ifa
is an element ofA
andb
is an element ofB
then the multiplicity of(a,b)
inA*B
isA[x]+B[x]
. 
The difference of multisets is computed as
AB
. In the result, the multiplicity ofx
ismax(0, A[x]B[x])
. This is not the same assetdiff
becausesetdiff(A,B)
gives a multiset in which any element ofB
is completely removed fromA
.
julia> A = Multiset(1,1,2,3)
{1,1,2,3}
julia> B = Multiset(1,2)
{1,2}
julia> AB
{1,3}
julia> setdiff(A,B)
{3}
Cardinality
The function length
computes the cardinality (number of elements)
in a multiset (including multiplicities).
The function isempty
returns true
exactly when length(M)==0
.
Comparison
The operator A==B
and the function issubset(A,B)
are provided to determine
if A
and B
are equal or A
is a submultiset of B
. This can also be expressed
as A ⊆ B
or B ⊇ A
.
Note that A==B
holds when A[x]==B[x]
for all x
and issubset(A,B)
holds when A[x] <= B[x]
for all x
.
To test if x
is an element of a multiset A
, one may use any of the following:
x ∈ A
in(x,A)
A ∋ x
A[x] > 0
Iteration
When iterating over a Multiset
each element is repeated according to its
multiplicity.
julia> A = Multiset(1,2,1,2,3)
{1,1,2,2,3}
julia> for a in A
println(a)
end
2
2
3
1
1
julia> sum(A)
9
Multisets as Counters
Multisets are useful devices for counting. For example, suppose a program
reads in words from a text file and we want to count how often each word
appears in that file. We can let M = Multiset{String}()
and then
step through the words in the file pushing each instance into M
.
The basic structure looks like this:
for word in FILE
push!(M,word)
end
In the end, M[word]
will return how often word
was seen in the file.
See also my Counters
module.
Miscellaneous
Implementation
A Multiset
consists of a single data field called data
that is a
dictionary mapping elements to their multiplicities. The various
Multiset
functions ensure the integrity of data
(enforcing nonnegativity).
The function clean!
purges the data
field of any elements with multiplicity
equal to 0
. This is used by the hash
function which is provided so a Multiset
can be used as a key in a dictionary, etc. The hash of a
Multiset
is simply the hash of its cleaned data
field.
Note: The clean!
function is not exported. There probably should be no
reason for the user to invoke it, but if desired use
Multisets.clean!(M)
.
Multiset
Making a copy of a If A
is a Multiset
, then B=A
assigns B
to be the same object as A
. That is,
any changes to one affects the other:
julia> A = Multiset(1,1,2,3,5)
{1,1,2,3,5}
julia> B = A
{1,1,2,3,5}
julia> A[8]=2;
julia> A
{1,1,2,3,5,8,8}
julia> B
{1,1,2,3,5,8,8}
To make an independent copy, use deepcopy
:
julia> A = Multiset(1,1,2,3,5)
{1,1,2,3,5}
julia> B = deepcopy(A)
{1,1,2,3,5}
julia> A[8]=2;
julia> A
{1,1,2,3,5,8,8}
julia> B
{1,1,2,3,5}