Count things easily.
Author scheinerman
2 Stars
Updated Last
6 Months Ago
Started In
September 2017


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We often want to count things and a way to do that is to create a dictionary that maps objects to their counts. A Counter object simplifies that process. Say we want to count values of type String. We would create a counter for that type like this:

julia> c = Counter{String}()
Counter{String} with 0 entries

The two primary operations for a Counter are value increment and value retrieval. To increment the value of a counter we do this:

julia> c["hello"] += 1

To access the count, we use square brackets:

julia> c["hello"]

julia> c["bye"]

Notice that we need not worry about whether or not a key is already known to the Counter. If presented with an unknown key, the Counter assumes its value is 0.

A Counter may be assigned to like this c["alpha"]=4 but the more likely use case is c["bravo"]+=1 invoked each time a value, such as "bravo" is encountered.

Counting the elements of a list

The function counter (lowercase 'c') counts the element of a list/array or set. The multiplicity of an element is the number of times it appears in the list.

julia> A = [ "alpha", "bravo", "alpha", "gamma" ];

julia> C = counter(A);

julia> showall(C)
Counter{String} with these nonzero values:
alpha ==> 2
bravo ==> 1
gamma ==> 1

julia> counter(eye(3))
SimpleTools.Counter{Float64} with 2 entries:
  0.0 => 6
  1.0 => 3

Addition of counters

If c and d are counters (of the same type of object) their sum c+d creates a new counter by adding the values in c and d. That is, if a=c+d and k is any key, then a[k] equals c[k]+d[k].


To increment the count of an item x in a counter c we may either use c[x]+=1 or the increment function like this: incr!(c,x).

The increment function incr! is more useful for incrementing a collection of items. Use incr!(c,items) to add 1 to the count for each element held in items. If an element is present in items multiple times, its count is incremented for each occurrence.

julia> c = Counter{Int}()
SimpleTools.Counter{Int64} with 0 entries

julia> items = [1,2,3,4,1,2,1]
7-element Array{Int64,1}:

julia> incr!(c,items)

julia> showall(c)
Counter{Int64} with these nonzero values:
Counter{Int64} with these nonzero values:
1 ==> 3
2 ==> 2
3 ==> 1
4 ==> 1

In addition, incr! may be used to increment one counter by the amount held in another. Note that it's the first argument c that gets changed; there is no effect on the second argument d.

Note: incr!(c,d) and c += d have the same effect, but the first is more efficient.

julia> d = Counter{Int}();

julia> d[1] = 1;;

julia> d[5] = 1;

julia> incr!(c,d)

julia> showall(c)
Counter{Int64} with these nonzero values:
1 ==> 4
2 ==> 2
3 ==> 1
4 ==> 1
5 ==> 1

More functions

  • sum(c) returns the sum of the values in c; that is, the total of all the counts.
  • length(c) returns the number of values held in c. Note that this might include objects with value 0.
  • nnz(c) returns the number of nonzero values held in c.
  • keys(c) returns an iterator for the keys held by c.
  • values(c) returns an iterator for the values held by c.
  • showall(c) gives a print out of all the keys and their nonzero values in c.
  • clean!(c) removes all keys from c whose value is 0. This won't change its behavior, but will free up some memory.

In addition, we can convert a Counter into a one-dimensional array in which each element appears with its appropriate multiplicity using collect:

julia> C = Counter{Int}()
SimpleTools.Counter{Int64} with 0 entries

julia> C[3] = 4

julia> C[5] = 0

julia> C[-2] = 2

julia> collect(C)
6-element Array{Int64,1}:

julia> collect(keys(C))
3-element Array{Int64,1}:

Average value

If the objects counted in C are numbers, then we compute the weighted average of those numbers with mean(C).

julia> C = Counter{Int}()
SimpleTools.Counter{Int64} with 0 entries

julia> C[2] = 3

julia> C[3] = 7

julia> mean(C)


hash(C::Counter) returns a hash value for the C. Note that clean! is applied to C before computing the hash. This is done to ensure that equal counters give the same hash value.

May also be invoked as hash(C::Counter, h::Uint).

It's Associative

A Counter is a subtype of Associative and therefore we can use methods such as keys and/or values to get iterators to those items.

CSV Printing

The function csv_print writes a Counter to the screen in comma-separated format. This can be readily used for importing into a spreadsheet.

julia> C = Counter{Float64}()
SimpleTools.Counter{Float64} with 0 entries

julia> C[3.4]=10

julia> C[2.2]=3

julia> csv_print(C)
2.2, 3
3.4, 10

Counting in parallel

See the parallel-example directory for an illustration of how to use Counters in multiple parallel processes.