Iterators.jl

Common functional iterator patterns.

DEPRECATION

Iterators.jl has been deprecated in favour of IterTools.jl. Please update your package dependencies: Iterators 0.3.1 maps to IterTools 0.1.0.

See #104 for more information.

Installation

Install this package with Pkg.add("Iterators")

Usage

• takestrict(xs, n)

  Equivalent to take, but will throw an exception if fewer than n items are encountered in xs.

• repeatedly(f, [n])

  Call a function n times, or infinitely if n is omitted.

  Example:

  for t in repeatedly(time_ns, 3)
      @show t
  end

  t = 0x0000592ff83caf87
  t = 0x0000592ff83d8cf4
  t = 0x0000592ff83dd11e
  

• chain(xs...)

  Iterate through any number of iterators in sequence.

  Example:

  for i in chain(1:3, ['a', 'b', 'c'])
      @show i
  end

  i = 1
  i = 2
  i = 3
  i = 'a'
  i = 'b'
  i = 'c'
  

• product(xs...)

  Iterate over all combinations in the cartesian product of the inputs.

  Example:

  for p in product(1:3,1:2)
      @show p
  end

  yields

  p = (1,1)
  p = (2,1)
  p = (3,1)
  p = (1,2)
  p = (2,2)
  p = (3,2)
  

• distinct(xs)

  Iterate through values skipping over those already encountered.

  Example:

  for i in distinct([1,1,2,1,2,4,1,2,3,4])
      @show i
  end

  i = 1
  i = 2
  i = 4
  i = 3
  

• nth(xs, n)

  Return the n'th element of xs. Mostly useful for non indexable collections.

  Example:

  nth(1:3, 3)

  3
  

• takenth(xs, n)

  Iterate through every n'th element of xs

  Example:

  collect(takenth(5:15,3))

  3-element Array{Int32,1}:
    7
   10
   13
  

• partition(xs, n, [step])

  Group values into n-tuples.

  Example:

  for i in partition(1:9, 3)
      @show i
  end

  i = (1,2,3)
  i = (4,5,6)
  i = (7,8,9)
  

  If the step parameter is set, each tuple is separated by step values.

  Example:

  for i in partition(1:9, 3, 2)
      @show i
  end

  i = (1,2,3)
  i = (3,4,5)
  i = (5,6,7)
  i = (7,8,9)
  

• groupby(f, xs)

  Group consecutive values that share the same result of applying f.

  Example:

  for i in groupby(x -> x[1], ["face", "foo", "bar", "book", "baz", "zzz"])
      @show i
  end

  i = ASCIIString["face","foo"]
  i = ASCIIString["bar","book","baz"]
  i = ASCIIString["zzz"]
  

• imap(f, xs1, [xs2, ...])

  Iterate over values of a function applied to successive values from one or more iterators.

  Example:

  for i in imap(+, [1,2,3], [4,5,6])
       @show i
  end

  i = 5
  i = 7
  i = 9
  

• subsets(xs)

  Iterate over every subset of a collection xs.

  Example:

  for i in subsets([1,2,3])
   @show i
  end

  i = []
  i = [1]
  i = [2]
  i = [1,2]
  i = [3]
  i = [1,3]
  i = [2,3]
  i = [1,2,3]
  

• subsets(xs, k)

  Iterate over every subset of size k from a collection xs.

  Example:

  for i in subsets([1,2,3],2)
   @show i
  end

  i = [1,2]
  i = [1,3]
  i = [2,3]
  

• peekiter(xs)

  Add possibility to peek head element of an iterator without updating the state.

  Example:

  it = peekiter(["face", "foo", "bar", "book", "baz", "zzz"])
  s = start(it)
  @show peek(it, s)
  @show peek(it, s)
  x, s = next(it, s)
  @show x
  @show peek(it, s)

  peek(it,s) = Nullable("face")
  peek(it,s) = Nullable("face") # no change
  x = "face"
  peek(it,s) = Nullable("foo")
  

• ncycle(xs,n)

  Cycles through an iterator n times

  Example:

  for i in ncycle(1:3, 2)
      @show i
  end

  i = 1
  i = 2
  i = 3
  i = 1
  i = 2
  i = 3
  

• iterate(f, x)

  Iterate over successive applications of f, as in f(x), f(f(x)), f(f(f(x))), ....

  Example:

  for i in take(iterate(x -> 2x, 1), 5)
      @show i
  end

  i = 1
  i = 2
  i = 4
  i = 8
  i = 16
  

The @itr macro for automatic inlining in for loops

Using functional iterators is powerful and concise, but may incur in some overhead, and manually inlining the operations can typically improve performance in critical parts of the code. The @itr macro is provided to do that automatically in some cases. Its usage is trivial: for example, given this code:

for (x,y) in zip(a,b)
    @show x,y
end

the automatically inlined version can be obtained by simply doing:

@itr for (x,y) in zip(a,b)
    @show x,y
end

This typically results in faster code, but its applicability has limitations:

• it only works with for loops;
• if multiple nested iterators are used, only the outermost is affected by the transformation;
• explicit expressions are required (i.e. when a Tuple is expected, an explicit tuple must be provided, a tuple variable won't be accepted);
• splicing is not supported;
• multidimensional loops (i.e. expressions such as for x in a, y in b) are not supported

The @itr macro can be used with the following supported iterators:

• zip
• enumerate
• take
• takestrict
• drop
• chain