Iterators

Iterators is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Iterators are a design pattern that can be used to access elements of a container without depending on the implementation or type of the container.

Task

• Create an array like container to hold the days of the week and a linked-list like container to hold colors.
• Print all of the elements of each container.
• Print the first, fourth, and fifth elements of each container.
• Print the last, fourth to last, and fifth to last of each container.

If you language supports iterators, use them. Otherwise show how access to elements can be separated from the containers that hold them.

C++

<lang cpp>#include <iostream>

1. include <list>
2. include <string>
3. include <vector>

using namespace std;

// Use iterators to print all of the elements of any container that supports // iterators. It print elements starting at 'start' up to, but not // including, 'sentinel'. void PrintContainer(forward_iterator auto start, forward_iterator auto sentinel) {

 for(auto it = start; it != sentinel; ++it)
 {
   cout << *it << " "; 
 }
 cout << "\n";

}

// Use an iterator to print the first, fourth, and fifth elements void FirstFourthFifth(input_iterator auto it) {

 cout << *it;
 advance(it, 3);
 cout << ", " << *it;
 advance(it, 1);
 cout << ", " << *it;
 cout << "\n";

}

int main() {

 // Create two differnt kinds of containers of strings
 vector<string> days{"Sunday", "Monday", "Tuesday", "Wednesday",
  "Thursday", "Friday", "Saturday"};
 list<string> colors{"Red", "Orange", "Yellow", "Green", "Blue", "Purple"};

 cout << "All elements:\n";
 PrintContainer(days.begin(), days.end());
 PrintContainer(colors.begin(), colors.end());
 
 cout << "\nFirst, fourth, and fifth elements:\n";
 FirstFourthFifth(days.begin());
 FirstFourthFifth(colors.begin());

 cout << "\nReverse first, fourth, and fifth elements:\n";
 FirstFourthFifth(days.rbegin());
 FirstFourthFifth(colors.rbegin());

} </lang>

Output:

All elements:
Sunday Monday Tuesday Wednesday Thursday Friday Saturday 
Red Orange Yellow Green Blue Purple 

First, fourth, and fifth elements:
Sunday, Wednesday, Thursday
Red, Green, Blue

Reverse first, fourth, and fifth elements:
Saturday, Wednesday, Tuesday
Purple, Yellow, Orange

J

J's operations are designed to be applied to the data structure as a whole, and this explicitly includes mapping between representations. Also, all data in J is array-like, and type is data. And this necessarily includes linked lists (though we can introduce arbitrarily complex mechanisms to obfuscate a linked list structure).

Still, one approach here might be:

<lang J>dow=: ;:'monday tuesday wednesday thursday friday saturday sunday' col=: (,<)/;:'red orange yellow green blue purple'</lang>

This gives us:

<lang J>

  dow

┌──────┬───────┬─────────┬────────┬──────┬────────┬──────┐ │monday│tuesday│wednesday│thursday│friday│saturday│sunday│ └──────┴───────┴─────────┴────────┴──────┴────────┴──────┘

  col

┌───┬─────────────────────────────────────────┐ │red│┌──────┬────────────────────────────────┐│ │ ││orange│┌──────┬───────────────────────┐││ │ ││ ││yellow│┌─────┬───────────────┐│││ │ ││ ││ ││green│┌────┬────────┐││││ │ ││ ││ ││ ││blue│┌──────┐│││││ │ ││ ││ ││ ││ ││purple││││││ │ ││ ││ ││ ││ │└──────┘│││││ │ ││ ││ ││ │└────┴────────┘││││ │ ││ ││ │└─────┴───────────────┘│││ │ ││ │└──────┴───────────────────────┘││ │ │└──────┴────────────────────────────────┘│ └───┴─────────────────────────────────────────┘</lang>

Here, language's array indexing would see the linked list representation as a two element array. To index arbitrary elements from the linked list, we might map back from the linked list representation to a flat array representation, perhaps using <S:0 (which is a no-op on our array of days of week).

<lang J> echo ;:inv <S:0 dow monday tuesday wednesday thursday friday saturday sunday

  echo ;:inv <S:0 col

red orange yellow green blue purple

  echo ;:inv 0 3 4 {<S:0 dow 

monday thursday friday

  echo ;:inv 0 3 4 {<S:0 col

red green blue

  echo ;:inv _1 _3 _4 {<S:0 dow 

sunday friday thursday

  echo ;:inv _1 _3 _4 {<S:0 col

purple green yellow</lang>

The downside of this approach is that the programmer must understand the data.

However, given the intrinsic value of comprehension when programming, this is frequently a worthwhile price. (There's also an execution cost issue, which can be especially significant in contexts where comprehension is difficult.)

Julia

Translation of: C++

Julia has a iteration interface. Any Julia type which implements the `iterate` function is considered an iterable. In base Julia, this includes any array type or collection, including any subclass of AbstractRange, UnitRange, Tuple, Number, AbstractArray, BitSet, IdDict, Dict, WeakKeyDict, EachLine, AbstractString, Set, Pair, and NamedTuple. In particular, Julia has an extensive set of functions which act on lists and vectors. Julia's Iterators can implement the C++ example: <lang julia>using DataStructures

function PrintContainer(iterator)

   iter = Iterators.Stateful(iterator)
   foreach(x -> print(x, ", "), Iterators.take(iter, length(iter) -1))
   foreach(println, Iterators.take(iter, 1))

end

function FirstFourthFifth(iterator)

   iter = Iterators.Stateful(iterator)
   foreach(x -> print(x, ", "), Iterators.take(iter, 1))
   popfirst!(iter); popfirst!(iter)
   foreach(x -> print(x, ", "), Iterators.take(iter, 1))
   foreach(println, Iterators.take(iter, 1))

end

const days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"] const colors = list("Red", "Orange", "Yellow", "Green", "Blue", "Purple") # this is a linked list

println("All elements:") PrintContainer(days) PrintContainer(colors)

println("\nFirst, fourth, and fifth elements:") FirstFourthFifth(days) FirstFourthFifth(colors)

println("\nReverse first, fourth, and fifth elements:") FirstFourthFifth(reverse(days)) FirstFourthFifth(reverse(colors))

</lang>

Output:

All elements:
Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday
Red, Orange, Yellow, Green, Blue, Purple

First, fourth, and fifth elements:
Sunday, Wednesday, Thursday
Red, Green, Blue

Reverse first, fourth, and fifth elements:
Saturday, Wednesday, Tuesday
Purple, Yellow, Orange

Phix

Phix does not have iterators or for that matter design patterns. Since there are only five builtin data types it is not an issue for a routine to "know" what it is doing.
Something along the lines of Same_fringe#Phix could perhaps also be used to implement custom iterators.

with javascript_semantics
procedure print_all(object s)
    if integer(s) then -- (a dictionary)
        s = apply(true,getd,{getd_all_keys(s),s})
    end if
    printf(1,"%s\n",join(s))
end procedure
 
procedure printFirstFourthFifth(object s, integer d=+1)
    if integer(s) then -- (a dictionary)
        s = apply(true,getd,{getd_all_keys(s),s})
    end if
    printf(1,"%s\n",join(extract(s,sq_mul({1,4,5},d))))
end procedure
 
sequence days = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"}
integer colors = new_dict({{2,"Red"}, {2.5,"Orange"}, {3,"Yellow"}, {"a","Green"}, {"b","Blue"}, {{#CD},"Purple"}})
 
printf(1,"All elements:\n")
print_all(days)
print_all(colors)
 
printf(1,"\nFirst, fourth, and fifth elements:\n")
printFirstFourthFifth(days)
printFirstFourthFifth(colors)
 
printf(1,"\nReverse first, fourth, and fifth elements:\n")
printFirstFourthFifth(days,-1)
printFirstFourthFifth(colors,-1)

Originally I used keys of 1..6 on the colours dictionary, but that looked suspicious. Note that the keys here are a mix of int/flt/string/seq, but still carefully "in order".

Output:

All elements:
Sunday Monday Tuesday Wednesday Thursday Friday Saturday
Red Orange Yellow Green Blue Purple

First, fourth, and fifth elements:
Sunday Wednesday Thursday
Red Green Blue

Reverse first, fourth, and fifth elements:
Saturday Wednesday Tuesday
Purple Yellow Orange

Raku

Raku has iterators, but is rare for casual users to ever explicitly use them. Operators and functions that are designed to work on Iterable objects generally have the iteration semantics built in; so don't need an iterator to be explicitly called. It is far, far more common to use object slices to do the task example operations.

Rakus iterators are one direction only (not reversible), since they are designed to be able to work with infinite streams. It is difficult to reverse a stream that has no end. If the object / collection is finite, it may be reversed, but that is a separate operation from iteration.

Once an iteration has been reified, it is discarded unless explicitly cached. This allows effortless iteration through multi-gigabyte sized data objects and streams without filling up main memory.

The following example iterates though a hash of Positional Iterable objects and demonstrates both using explicit iterators, and object slice operations on each; then has a semi contrived example of where directly using iterators may be actually useful in Raku; collating unique ascending values from several infinite sequence generators.

<lang perl6>my %positional-iterable-types =

   array    => [<Sunday Monday Tuesday Wednesday Thursday Friday Saturday>],
   list     => <Red Orange Yellow Green Blue Purple>,
   range    => 'Rako' .. 'Raky',
   sequence => (1.25, 1.5, * × * … * > 50),

sub first-fourth-fifth ($iterable) {

   my $iterator = $iterable.iterator;
   gather {
       take $iterator.pull-one;
       $iterator.skip-one xx 2;
       take $iterator.pull-one;
       take $iterator.pull-one;
   }

}

say "Note: here we are iterating over the %positional-iterable-types hash, but the order we get elements out is not the same as the order they were inserted. Hashes are not guaranteed to be in any specific order, in fact, they are guaranteed to _not_ be in any specific order.";

for %positional-iterable-types.values {

   say "\nType " ~ .^name ~ ', contents: ' ~ $_ ~
   "\nUsing iterators    : first, fourth and fifth from start: " ~ first-fourth-fifth($_) ~ ', and from end: ' ~ first-fourth-fifth(.reverse) ~
   "\nUsing object slices: first, fourth and fifth from start: " ~ .[0, 3, 4] ~ ', and from end: ' ~ .[*-1, *-4, *-5] ~ "\n";

};

say "\nWhere iterators really shine; when you are collating the values from several infinite generators."; my @i = (1, * × 2 … *).iterator, (1, * × 3 … *).iterator, (1, * × 5 … *).iterator; my @v = @i[0].pull-one, @i[1].pull-one, @i[2].pull-one;

my @seq = lazy gather loop {

   take my $min := @v.min;
   for ^@v { @v[$_] = @i[$_].pull-one if @v[$_] == $min };

}

say @seq[^25];</lang>

Output:

Note: here we are iterating over the %positional-iterable-types hash, but
the order we get elements out is not the same as the order they were inserted.
Hashes are not guaranteed to be in any specific order, in fact, they are
guaranteed to _not_ be in any specific order.

Type Range, contents: Rako Rakp Rakq Rakr Raks Rakt Raku Rakv Rakw Rakx Raky
Using iterators    : first, fourth and fifth from start: Rako Rakr Raks, and from end: Raky Rakv Raku
Using object slices: first, fourth and fifth from start: Rako Rakr Raks, and from end: Raky Rakv Raku


Type Seq, contents: 1.25 1.5 1.875 2.8125 5.273438 14.831543 78.2132149
Using iterators    : first, fourth and fifth from start: 1.25 2.8125 5.273438, and from end: 78.2132149 2.8125 1.875
Using object slices: first, fourth and fifth from start: 1.25 2.8125 5.273438, and from end: 78.2132149 2.8125 1.875


Type Array, contents: Sunday Monday Tuesday Wednesday Thursday Friday Saturday
Using iterators    : first, fourth and fifth from start: Sunday Wednesday Thursday, and from end: Saturday Wednesday Tuesday
Using object slices: first, fourth and fifth from start: Sunday Wednesday Thursday, and from end: Saturday Wednesday Tuesday


Type List, contents: Red Orange Yellow Green Blue Purple
Using iterators    : first, fourth and fifth from start: Red Green Blue, and from end: Purple Yellow Orange
Using object slices: first, fourth and fifth from start: Red Green Blue, and from end: Purple Yellow Orange


Where iterators really shine; when you are collating the values from several infinite generators.
(1 2 3 4 5 8 9 16 25 27 32 64 81 125 128 243 256 512 625 729 1024 2048 2187 3125 4096)

Ring

This example is incorrect. Please fix the code and remove this message. Details: The task is specifically about iterators, not counting loops with indexing.

<lang ring> list = [["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"],

         ["Red","Orange","Yellow","Green","Blue","Purple"]]

ind = [1,4,5] revInd = reverse(ind)

see "working..." +nl see "All elements:" + nl

for n = 1 to len(list)

    for m = 1 to len(list[n])
         see list[n][m] + " "
    next
    see nl

next

see nl + "First, fourth, and fifth elements:" + nl

for n = 1 to len(list)

    for m = 1 to len(ind)
         see list[n][m] + " "
    next
    see nl

next

see nl +"Reverse first, fourth, and fifth elements:" + nl

for n = 1 to len(list)

    for m = 1 to len(revInd)
         see list[n][m] + " "
    next
    see nl

next

see "done..." +nl </lang>

Output:

All elements:
Sunday Monday Tuesday Wednesday Thursday Friday Saturday 
Red Orange Yellow Green Blue Purple 

First, fourth, and fifth elements:
Sunday Monday Tuesday 
Red Orange Yellow 

Reverse first, fourth, and fifth elements:
Sunday Monday Tuesday 
Red Orange Yellow 

Wren

Translation of: C++

Library: Wren-llist

In Wren an iterable object is a sequence whose class implements the iterate and iteratorValue methods. These methods enable one to walk through the sequence and look up the value of the each element.

Iterable objects, which include the built-in classes: List, Range and String, generally inherit from the Sequence class which provides a number of useful methods including: map, where and reduce.

Wren has no built-in linked list class but the Wren-llist module provides singly and doubly-linked implementations of them. Nor does it have a built-in way to iterate through a sequence in reverse though it is possible to write one. Here, we simply reverse the sequence first to do this.

The iterator protocol methods are not usually called directly as Wren's 'for' statement (and the Sequence methods) call them automatically under the hood. However, in the spirit of this task, they are called directly. <lang ecmascript>import "./llist" for DLinkedList

// Use iterators to print all elements of the sequence. var printAll = Fn.new { |seq|

   var iter = null
   while (iter = seq.iterate(iter)) System.write("%(seq.iteratorValue(iter)) ")
   System.print()

}

// Use iterators to print just the first, fourth and fifth elements of the sequence. var printFirstFourthFifth = Fn.new { |seq|

   var iter = null
   iter = seq.iterate(iter)
   System.write("%(seq.iteratorValue(iter)) ")  // first
   for (i in 1..3) iter = seq.iterate(iter)
   System.write("%(seq.iteratorValue(iter)) ")  // fourth
   iter = seq.iterate(iter)
   System.print(seq.iteratorValue(iter))        // fifth

}

// built in list (elements stored contiguously) var days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]

// custom doubly linked list var colors = DLinkedList.new(["Red", "Orange", "Yellow", "Green", "Blue", "Purple"])

System.print("All elements:") printAll.call(days) printAll.call(colors)

System.print("\nFirst, fourth, and fifth elements:") printFirstFourthFifth.call(days) printFirstFourthFifth.call(colors)

System.print("\nReverse first, fourth, and fifth elements:") printFirstFourthFifth.call(days[-1..0]) printFirstFourthFifth.call(colors.reversed)</lang>

Output:

All elements:
Sunday Monday Tuesday Wednesday Thursday Friday Saturday 
Red Orange Yellow Green Blue Purple 

First, fourth, and fifth elements:
Sunday Wednesday Thursday
Red Green Blue

Reverse first, fourth, and fifth elements:
Saturday Wednesday Tuesday
Purple Yellow Orange

XPL0

Translation of: C++

XPL0 doesn't have iterators as intended here. It has no built-in linked lists, thus an array is used instead. This translation is about as close as XPL0 can come to meeting the requirements of the task.

<lang XPL0> \\ Use iterators to print all of the elements of any container that supports \\ iterators. It print elements starting at 'start' up to, but not \\ including, 'sentinel'. proc PrintContainer(Start, Sentinel); int Start, Sentinel, It; [It:= 0; while Start(It) # Sentinel do

   [Text(0, Start(It));  Text(0, " ");
   It:= It+1;
   ];

Text(0, "^m^j"); ];

\\ Use an iterator to print the first, fourth, and fifth elements proc FirstFourthFifth(Start, Dir); int Start, Dir, It; [It:= 0; Text(0, Start(It)); It:= It + 3*Dir; Text(0, ", "); Text(0, Start(It)); It:= It + 1*Dir; Text(0, ", "); Text(0, Start(It)); Text(0, "^m^j"); ];

\\ Create two different kinds of containers of strings int Days, Colors; [Days:= ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday",

   "Friday", "Saturday", 0];

Colors:= ["Red", "Orange", "Yellow", "Green", "Blue", "Purple", 0];

Text(0, "All elements:^m^j"); PrintContainer(Days, 0); PrintContainer(Colors, 0);

Text(0, "^m^jFirst, fourth, and fifth elements:^m^j"); FirstFourthFifth(Days, +1); FirstFourthFifth(Colors, +1);

Text(0, "^m^jReverse first, fourth, and fifth elements:^m^j"); FirstFourthFifth(@Days(7-1), -1); FirstFourthFifth(@Colors(6-1), -1); ]</lang>

Output:

All elements:
Sunday Monday Tuesday Wednesday Thursday Friday Saturday 
Red Orange Yellow Green Blue Purple 

First, fourth, and fifth elements:
Sunday, Wednesday, Thursday
Red, Green, Blue

Reverse first, fourth, and fifth elements:
Saturday, Wednesday, Tuesday
Purple, Yellow, Orange