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# 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.

• 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.

## BASIC256

Translation of: FreeBASIC
`arraybase 1dim list\$ = {{"Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"}, {"Red","Orange","Yellow","Green","Blue","Purple"}}dim ind = {1,4,5} print "All elements:"for n = 1 to list\$[?,]    for m = 1 to list\$[,?]	print list\$[n, m]; " ";    next m    printnext nprint print "First, fourth, and fifth elements:"for n = 1 to list\$[?,]    for m = 1 to ind[?]	print list\$[n, ind[m]]; " ";    next m    printnext nprint print "Reverse first, fourth, and fifth elements:"for m = 1 to ind[?]    print list\$[1, list\$[,?]+1-ind[m]]; " ";next mprintfor m = 1 to ind[?]    print list\$[2, list\$[,?]-ind[m]]; " ";next mprintend `
`Igual que la entrada de FreeBASIC.`

## C++

`#include <iostream>#include <list>#include <string>#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 elementsvoid 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());} `
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
```

## F#

` //Iterators. Nigel Galloway: Januuary 30th., 2022let N,G=[|"Sunday"; "Monday"; "Tuesday"; "Wednesday"; "Thursday"; "Friday"; "Saturday"|],["Red"; "Orange"; "Yellow"; "Green"; "Blue"; "Purple"]N|>Array.iter(printf "%s "); printfn ""N|>Seq.iter(printf "%s "); printfn ""G|>List.iter(printf "%s "); printfn ""G|>Seq.iter(printf "%s "); printfn "" let rec advance(n:System.Collections.IEnumerator)=function 0->() |g->n.MoveNext(); advance n (g-1)let next(n:System.Collections.IEnumerator)=n.MoveNext(); n.Current let X=(N|>Seq.ofArray).GetEnumerator() in printfn \$"{next X} {(advance X 2; next X)} {next X}"let X=(G|>Seq.ofList).GetEnumerator() in printfn \$"{next X} {(advance X 2; next X)} {next X}"let X=(N|>Array.rev|>Seq.ofArray).GetEnumerator() in printfn \$"{next X} {(advance X 3; next X)} {next X}"let X=(G|>List.rev|>Seq.ofList).GetEnumerator() in printfn \$"{next X} {(advance X 3; next X)} {next X}" `
Output:
```Sunday Monday Tuesday Wednesday Thursday Friday Saturday
Sunday Monday Tuesday Wednesday Thursday Friday Saturday
Red Orange Yellow Green Blue Purple
Red Orange Yellow Green Blue Purple
Sunday Wednesday Thursday
Red Green Blue
Saturday Tuesday Monday
Purple Orange Red
```

## FreeBASIC

`Dim As Integer n, mDim As String list(1 To 2, 1 To 7) = {_{"Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"}, _{"Red","Orange","Yellow","Green","Blue","Purple"}}Dim As Integer ind(1 To 3) = {1,4,5} Print "All elements:"For n = 1 To Ubound(list)    For m = 1 To Ubound(list, 2)        Print list(n, m); " ";    Next m    PrintNext n Print !"\nFirst, fourth, and fifth elements:"For n = 1 To Ubound(list)    For m = 1 To Ubound(ind)        Print list(n, ind(m)); " ";    Next m    PrintNext n Print !"\nReverse first, fourth, and fifth elements:"For m = 1 To Ubound(ind)    Print list(1, Ubound(list,2)+1-ind(m)); " ";Next mPrintFor m = 1 To Ubound(ind)    Print list(2, Ubound(list,2)-ind(m)); " ";Next mSleep`
Output:
```Same as C++ input, Julia, Phix or Wren.
```

## J

J's operations are designed to be applied to the data structure as a whole, and this explicitly includes mapping between representations. Also, in J, all data 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:

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

This gives us:

`    dow┌──────┬───────┬─────────┬────────┬──────┬────────┬──────┐│monday│tuesday│wednesday│thursday│friday│saturday│sunday│└──────┴───────┴─────────┴────────┴──────┴────────┴──────┘   col┌───┬─────────────────────────────────────────┐│red│┌──────┬────────────────────────────────┐││   ││orange│┌──────┬───────────────────────┐│││   ││      ││yellow│┌─────┬───────────────┐││││   ││      ││      ││green│┌────┬────────┐│││││   ││      ││      ││     ││blue│┌──────┐││││││   ││      ││      ││     ││    ││purple│││││││   ││      ││      ││     ││    │└──────┘││││││   ││      ││      ││     │└────┴────────┘│││││   ││      ││      │└─────┴───────────────┘││││   ││      │└──────┴───────────────────────┘│││   │└──────┴────────────────────────────────┘│└───┴─────────────────────────────────────────┘`

Here, the implementation'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).

`  echo ;:inv <S:0 dow monday tuesday wednesday thursday friday saturday sunday   echo ;:inv <S:0 colred orange yellow green blue purple    echo ;:inv 0 3 4 {<S:0 dow monday thursday friday   echo ;:inv 0 3 4 {<S:0 colred green blue    echo ;:inv _1 _3 _4 {<S:0 dow sunday friday thursday   echo ;:inv _1 _3 _4 {<S:0 colpurple green yellow`

The downside of this approach is that the programmer must understand the data (to know to map all relevant list structures to the desired form). For example, we might instead say that a linked list is not merely an unbalanced binary tree, but must always have its data elements in the left node. That would give us this implementation for col:

` col=: '' ]F..(,<) ;:'red orange yellow green blue purple' `
`   col┌──────┬───────────────────────────────────────┐│purple│┌────┬────────────────────────────────┐││      ││blue│┌─────┬────────────────────────┐│││      ││    ││green│┌──────┬───────────────┐││││      ││    ││     ││yellow│┌──────┬──────┐│││││      ││    ││     ││      ││orange│┌───┬┐││││││      ││    ││     ││      ││      ││red││││││││      ││    ││     ││      ││      │└───┴┘││││││      ││    ││     ││      │└──────┴──────┘│││││      ││    ││     │└──────┴───────────────┘││││      ││    │└─────┴────────────────────────┘│││      │└────┴────────────────────────────────┘│└──────┴───────────────────────────────────────┘`

This creates an issue that a single element nested list looks very much like a flat array. To prevent that from being a problem, we include an empty element at the end of our flat array:

` dow=: a:,~;:'monday tuesday wednesday thursday friday saturday sunday' `
`   dow┌──────┬───────┬─────────┬────────┬──────┬────────┬──────┬┐│monday│tuesday│wednesday│thursday│friday│saturday│sunday││└──────┴───────┴─────────┴────────┴──────┴────────┴──────┴┘`

Now everything is the same as before, except that we need to explicitly ignore the empty trailing element:

`   echo ;:inv _2 _4 _5 {<S:0 dow sunday friday thursday   echo ;:inv _2 _4 _5 {<S:0 colred yellow green`

That said, we now also have the opportunity to implement a different kind of normalization routine, which takes advantage of the difference in representation (if we had a need for that...).

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

That said, we could implement other approaches which are more similar to some of the other approaches here. These tend to be bulky and inefficient, but some people like that kind of thing. (Some insist on that kind of thing.)

For example:

`nextItem=: {{ (x+1) -.#y }}nextLink=: {{ (1;x) #~ (L.y) > #x }} makeIterator=: {{ x;y;0 }} iterate=: {{'`next val more'=. ops['ops list position'=. y  (}:y),<position next list}} value=: {{'`next val more'=. ops['ops list position'=. y  position val list}} more=: {{'`next val more'=. ops['ops list position'=. y  more position}}`

With this approach, one of the task examples could look like this:

` printAll=: {{   while. more y do.     echo value y     y=.  iterate y  end.EMPTY}}   DOW=:  nextItem`{::`# makeIterator ;:'monday tuesday wednesday thursday friday saturday sunday'   COL=:  nextLink`{::`# makeIterator (,<)/;:'red orange yellow green blue purple'     printAll DOWmondaytuesdaywednesdaythursdayfridaysaturdaysunday   printAll COLredorangeyellowgreenbluepurple   value iterate^:0 DOWmonday   value iterate^:3 DOWthursday`
etc.

(Except that this approach does not support negative indexing, so indexing from the end of a list would, for the general case, require either extending the system with explicit support for a length operation or iterating once over the list to determine the list length.)

## 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:

`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)) `
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.

`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, butthe 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 areguaranteed 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];`
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.
` list = [["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"],          ["Red","Orange","Yellow","Green","Blue","Purple"]]ind = [1,4,5]revInd = reverse(ind) see "working..." +nlsee "All elements:" + nl for n = 1 to len(list)     for m = 1 to len(list[n])          see list[n][m] + " "     next     see nlnext 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 nlnext 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 nlnext see "done..." +nl `
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.

`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 listvar 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)`
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.

` \\ 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 elementsproc 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 stringsint 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);]`
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
```