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

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.

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"}, {{#CF},"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 directly use them. Operators and functions that are designed to work on Iterable objects generally have the iteration semantics built in; so don't need 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), and once the 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 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),

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: ' ~ $_ ~ "\nFirst, fourth and fifth from start; " ~
   .[0, 3, 4] ~ ', and from end: ' ~ .[*-1, *-4, *-5];

};

say "\nWhere iterators really shine; when you are trying to collate 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 List, contents: Red Orange Yellow Green Blue Purple
First, fourth and fifth from start; Red Green Blue, and from end: Purple Yellow Orange

Type Range, contents: Rako Rakp Rakq Rakr Raks Rakt Raku Rakv Rakw Rakx Raky
First, fourth and fifth from start; Rako Rakr Raks, and from end: Raky Rakv Raku

Type Array, contents: Sunday Monday Tuesday Wednesday Thursday Friday Saturday
First, fourth and fifth from start; Sunday Wednesday Thursday, and from end: Saturday Wednesday Tuesday

Type Seq, contents: 1.25 1.5 1.875 2.8125 5.273438 14.831543 78.2132149
First, fourth and fifth from start; 1.25 2.8125 5.273438, and from end: 78.2132149 2.8125 1.875

Where iterators really shine; when you are trying to collate 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)

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