yield examples

change-count.php

<?php

function countChange($amount) {
  return cc($amount, 5);
}

function cc($amount, $kindOfCoins) {
  if ($amount === 0) return 1;
  if ($amount < 0) return 0;
  if ($kindOfCoins === 0) return 0;

  return cc($amount, $kindOfCoins - 1) + cc($amount - firstDenomination($kindOfCoins), $kindOfCoins);
}

function firstDenomination($kindOfCoins) {
  if ($kindOfCoins === 1) return 1;
  if ($kindOfCoins === 2) return 5;
  if ($kindOfCoins === 3) return 10;
  if ($kindOfCoins === 4) return 25;
  if ($kindOfCoins === 5) return 50;
}

var_dump(countChange(100));

change-list.php

<?php

function countChange($amount) {
  return cc($amount, 5);
}

function cc($amount, $kindOfCoins) {
  if ($amount === 0) { yield []; return; }
  if ($amount < 0) return;
  if ($kindOfCoins === 0) return;


  foreach (cc($amount, $kindOfCoins - 1) as $c)
    yield $c;

  foreach (cc($amount - firstDenomination($kindOfCoins), $kindOfCoins) as $c)
    yield array_merge([firstDenomination($kindOfCoins)], $c);
}

function firstDenomination($kindOfCoins) {
  if ($kindOfCoins === 1) return 1;
  if ($kindOfCoins === 2) return 5;
  if ($kindOfCoins === 3) return 10;
  if ($kindOfCoins === 4) return 25;
  if ($kindOfCoins === 5) return 50;
}

var_dump(count(iterator_to_array(countChange(100))));
foreach (countChange(100) as $cc) {
  echo join(" ", $cc), "\n";
}

sieve.php

<?php

// Sieve of Eratosthenes
function ints($n) {
  for ($i = $n;; $i++) yield $i;
}

function headTail($gen) {
  $head = $gen->current();
  $gen->next();
  return [$head, $gen];
}

function primes($nums) {
  list($head, $tail) = headTail($nums);
  yield $head;
  foreach (primes($tail) as $p) {
    if ($p % $head != 0)
      yield $p;
  }
}

$ps = primes(ints(2));
foreach ($ps as $p) {
  if ($p > 1000) break;
  echo $p." ";
}

tree-iterator.php

<?php

interface Tree {
  function preOrder();
  function inOrder();
  function postOrder();
}

class Fork implements Tree {
  private $left, $el, $right;

  function __construct(Tree $left, $el, Tree $right) {
    list($this->left, $this->el, $this->right) = func_get_args();
  }

  function preOrder() {
    yield $this->el;
    foreach ($this->left->preOrder() as $x) yield $x;
    foreach ($this->right->preOrder() as $x) yield $x;
  }

  function inOrder() {
    foreach ($this->left->inOrder() as $x) yield $x;
    yield $this->el;
    foreach ($this->right->inOrder() as $x) yield $x;
  }

  function postOrder() {
    foreach ($this->left->postOrder() as $x) yield $x;
    foreach ($this->right->postOrder() as $x) yield $x;
    yield $this->el;
  }
}

class Leaf implements Tree {
  function preOrder()  { return; yield; }
  function inOrder()   { return; yield; }
  function postOrder() { return; yield; }
}

// ***

$leaf = new Leaf();

$t = new Fork(new Fork($leaf, 1, $leaf), 3, new Fork($leaf, 2, $leaf));
foreach ($t->inOrder() as $x)
  echo $x, "\n";