getallenreeks (Opgelost)

$index = ...;
$counted = 0;
$the_number = 0; // init op 0
$are_we_there_yet = false;

while(!$are_we_there_yet) {
  $counted += $the_number;
  $the_number++;
  // zit de gezochte index in het volgende interval?
  $are_we_there_yet = $index >= $counted && $index < $counted+$the_number+1;
}

$index = ...;
$counted = 0;
$the_number = 0; // init op 0
$are_we_there_yet = false;
 
while(!$are_we_there_yet) {
  $counted += $the_number;
  $the_number++;
  // zit de gezochte index in het volgende interval?
  $are_we_there_yet = $index >= $counted && $index < $counted+$the_number+1;
}

<?php

for($i = 1; $i < 2007; $i++)
{
	for($x = $i; $x > 0; $x--)
	{
		$var .= $i.";";
	}
}

$array = explode(";", $var);
echo $array[2005];

<?php
 
for($i = 1; $i < 2007; $i++)
{
	for($x = $i; $x > 0; $x--)
	{
		$var .= $i.";";
	}
}
 
$array = explode(";", $var);
echo $array[2005];

<?php

$i = 1;

while($aantal < 2007)
{
 	$i++;
	for($x = $i; $x > 0; $x--)
	{
		$var .= $i.";";
				
	}
	$array = explode(";", $var);
	$aantal = count($array);
}

echo $array[2005];

?>

<?php
 
$i = 1;
 
while($aantal < 2007)
{
 	$i++;
	for($x = $i; $x > 0; $x--)
	{
		$var .= $i.";";
 
	}
	$array = explode(";", $var);
	$aantal = count($array);
}
 
echo $array[2005];
 
?>

<?
$index = 2006;

// Formule: 0.5n^2 + 0.5n - $index = 0 (ABC formule)
$D = 0.25 + 2 * $index; // Discriminant
$opl = -0.5 + sqrt($D); // (-b + D^0.5) / 2a

print ceil($opl);
?>

<?
$index = 2006;
 
// Formule: 0.5n^2 + 0.5n - $index = 0 (ABC formule)
$D = 0.25 + 2 * $index; // Discriminant
$opl = -0.5 + sqrt($D); // (-b + D^0.5) / 2a
 
print ceil($opl);
?>

<?
$looking_for_index = 2006; // de te zoeken index
$counted_indexes = 0;
$current_index = 0;

while($looking_for_index < $counted_indexes)
{
  $current_index++;
  $counted_indexes += $current_index;
}
// nu geldt $looking_for_index >= $counted_indexes,
// dus $current_index bevat (hopelijk) de goede waarde
?>

<?
$looking_for_index = 2006; // de te zoeken index
$counted_indexes = 0;
$current_index = 0;
 
while($looking_for_index < $counted_indexes)
{
  $current_index++;
  $counted_indexes += $current_index;
}
// nu geldt $looking_for_index >= $counted_indexes,
// dus $current_index bevat (hopelijk) de goede waarde
?>