How to correctly apply the isset() function, PLEASE HELP,

I am required to Use the function isset to test if the incremented value equals one of the keys in the $facts array. If there is a key that matches, display the value AFTER the number. NOTE: all numbers between 1 and 100 should still be displayed

This is my code (I could only manage to use the switch, since I am yet to grasp the key concept of the isset(), please kindly direct me on how to go about this)

<?php $facts = array( 57 => ' on Heinz ketchup bottles represents the number of varieties of pickles the company once had.', 2 => ' is the approximate hours a day Giraffes sleeps', 18 => ' is the average hours a Python sleeps per day', 10 => ' per cent of the world is left-handed.', 11 => ' Empire State Buildings, stacked one on top of the other, would be required to measure the Gulf of Mexico at its deepest point.', 98 => '% of the atoms in your body are replaced every year', 69 => ' is the largest number of recorded children born to one woman', ); //add your loop below this line for ($i = 1; $i<=100; $i++) { echo $i."\n";

switch ($i)

{ case 57: echo "$facts[$i]\n"; break;

 case 2:
echo "$facts[$i]\n";
break;

case 18:
echo "$facts[$i]\n";
break;

case 10:
echo "$facts[$i]\n";
break;

case 11:
echo "$facts[$i]\n";
break;

case 98:
echo "$facts[$i]\n";
break;

case 69:
echo "$facts[$i] \n";
break;

  default:
    break;

}

} ?>

<?php
$facts = array(
    57 => ' on Heinz ketchup bottles represents the number of varieties of pickles the company once had.',
    2 => ' is the approximate hours a day Giraffes sleeps',
    18 => ' is the average hours a Python sleeps per day',
    10 => ' per cent of the world is left-handed.',
    11 => ' Empire State Buildings, stacked one on top of the other, would be required to measure the Gulf of Mexico at its deepest point.',
    98 => '% of the atoms in your body are replaced every year',
    69 => ' is the largest number of recorded children born to one woman',
);
//add your loop below this line
for ($i = 1; $i<=100; $i++)
{
  echo $i."\n";

    switch ($i)
  {
    case 57:
    echo "$facts[$i]\n";
    break;

     case 2:
    echo "$facts[$i]\n";
    break;

    case 18:
    echo "$facts[$i]\n";
    break;

    case 10:
    echo "$facts[$i]\n";
    break;

    case 11:
    echo "$facts[$i]\n";
    break;

    case 98:
    echo "$facts[$i]\n";
    break;

    case 69:
    echo "$facts[$i] \n";
    break;

      default:
        break;

  }

}