How to lower the chance of getting a number?

Question

PHP

For example, we have numbers from 0 to 15. I do getrandomint(0.15) and get a random number from this sample. But here, under certain conditions, I need to lower the chance of numbers falling out from 1 to 7. That is, I also need a random number from 0 to 15, but so that the chance of numbers falling out from 1 to 7 is less. The first thing that came to mind was to fill the array with these numbers and make a random selection from there, but this idea did not work very well. Can you suggest your implementation algorithm? It is possible with an array, but what would work)

Answer 1

// set the probabilities of getting a number in each interval. Total prob must be equal to 100

$data = [
  [
    'start' => 0,	// начало промежутка
    'end' => 7,		// окончание промежутка
    'prob' => 20, 	// вероятность попадания, %
  ],
  [
    'start' => 8,	// начало промежутка
    'end' => 15,	// окончание промежутка
    'prob' => 80, 	// понижающий коээффициент на вероятность, %
  ]
];

// call the function to get an arbitrary number, taking into account the probability of falling into the interval

function calcRand($data) {
  $prob = rand(0, 100);
  $pr = 0;
  $out = 0;
  foreach ($data as $info) {
    if ($prob >= $pr && $prob < $pr + $info['prob']) {
      $out = rand($info['start'], $info['end']);
      break;
    }
    $pr += $info['prob'];
  }
  return $out;
}

// verification code to demonstrate distribution

$numbers = [];
for ($i = 0; $i < 10000; $i++) {
  $number = calcRand($data);

  $numbers[$number] = (isset($numbers[$number]) ? $numbers[$number] + 1 : 1);
}
ksort($numbers);

echo '<pre>';
var_export($numbers);
var_export(array_map(function($a) { return round($a / 100); }, $numbers));