CODEWITHSUNDEEP
phprandomfor-loop
Psyche
Psyche

Reputation: 8773

Generating random values and keeping track of their sum

I have more than 200 entries in a database table and I would like to generate a random value for each entry, but in the end, the sum of entries values must equal 100. Is it possible to do this using a for loop and rand() in PHP?

Upvotes: 2

Views: 1066

Answers (5)

Szocske
Szocske

Reputation: 7661

Original question yesterday had exactly 200 entries and the sum "not greater than 100". My original answer from yesterday:

Use random numbers not greater than 0.5 to be sure.

Alternatively, depending on how "random" those numbers need to be (how much correlation is allowed), you could keep a running total, and if it gets disproportionately high, you can mix in a bunch of smaller values.

Edit:

Way to go changing the question, making me look stupid and get downvoted.

To get the exact sum you have to normalize, and better use exact fractions instead of floats to avoid rounding errors.

Upvotes: -2

Sammitch
Sammitch

Reputation: 32262

Do a little over/under:

$size = 200;
$sum = 100;
$places = 3;

$base = round($sum/$size, $places);

$values = array_fill(0, $size, $base);

for($i=0; $i<$size; $i+=2) {
    $diff = round((rand()/getrandmax()) * $base, $places);
    $values[$i]   += $diff;
    $values[$i+1] -= $diff;
}

//optional: array_shuffle($values);

$sum = 0;

foreach($values as $item) {
    printf("%0.3f ", $item);
    $sum += $item;
}

echo $sum;

Output:

0.650 0.350 0.649 0.351 0.911 0.089 0.678 0.322 0.566 0.434 0.563 0.437 0.933 0.067 0.505 0.495 0.503 0.497 0.752 0.248 0.957 0.043 0.856 0.144 0.977 0.023 0.863 0.137 0.766 0.234 0.653 0.347 0.770 0.230 0.888 0.112 0.637 0.363 0.716 0.284 0.891 0.109 0.549 0.451 0.629 0.371 0.501 0.499 0.652 0.348 0.729 0.271 0.957 0.043 0.769 0.231 0.767 0.233 0.513 0.487 0.647 0.353 0.612 0.388 0.509 0.491 0.925 0.075 0.797 0.203 0.799 0.201 0.588 0.412 0.788 0.212 0.693 0.307 0.688 0.312 0.847 0.153 0.903 0.097 0.843 0.157 0.801 0.199 0.538 0.462 0.954 0.046 0.541 0.459 0.893 0.107 0.592 0.408 0.913 0.087 0.711 0.289 0.679 0.321 0.816 0.184 0.781 0.219 0.632 0.368 0.839 0.161 0.568 0.432 0.914 0.086 0.991 0.009 0.979 0.021 0.666 0.334 0.678 0.322 0.705 0.295 0.683 0.317 0.869 0.131 0.837 0.163 0.792 0.208 0.618 0.382 0.606 0.394 0.574 0.426 0.927 0.073 0.661 0.339 0.986 0.014 0.759 0.241 0.547 0.453 0.804 0.196 0.681 0.319 0.960 0.040 0.708 0.292 0.558 0.442 0.605 0.395 0.986 0.014 0.621 0.379 0.992 0.008 0.622 0.378 0.937 0.063 0.884 0.116 0.840 0.160 0.607 0.393 0.765 0.235 0.632 0.368 0.898 0.102 0.946 0.054 0.794 0.206 0.561 0.439 0.801 0.199 0.770 0.230 0.843 0.157 0.681 0.319 0.794 0.206 100

The rounding gets a bit squiffy if you're not using nice numbers like 100 and 200, but never more than 0.1 off.

Upvotes: 1

Honk der Hase
Honk der Hase

Reputation: 2488

How exact has the 100 to be? Just curious, because all hints end at using floating point values, which tend to be inacurate.

I'd propose using fractions... lets say 10000 fractions, each count 1/100 point (10000 * 1/100 = 100 points). Distribute 10000 points to 200 elements, using integers - and be absolutely sure, that the sum of all integers divided by 10000 is 100. There is no need for floats, just think around the corner...

Upvotes: 1

Yoshi
Yoshi

Reputation: 54649

You could simply normalize a set of numbers, like:

$numbers = array();
for ($i = 0; $i < 200; $i += 1) {
  $numbers[] = rand();
}

$sum = array_sum($numbers);

// divide $sum by the target sum, to have an instant result, e.g.:
// $sum = array_sum($numbers) / 100;
// $sum = array_sum($numbers) / 42;
// ...

$numbers = array_map(function ($n) use($sum) {
  return $n / $sum;
}, $numbers);

print_r($numbers);
print_r(array_sum($numbers)); // ~ 1

demo: http://codepad.viper-7.com/RDOIvX

Upvotes: 3

Robert
Robert

Reputation: 20286

The solution for your problem is to rand number from 0 to 200 then put in array, then sum the values and divide it by 200 after that. Loop through elements and divide every element by result of previous equatation it will give you the answer

$sum = 0;
$max = 100; //max value to be sumed
$nr_of_records = 200; // number of records that should sum to $max
$arr = array();
for($i=0;$i<$nr_of_records;++$i)
{
   $arr[$i] = rand(0,$max);
}

$div = array_sum($arr) / $max;


for($i=0;$i<$nr_of_records;++$i)
{
  $arr[$i] /= $div;
  echo $arr[$i].'<br>';
}

echo array_sum($arr);

Created living example

Upvotes: 1

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