Question

How to select N unique random values that appear exactly once in an existing array with PHP

Answer 1

/*
    Goal:
    - Select N random values that appear exactly once in the array.
    - The array includes duplicates, but only values with frequency = 1
      are eligible for selection.

    Approach:
    1. Count frequencies using array_count_values().
    2. Collect values that appear exactly once.
    3. Shuffle the unique list.
    4. Take the first N values.
    5. Return them and print them.

    Notes:
    - Uses built-in PHP functions for clarity and efficiency.
    - Uses expressive, structured functions.
*/

#---------------------------------------------------------------
# Build a frequency map: value -> count
#---------------------------------------------------------------
function buildFrequencyMap(array $data): array {
    // array_count_values returns an associative array: value => count
    return array_count_values($data);
}

#---------------------------------------------------------------
# Collect values that appear exactly once
#---------------------------------------------------------------
function collectGloballyUniqueValues(array $data, array $freq): array {
    $unique = [];

    foreach ($data as $value) {
        if ($freq[$value] === 1) {
            $unique[] = $value;
        }
    }

    return $unique;
}

#---------------------------------------------------------------
# Randomly select N values from the unique list
#---------------------------------------------------------------
function selectRandomUnique(array $unique, int $N): array {
    if ($N > count($unique)) {
        $N = count($unique);   // clamp
    }

    // Shuffle the list to randomize order
    $temp = $unique;
    shuffle($temp);

    // Take first N elements
    return array_slice($temp, 0, $N);
}

#---------------------------------------------------------------
# Print helper
#---------------------------------------------------------------
function printList(array $list): void {
    foreach ($list as $v) {
        echo $v . " ";
    }
    echo PHP_EOL;
}

#---------------------------------------------------------------
# Main program
#---------------------------------------------------------------

$data = [
    5, 12, 5, 19, 5, 33, 19, 5, 8, 8, 8, 59, 61, 17, 3, 5, 3, 74, 83, 90, 3, 1
];

# Step 1: Build frequency map
$freq = buildFrequencyMap($data);

# Step 2: Collect values that appear exactly once
$uniqueValues = collectGloballyUniqueValues($data, $freq);

# Step 3: Choose how many unique random values to select
$N = 5;

# Step 4: Select N random unique values
$randomSelection = selectRandomUnique($uniqueValues, $N);

# Step 5: Print results
echo "Values that appear exactly once:\n";
printList($uniqueValues);

echo "\nRandom selection ($N values):\n";
printList($randomSelection);



/*
run:

Values that appear exactly once:
12 33 59 61 17 74 83 90 1 

Random selection (5 values):
1 74 83 90 59 

*/

 

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