Question

How to display string letters sorted by frequency in PHP

Answer 1

// sortByFrequency — counts how often each letter appears and returns a sorted list
function sortByFrequency(string $s): array {
    // Create a frequency array for 26 lowercase letters, initialized to 0
    $freq = array_fill(0, 26, 0);

    // Iterate through the string and count only alphabetic characters
    foreach (str_split($s) as $c) {
        if (ctype_alpha($c)) {
            // Convert to lowercase and map 'a'..'z' to 0..25
            $freq[ord(strtolower($c)) - ord('a')]++;
        }
    }

    // Build an array of (letter, frequency) pairs
    $result = [];
    for ($i = 0; $i < 26; $i++) {
        $result[] = [
            'letter' => chr(ord('a') + $i),
            'freq'   => $freq[$i]
        ];
    }

    // Sort pairs by frequency in descending order
    usort($result, function($a, $b) {
        return $b['freq'] <=> $a['freq'];   // spaceship operator
    });

    return $result;
}

// Build a string sorted by frequency: ddddddddddddccccccbbbbbaaaafffeehhgs
function buildSortedString(array $sorted): string {
    $out = '';

    foreach ($sorted as $p) {
        // append 'p["letter"]' repeated p["freq"] times
        if ($p['freq'] > 0) {
            $out .= str_repeat($p['letter'], $p['freq']);
        }
    }

    return $out;
}

// Input text to analyze
$text = "bbcabddddccafffadbbcdccsedddddhhgade";

// Get sorted frequency list
$sorted = sortByFrequency($text);

// Print each letter and its frequency
foreach ($sorted as $p) {
    if ($p['freq'] != 0) {
        echo $p['letter'] . ": " . $p['freq'] . PHP_EOL;
    }
}

// Print the reconstructed sorted string
$letters_sorted_by_frequency = buildSortedString($sorted);
echo PHP_EOL . "Sorted string: " . $letters_sorted_by_frequency . PHP_EOL;


/*
run:

d: 12
c: 6
b: 5
a: 4
f: 3
e: 2
h: 2
g: 1
s: 1

Sorted string: ddddddddddddccccccbbbbbaaaafffeehhgs

*/

 

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