Look-and-Say Sequence php

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I need to write a function who prints Look-and-Say Sequence hat takes a number as nth iteration. The first iteration is 0 and output has to contain every previous lines. For example, if we test sequence(3), the result need to be :

1
11
21
1211

SO I made a function for each line and another to stack them but it doesn't work =/ Here's my code

<?php
function Conway($n)
{
if ($n == 1)
    return "1";
if ($n == 2)
    return "11";


for ($i = 3; $i <= $n; $i++) {
    $str = "11";
    $str = $str.'$';
    $len = strlen($str);
    $cnt = 1;
    $tmp = "";

    for ($j = 1; $j < $len; $j++)
    {
        if ($str[$j] != $str[$j - 1])
        {
            $tmp = $tmp.$cnt + 0;
            $tmp = $tmp. $str[$j - 1];
            $cnt = 1;
        }

        else $cnt++;
        }

        $str = $tmp;
    }
    return $str;
}

function sequence($nth) {
    for ($i = 1; $i <= $nth+1; $i++)
    do {
        echo Conway($i)."\n";
    } while ($i > $nth+1);

}
php
2 Answers

As explained here en.wikipedia.org/wiki/Look-and-say_sequence, the idea is to construct a string based on the previous constructed string (in a loop):

To generate a member of the sequence from the previous member, read off the digits of the previous member, counting the number of digits in groups of the same digit. For example:

  • 1 is read off as "one 1" or 11.
  • 11 is read off as "two 1s" or 21.
  • 21 is read off as "one 2, one 1" or 1211.
  • 1211 is read off as "one 1, one 2, two 1s" or 111221.
  • 111221 is read off as "three 1s, two 2s, one 1" or 312211.

An attempt to solve this problem, we'll have a function called lookAndSay($n) that accepts a number $n. Basically, that function will have a loop that runs $n times.

To simplify things, we'll have a another function, let's call it tokenize($str), that will basically count the occurrence of a number, concatenates that count to the number (like 31 where 3 is the number of occurrences of the number 1), repeat until all the numbers in the string $str are dealt with.
A usage of that function can result in (for example):

tokenize('11'); // returns: '21'

Our main function, lookAndSay($n), will call tokenize($str) after making some initialization and in the end will return the resulting string.

Here's a code sample illustrating what's being said:

/**
 * A function that generate look-and-say sequence based on a number "$n"
 * 
 * @param int $n
 * @return string
 */
function lookAndSay(int $n): string {
    if ($n <= 1) return '1';
    /** "$lastSeq" will hold the latest sequence tokenized by "tokenize" function */
    $r = $lastSeq = '1';
    /** loop based on "$n" and construct the final string along the way */
    while($n-- > 0) $r .= ' '.($lastSeq = tokenize($lastSeq));
    /** return the look-and-say string for the number "$n" */
    return $r;
}

/**
 * A function that "tokenizes" a string of numbers 
 * and return a string having the format "CN" where C is the number of occurrences of a number N in the string
 * 
 * @param string $str
 * @return string
 */
function tokenize(string $str): string {
    /** will hold the string of tokens that will be returned after the function finishes */
    $r = '';
    /** holds the count of a number in the string $str */
    $c = 1;
    for ($i = 0; $i < strlen($str); $i++) {
        /** we still count the number of occurrences of the same number "$str[$i]" ... */
        if($str[$i] == ($str[$i + 1] ?? '')) {
            /** increment the count and skip the rest of the loop */
            $c++;
            continue;
        }
        /** at this point we have the count for "$str[$i]" number so we concatenate the result to the final string and reset the count to "1" to prepare it for the next number (if any) */
        ($r .= $c.$str[$i]) && ($c = 1);
    }
    /** return the "tokenized" string */
    return $r;
}

A call to lookAndSay could result in:

echo lookAndSay(3); // prints: 1 11 21 1211

Also here's a live demo of the above attempt.

ths0

Is that you looking for ?

function sequence($str) {
 $str .= ''; 
 $len = strlen($str); 
 $count= 0; 
 $result= ''; 
 $temp= $str[0]; 

for($i=0;$i<$len;$i++) { 
    if($temp!=$str[$i]) { 
        $result.=$count.$temp; 
        $temp = $str[$i]; 
        $count=1; 
    } else { 
        $count++; 
    } 
} 

$result.=$count.$temp; 
return $result; 
} 

$num = 1; 
echo "$num\n"; 

for($i=0;$i<3;$i++) { 
  $num=sequence($num); 
  print $num."\n"; 
}

output 1 11 21 1211

bobi0
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