Look-and-Say Sequence php - php

Question

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);
}

Answer 1

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.

Answer 2

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