Recipe 1.4. Processing a String One Byte at a Time


1.4.1. Problem

You need to process each byte in a string individually.

1.4.2. Solution

Loop through each byte in the string with for. Example 1-17 counts the vowels in a string .

Processing each byte in a string

<?php $string = "This weekend, I'm going shopping for a pet chicken."; $vowels = 0; for ($i = 0, $j = strlen($string); $i < $j; $i++) {     if (strstr('aeiouAEIOU',$string[$i])) {         $vowels++;     } } ?>

1.4.3. Discussion

Processing a string a character at a time is an easy way to calculate the "Look and Say" sequence, as shown in Example 1-18.

The "Look and Say" sequence

<?php function lookandsay($s) {     // initialize the return value to the empty string     $r = '';     // $m holds the character we're counting, initialize to the first     // character in the string     $m = $s[0];     // $n is the number of $m's we've seen, initialize to 1     $n = 1;     for ($i = 1, $j = strlen($s); $i < $j; $i++) {         // if this character is the same as the last one         if ($s[$i] == $m) {             // increment the count of this character             $n++;         } else {             // otherwise, add the count and character to the return value             $r .= $n.$m;             // set the character we're looking for to the current one             $m = $s[$i];             // and reset the count to 1             $n = 1;         }     }     // return the built up string as well as the last count and character     return $r.$n.$m; } for ($i = 0, $s = 1; $i < 10; $i++) {     $s = lookandsay($s);     print "$s <br/>\n"; } 

Example 1-18 prints:

1 11 21 1211 111221 312211 13112221 1113213211 31131211131221 13211311123113112211

It's called the "Look and Say" sequence because each element is what you get by looking at the previous element and saying what's in it. For example, looking at the first element, 1, you say "one one." So the second element is "11." That's two ones, so the third element is "21." Similarly, that's one two and one one, so the fourth element is "1211," and so on.

1.4.4. See Also

Documentation on for at http://www.php.net/for; more about the "Look and Say" sequence at http://mathworld.wolfram.com/LookandSaySequence.html.




PHP Cookbook, 2nd Edition
PHP Cookbook: Solutions and Examples for PHP Programmers
ISBN: 0596101015
EAN: 2147483647
Year: 2006
Pages: 445

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