Processing a String One Byte at a Time






Processing a String One Byte at a Time

Problem

You need to process each byte in a string individually.

Solution

Loop through each byte in the string with for. Figure 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++;
    }
}
?>

Discussion

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

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";
}

Figure 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.

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.



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