DIFFICULT Anonymous Function. Plz Help!!!!

Paul_help_

I am having trouble understanding the following code:

<?php
$myText = <<<END_TEXT
But think not that this famous town has
only harpooneers, cannibals, and
bumpkins to show her visitors. Not at
all. Still New Bedford is a queer place.
Had it not been for us whalemen, that
tract of land would this day perhaps
have been in as howling condition as the
coast of Labrador.
END_TEXT;
echo “<h2>The text:</h2>”;
echo “<div style=\”width: 30em;\”>$myText</div>”;
$myText = preg_replace( “/[\,.]/”, “”, $myText );
$words = array_unique( preg_split( “/[ \n\r\t]+/”, $myText ) );
usort( $words, create_function( ‘$a, $b’, ‘return strlen($a) - strlen($b);
’ ) );
echo “<h2>The sorted words:</h2>”;
echo “<div style=\”width: 30em;\”>”;
foreach ( $words as $word ) {
echo “$word “;
}
echo “</div>”;
?>

Basically, what this (above) code is suppose to do is display an unedited version of the paragraph stored in the $myText and beneath that it displays an edited version of the paragraph which rearranges the words in order of size (smallest to biggest).

I don't have any visuals to show you but I have a link to the author's book (which contains visuals ( which you can view online for free),,,, page 155. http://www.cs.karelia.ru/~musen/php/...tt%20Doyle.pdf

I think i understand the majority of the code (above).
But i am kind of confused with how the 'anonymous function' (shown below) works in the above code:

usort( $words, create_function( ‘$a, $b’, ‘return strlen($a) - strlen($b);

here are the things i don't understand:

1) how does this function even work if its not even referenced by any of the other code?
2) and why on earth are $a and $b used and what does passing them in the function actually do, since they are blank variables?

Please help me understand.

Paul.

bradgrafelman

Have you read the manual page for [man]create_function/man? To answer your two questions...

1) It works because the anonymous function is called - by [man]usort/man.
2) $a and $b are used because there's not really a better name to give those variables. You could call them $string1 and $string2, or you could call them $watermelon and $blueberry if you really wanted. They won't be blank, they'll hold the values of the two objects being compared by usort().

Paul_help_

but what exactly is being compared?

bradgrafelman

Two items from the array that usort() is sorting.

Paul_help_

so to begin with,, $a is equal to the word 'but' and $b is equal to the word 'think' (because they are the 1st 2 words in the paragraph?? and then after comparing the first to words they move onto comparing 'not' and 'that'? and so on?

bradgrafelman

You could add print statements to the comparator function to see the elements passed to it in each call. For example, see this user comment on the usort() manual page.

Paul_help_

but am i correct in the comment i made just above above?

bradgrafelman

Paul help!;11032375 wrote:
but am i correct in the comment i made just above above?

No, because of the algorithm PHP uses to sort the array.

Using code similar to what was posted in the user comment above, the first few comparisons I see being made are:

a (is) is shorter than b (think), moving b down array
b (is) is shorter than a (Labrador), moving b up array
b (is) is shorter than a (coast), moving b up array
b (is) is shorter than a (the), moving b up array
b (is) is shorter than a (condition), moving b up array
b (is) is shorter than a (howling), moving b up array
a (as) is same length as b (is), keeping the same

Paul_help_

thanks. this makes better sense now.

Paul_help_

This algorithm that you speak of. Is that the algorithm of the usort() function?

Weedpacket

Yes; it's an implementation of quicksort.

Paul_help_

Hi guys.

There is just one little thing i can't understand about this code.

Basically, the above extract of code is from a book i am reading. And in the book it states:

In this case, the comparison function needs to determine if the length of the string $a is less than, equal
to, or greater than the length of the string $b . It can do this simply by subtracting the length of $a from
the length of $b and returning the result. (Remember from Chapter 5 that PHP ’ s strlen() function
returns the length of a string.)
Here, then, is the complete line of code to sort the array:
usort( $words, create_function( ‘$a, $b’, ‘return strlen($a) - strlen($b);’ ) );

I now understand virtually all of this code. The only thing i don't understand is how, in the book i am reading, it says:

subtracting the length of $a from the length of $b

now, to me, it looks like it is the other way around: $a - $b.

Am I missing something here?

Paul.

NogDog

Paul help!;11049443 wrote:
...

now, to me, it looks like it is the other way around: $a - $b.

Am I missing something here?

Paul.

Frankly, I never remember, so on the rare occasions when I need to do something like that, I try it one way with a small set of test data, and if it comes out in the wrong order, then I swap the 2 variables. 🙂

laserlight

Paul help! wrote:
now, to me, it looks like it is the other way around: $a - $b.

Am I missing something here?

NogDog wrote:
Frankly, I never remember, so on the rare occasions when I need to do something like that, I try it one way with a small set of test data, and if it comes out in the wrong order, then I swap the 2 variables.

I find it easy to remember by recalling how such a function would be used: comparison with 0 such that foo($x, $y) COMPARISON 0 would mean that $x COMPARISON $y is true, e.g., foo($x, $y) < 0 means that $x is less than $y for whatever "less than" means according to foo.

So, if you want to do the subtraction trick with $a and $b, then it means that to correctly test for $a less than $b, you must make it such that foo($a, $b) < 0. Given $a = 0, $b = 1, it is easy to see that 0 - 1 == -1, so you would return $a - $b.

Paul_help_

I suppose what i am trying to ask you is - has the author of the book i am reading got his words mixed up?

because in the code i tested (below), it subtracts the length of variable $b from $a,,,, not $a from $b.

$a = "hello world";

$b = "hi";

echo strlen($a) - strlen($b); // outputs: 9

laserlight

Yes, the words were swapped though the code is correct.