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Standard Pattern for Iterating Over Loop and Printing Start and End Tags

Answer 1

+1 A:

You should print the closing </div> at the beginning of the loop, if the freshly read id is different from last one (and the last one is not null).

Additionally, after the loop (again, if last id is not null, which would mean that there were no groups at all) you need to close the last group.

Rafał Dowgird 2010-01-11 20:41:58

Thanks! I was actually doing that except I wasn't checking to see if $last_id was null before printing. I'll update the code. Do you know of a better solution for this though?

Topher Fangio 2010-01-11 20:44:07

Not really - I believe this is the standard way to do this. You can reduce code repetition (printing the div twice) by putting the stuff into functions like `groupstart()` and `groupend()` and call those functions within the loop.

Rafał Dowgird 2010-01-11 20:48:50

Answer 2

A:

Been a while since I've done any PHP but I'd try something like this...

// print leading div for very first in array
echo "<div>";

$last_question = null;
while ($choice = pg_fetch_array($choices)) {

    // print choice info
    echo "<span ...>";

    if($last_question != $choice['question_id'])
    {
        // print trailing div for last one
        echo "</div>";

        // print leading div for next one
        echo "<div>";
    }


    // set last question
    $last_question = $choice['question_id'];

}

// print trailing div for very last in array
echo "</div>";

Might need improving to make sure it doesn't print an extra div at the end.

mdm 2010-01-11 20:50:20

That really isn't a whole lot better than my existing solution. Not to mention, the first `<div>` printing must be inside of the loop so that I can print the id of the question as part of the div.

Topher Fangio 2010-01-11 21:10:58

Answer 3

+1 A:

Grouping items by similar values can hardly be called an algorithm. It's more of a coding pattern if anything else.

A good way to code is to separate the mechanism from the intent. In this case the mechanism is how to keep track of the key values to find the grouping boundaries and the intent is to output HTML for each sequential group.

Python for instance has a library function called groupby to do exactly this. So in Python the code would look something like this (ignoring the fact that one would use a templating library for this):

from itertools import groupby

def question_from_row(row):
    return dict(id=row['question_id'], body=row['question_body'])

for question, choices in groupby(questions, key=question_from_row):
    print('<div class="question" id="%s">' % question['id'])
    print('  <p>%s</p>\n' % question['body'])
    print('  <div class="choices">')
    for choice in choices:
        print('<span class="choice">%s</span>' % choice['choice_body'])
    print('  </div>')
    print('</div>')

PHP to my knowledge doesn't have anything like that built in, but a naive implementation is pretty easy:

function array_groupby($input, $keyfunc) {
    $output = array();
    $last_key = null;
    $current = null;
    foreach ($input as $item) {
        $item_key = $keyfunc($item);
        if ($last_key === null || $item_key != $last_key) {
            if ($current !== null) {
                $output[] = $current;
            }
            $last_key = $item_key;
            $current = array();
        }
        $current[] = $item;
    }
    if ($current !== null) {
        $output[] = $current;
    }
    return $output;
}

This would be typical library code that you include in. The code that deals with the output then becomes rather trivial. Completely isolated from how the grouping is done. For instance you could change array_groupby to return an object that implements the Iterator interface and only lazily fetches from the input iterable.

$questions = array_groupby(pg_fetch_array($choices),
    function($choice) { return $choice['id']; });

foreach ($questions as $choices) {
    $question = $choices[0];
    echo '<div class="question" id="'.$question['id'].'">';
    echo '<p>'.$question['body'].'</p>';
    echo '<div class="choices">';
    foreach ($choices as $choice) {
        echo '<span class="choice">'.$choice['choice_body'].'</span>';
    }
    echo '</div>';
    echo '</div>';
}

This example is using the PHP 5.3 closures feature. On older versions specifying the function to extract the grouping key would be slightly uglier, perhaps calling for an object oriented approach.

Ants Aasma 2010-01-12 06:30:11

Thanks Ants! You're absolutely right. It is much more of a pattern than an algorithm. I had thought about the grouping, but only very briefly. This solidifies my thoughts very well. Thanks for such a well thought and well stated answer!

Topher Fangio 2010-01-12 15:07:32

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Standard Pattern for Iterating Over Loop and Printing Start and End Tags

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