Spot The Bug: The Year and Month Combination

Hi and welcome to the very first edition of Spot The Bug!

Are ya ready?!

Somewhere last year, I wrote a piece of PHP code that seemed all right initially, but somehow failed to work early January. It was a simple listing of featured birds for various months, grouped by Year and Month, starting from current Month and Year, and working its way through all records in descending order (latest first, least recent last), for 12 months. Seemed straightforward enough.

The data in MySQL: (tb_featuredbirds)

record_title	record_no	record_month	record_year
Cuckoo	1	4	2014
Pigeon	2	4	2014
Woodpecker	1	5	2014
Eagle	1	6	2014
Owl	1	7	2014
Nightingale	2	7	2014
Tit	3	7	2014
Thrush	1	8	2014
Pelican	1	9	2014
Stork	1	10	2014
Crane	1	11	2014
Raven	1	12	2014
Ostrich	2	12	2014
Dodo	3	12	2014
Mockingbird	1	1	2015
Hummingbird	2	1	2015

The code, in PHP using the mysqli extension:

<?php

$DBConn = new mysqli($DBServer, $DBUser, $DBPass, $DBName);

$year=intval(date("YYYY",strtotime("now")));
$month=intval(date("m",strtotime("now")));

$months=1;

$current_year=0;

while ($months<=12)
{
    if ($current_year!=$year)
    {
        echo "<b><u>";
        echo $year;
        echo "</u></b>";

        $current_year=$year;
    }

    echo "<b>";
    echo date("M",strtotime($year."-".$month."-01"));
    echo "</b>"; 

    $strsql = $DBConn->prepare("SELECT record_title FROM tb_featuredbirds WHERE record_year=? and record_month=? ORDER BY record_no");
    $strsql->bind_param("ii", $year,$month);
    $strsql->execute();

    $strsql->bind_result($bird);

    echo "<ul>";

    while ($strsql->fetch())
    {
        echo "<li>" . $bird . "</li>";
    }

    echo "</ul>";

    $strsql->close();

    $month=($month==1?12:$month-1);
    $year=($month==1?$year-1:$year);

    $months++;
}
?>

The result:

2015
Jan

• Mockingbird
• Hummingbird

Dec

Nov

Oct

Sep

Aug

Jul

Jun

May

Apr

Mar

Feb

What went wrong

Anything from last year and before, just didn't show up! It was January then, and only stuff that was dated January of this year, was displayed. There were no system error messages.

Why it went wrong

The error was at these lines. See the logical disconnect there?

    $month=($month==1?12:$month-1);
    $year=($month==1?$year-1:$year);

Of course, if you progress backwards, once the $month variable reaches January, the next value must be December. By that logic, once the $month variable reaches January, the $year variable should be decremented, to reflect December of the preceding year. I've highlighted the transition below.

Step 1: $month=1, $year=2015
Step 2: $month=12, $year=2014
Step 3: $month=11, $year=2014
Step 4: $month=10, $year=2014
.
.
.

But if $month variable turns to 12, then the next line, which checks if the $month variable is 1 before decrementing the $year variable, would never fire off! Instead, the progression would be more like:

Step 1: $month=1, $year=2015
Step 2: $month=12, $year=2015
Step 3: $month=11, $year=2015
Step 4: $month=10, $year=2015
.
.
.

It was all right in the middle or near the end of the current year (because the code would never have come into effect anyway), but once 2015 arrived, the logic bug kicked in. Because there were no records for the year of 2015 other than January, no records would be returned.

How I fixed it

I basically switched the two lines around, and it worked like a charm.

    $year=($month==1?$year-1:$year);
    $month=($month==1?12:$month-1);

The new result:

2015
Jan

• Mockingbird
• Hummingbird

2014
Dec

• Raven
• Ostrich
• Dodo

Nov

• Crane

Oct

• Stork

Sep

• Pelican

Aug

• Thrush

Jul

• Owl
• Nightingale
• Tit

Jun

• Eagle

May

• Woodpecker

Apr

• Cuckoo
• Pigeon

Moral of the story

Sequence matters. It matters a lot! Boy, did I feel like a noob when I debugged.

Thanks for joining me for Spot The Bug. Join me again next time. It's a date!
T___T