Group and sum values in associative array based on year substring in keys

Question

I am trying to sum all of the yearly amounts based on an array with keys in a month-year ("M y") date format.

$array_list = [
    "Jan 2016" => 2,
    "Feb 2016" => 4,
    "Mar 2016" => 2,
    "Apr 2016" => 0,
    "Jan 2017" => 9,
    "Feb 2017" => 2,
    "Mar 2017" => 5,
    "Jan 2018" => 4,
    "Feb 2018" => 6
];

Code:

<table border='1' cellpadding='5' cellspancing='3'>
    <tr>
        <td>SN</td>
        <td>Month </td>
        <td>Year </td>
        <td>is new? </td>
        <td>Data</td>
        <td>Total</td>
    </tr>
    <?php 
    $prev_year = 0; 
    $i = 1;
    $total_val = 0; 
    foreach ($array_list as $key => $val):
        $row_span = 0; 
        $MonthYear_label    = explode(" ", $key); 
        $year_label = $MonthYear_label[1];
        $month_label = $MonthYear_label[0];
        $curr_year = $year_label;

        if ($curr_year == $prev_year) {
            $str = "same";
            $total_val = $total_val + $val; 
        } else {
            if ($i == 1) {
                //skip for first time.. first iterator 
            } else {
                echo "<tr>
                          <td colspan='6'>&nbsp;&nbsp;</td>
                      </tr>";
            }
            $str = "--new--";
            $total_val = $val;      
       }
       echo '<tr> 
                 <td>' . $i . '</td>
                 <td>' . $month_label . '</td>
                 <td>' . $year_label . ' </td>
                 <td>' . $str . '</td>
                 <td>' . $val . '</td>
                 <td><b>' . $total_val . '</b></td>
             </tr>';
       $i++; 
       $prev_year = $curr_year; 
    endforeach; 
    ?> 
</table>

Current Output:

current output

What i expect ? By adding those data year wise..

desired output

Notes: all data appears year-wise & data column represents the total sum of data

Answer 1

    <?php 
    $array_list= array("Jan 2016"=>2, "Feb 2016"=>4, "Mar 2016"=>2,"Apr 2016"=>0, "Jan 2017"=>9,"Feb 2017"=>2,"Mar 2017"=>5,"Jan 2018"=>4,"Feb 2018"=>6);
    ?>
    <table border='1' cellpadding='5' cellspancing='3'>
        <tr>
            <td>SN</td>
            <td>Month/Year</td>
            <td>Data</td>
        </tr>

        <?php 
        $array = array();
        foreach($array_list as $key => $value):
            $MonthYear_label    = explode(" ",$key); 
            $year_label         = $MonthYear_label[1];
            $month_label        = $MonthYear_label[0];
            $curr_year          = $year_label; 
            $array[$curr_year]['count'] =  $array[$curr_year]['count'] + $value;
        endforeach; 
        $i = 1;
        foreach ($array as $key => $value) {
            echo '<tr> 
            <td>'.$i.'</td>
            <td>'.$key.'</td>
            <td>'.$value['count'].' </td>
            </td>';
        }
        ?>
    </table>

Answer 2

You can filter the array with two foreach arrays and a bunch of array_funtions.
I first find all the years and loop the unique values in the second loop.
Then I use array_sum and array_intersect to find the $year and sum the values.
The output array $res should be easy to output in your table.

$array_list= array("Jan 2016"=>2, "Feb 2016"=>4, "Mar 2016"=>2,"Apr 2016"=>0, "Jan 2017"=>9,"Feb 2017"=>2,"Mar 2017"=>5,"Jan 2018"=>4,"Feb 2018"=>6); 

foreach(array_keys($array_list) as $key){
    $years[] = explode(" ", $key)[1];
}

foreach(array_unique($years) as $year){
    $res[$year] = array_sum(array_intersect_key(array_values($array_list), array_intersect($years, [$year])));
}

var_dump($res);

Because I find the matching year with array_functions the array does not have to be in sorted order.

https://3v4l.org/kCBRE
Performance: https://3v4l.org/OgT0a

Answer 3

HTML aside, you can choose to create a new array of year totals or you can convert your one-dimensional array to a two-dimensional array containing month amounts and year totals. This is probably a programming style choice, since either technique is sensible.

1. Create a separate year totals array. This produces more lean data (no redundant data) than the next snippet, but later in your HTML markup generation, you'll need to re-extract the year substring to access the totals array.

   1. Loop a copy of the array.
   2. Extract the year from the key.
   3. Declare or update the year amount (depending on if it has been encountered previously).

   Code: (Demo)

   $totals = [];
   foreach ($array as $date => $amount) {
       $year = substr($date, -4);
       $totals[$year] = ($totals[$year] ?? 0) + $amount;
   }
   

2. Create rows of data in the original array to contain both the month amount and its year total. This stores redundant total amount data in your array, but means you won't need to re-extract the year value in order to access the year amount.

   1. Loop and modify the original array as you iterate.
   2. Extract the year from the key.
   3. Overwrite the $amount variable's integer type value with an array which contains the original integer value AND reference variable.
   4. Update the year-specific reference value by adding each encountered amount to the cached total.

   Code: (Demo)

   foreach ($array as $date => &$amount) {
       $year = substr($date, -4);
       $amount = ['month' => $amount, 'total' => &$ref[$year]];
       $ref[$year] = ($ref[$year] ?? 0) + $amount['month'];
   }