CODEWITHSUNDEEP
phparrays
Ben Wilson
Ben Wilson

Reputation: 2676

Getting running average for PHP arrays

I'm using a series of MySQL queries to pull back calculations stored by date for graphing via the Flot library. After the calculations are done, the echoed material looks like this (using UNIX timestamp dates):

Item 1: 
        [
            [1159765200000,-117.875], 
            [1159851600000,-117.25], 
            [1159938000000,-120.625], 
            [1160024400000,-122.125], 
            [1160110800000,-118.125], 
            [1160370000000,-121.125], 
            [1160456400000,-123.375], 
            [1160542800000,-115.625], 
            [1160629200000,-117.75], 
            [1160715600000,-112.75], 
            [1160974800000,-125.25], 
            [1161061200000,-135], 
            [1161147600000,-138.375], 
            [1161234000000,-137], 
            [1161320400000,-136.25], 
            [1161579600000,-139.875], 
            [1161666000000,-146.625], 
            [1161752400000,-143.625], 
            [1161838800000,-150.25], 
            [1161925200000,-152.875], 
            [1162188000000,-151.75], 
            [1162274400000,-149.75]
        ]


Item 2: 
        [
            [1104732000000,47.3913043478], 
            [1104818400000,45.5072463768], 
            [1104904800000,45.5797101449], 
            [1104991200000,45.115942029], 
            [1105077600000,44.1739130435], 
            [1105336800000,44.5362318841], 
            [1105423200000,45.9565217391], 
            [1105509600000,45.9420289855], 
            [1105596000000,46.0289855072], 
            [1105682400000,46.4347826087], 
            [1106028000000,48.347826087], 
            [1106114400000,46.8695652174], 
            [1106200800000,46.4927536232], 
            [1106287200000,45.6376811594], 
            [1106546400000,44.3768115942], 
            [1106632800000,44.0579710145], 
            [1106719200000,44.5942028986], 
            [1106805600000,45.0289855072], 
            [1106892000000,45.231884058], 
            [1107151200000,46.1449275362], 
            [1107237600000,46.5942028986], 
            [1107324000000,45.5652173913], 
            [1107410400000,45], 
            [1107496800000,46.2608695652], 
            [1107756000000,45.7391304348], 
            [1107842400000,46.3333333333]
        ]

Basically I'd like to calculate the average of the second value in each pair, controlling for the date. In other words, for each date that matches in each array, print the date and the average of all the second values in each array, e.g:

[Common Date, Average of all second values]

I've looked through a number of array merging techniques but can't seem to find a workable solution.

Thanks very much for any help.

Upvotes: 1

Views: 5564

Answers (2)

afuzzyllama
afuzzyllama

Reputation: 6548

For your merge

$merged_array = array();

function merge_by_time()
{
    $passed_arrays = func_get_args();

    $merged_array = array();
    foreach($passed_arrays as $array)
        foreach($array as $value_set){
            $merged_array[$value_set[0]][] = $value_set[1];
        }
    }

    return $merged_array;
}

Usage:

$new_array = merge_by_time($array1, $array2, $array3, ...)

Then you'll have an array based on timestamp that has all associated data values contained in it. I think you can take it from here to get the averages?

Second approach

function merge_by_time_and_get_average()
{
    $passed_arrays = func_get_args();

    $merged_array = array();
    foreach($passed_arrays as $array)
        foreach($array as $value_set){
            $merged_array[$value_set[0]]['data'][] = $value_set[1];

            $merged_array[$value_set[0]]['average'] = 0;
            foreach($merged_array[$value_set[0]]['data'] as $data_point){
                $merged_array[$value_set[0]]['average'] += $data_point;
            }
            $merged_array[$value_set[0]]['average'] = $merged_array[$value_set[0]]['average']/count($merged_array[$value_set[0]]['data'])
        }
    }

    return $merged_array;
}

Then you have $array[{timestamp}]['data'] containing your data points and $array[{timestamp}]['average'] containing your average of all data points. The nested foreachs are a little messy and expensive, but you can handle it all in one function call.

Upvotes: 0

Arnaud Le Blanc
Arnaud Le Blanc

Reputation: 99909

You could construct an array indexed by date in which you put a list of all values for the date:

$byDate = array();
foreach($item1 as $row) {
    $date = sprintf('%.0f', $row[0]);
    $byDate[$date][] = $row[1];
}
foreach($item2 as $row) {
    $date = sprintf('%.0f', $row[0]);
    $byDate[$date][] = $row[1];
}

Then you can easily compute the average for each list:

foreach($byDate as $date => $values) {
    $avg = array_sum($values) / count($value);
    printf("avg for %s: %f\n", $date, $avg);
}

Or compute all averages at once:

function array_avg($array) {
    return array_sum($array) / count($array);
}
$avgByDate = array_map('array_avg', $byDate);

Try it here: http://codepad.org/1S1HrYoB

Upvotes: 1

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