Getting running average for PHP arrays

Question

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.

I mean, each item has a series of dates. For each unique date, there should be a series of corresponding values (for each item). In other words, take the values for each item and group them by date. Does that make sense? — Ben Wilson
– Ben Wilson, Commented Aug 23, 2011 at 16:49

Arnaud Le Blanc · Accepted Answer · 2011-08-23 16:48:32Z

1

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

edited Aug 23, 2011 at 16:48

answered Aug 23, 2011 at 16:35

Arnaud Le Blanc

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6 Comments

piotrp Over a year ago

Just remember to pass the array through ksort(), so that in case of missing values in one of the arrays your result will be correctly ordered.

Arnaud Le Blanc Over a year ago

I don't think the order matters

piotrp Over a year ago

In your code it doesn't, but if Ben tries to chart it then it may.

Ben Wilson Over a year ago

Apparently I don't have enough "reputation" to vote you up, but thanks very much -- this was extremely helpful.

Ben Wilson Over a year ago

Hi Crack and or Arnaud -- how would I go about running these arrays through ksort? Indeed I am getting odd results when graphing because the numbers are out of order, however, does ksort work when you are using arrays like this? I thought it only worked for key-value type pairs

|

afuzzyllama · Accepted Answer · 2011-08-23 17:47:11Z

0

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.

edited Aug 23, 2011 at 17:47

answered Aug 23, 2011 at 16:56

afuzzyllama

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4 Comments

Ben Wilson Over a year ago

afuzzyllama -- thank you so much for your help. What should the arrays look like for your function? Should they be "date" as the key then the second number as the value?

afuzzyllama Over a year ago

It should work with the items you posted. Where it is an array which contains array pairs of [timestamp, value].

Ben Wilson Over a year ago

Thanks -- what I meant is that I'm trying to structure how I pull the data out of the MySQL database so that the array matches the structure used in your function. Heretofore I had been just been using a while loop and echoing out the date and calculated value. Thanks again so much.

afuzzyllama Over a year ago

Something like SELECT timestamp, value FROM data_table WHERE ... and then use mysql-fetch-assoc. You'll have to alter my function somewhat to use your column names instead of the 0 / 1 index

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