I am trying to show some values using json api. It returns a date and some density value in numbers. Successfully, I have printed all return values from url. Now, I want to find the difference between one density value to previous day density value.
I know how to calculate the difference in normal static numbers. But, I am not sure how do I achieve this inside a foreach loop.
PHP Code:
$json_url = "https://www.example.com/api";
$json = file_get_contents($json_url);
$obj = json_decode($json,true);
$data = $obj["data"];
foreach(array_slice($data, 0, 10) as $k) {
echo $k["date"]." - ".$k["density"]."<br/><br/>";
}
Current Output:
14-02-2020 - 110 13-02-2020 - 114 12-02-2020 - 112 11-02-2020 - 110 10-02-2020 - 105 07-02-2020 - 125 06-02-2020 - 122 05-02-2020 - 118 04-02-2020 - 119 03-02-2020 - 119
Expected Output:
14-02-2020 - 110 - Diff is -4 from Previous Date 13-02-2020 - 114 - Diff is +2 from Previous Date 12-02-2020 - 112 - Diff is +2 from Previous Date 11-02-2020 - 110 - Diff is +5 from Previous Date 10-02-2020 - 105 - Diff is -20 from Previous Date 07-02-2020 - 125 - Diff is +3 from Previous Date 06-02-2020 - 122 - Diff is +4 from Previous Date 05-02-2020 - 118 - Diff is -1 from Previous Date 04-02-2020 - 119 - No Difference - Same 03-02-2020 - 119 - No Difference - Same
To achieve the desired effect you will need to iterate the array two elements at a time. This can't easily be done with foreach outright. So instead I will implement a generator that returns pairs of values from the array at a time.
function getPairs(Array $array) {
reset($array);
do {
$a1 = current($array);
$key = key($array);
$a2 = next($array);
yield [$a1, $a2];
} while($key = key($array) !== null);
}
You can then use this generator implementation to traverse the array, attaining the desired effect.
$array = [
["date" => "14-02-2020", "density" => 110],
["date" => "13-02-2020", "density" => 114],
["date" => "12-02-2020", "density" => 112],
["date" => "11-02-2020", "density" => 110],
["date" => "10-02-2020", "density" => 105],
["date" => "07-02-2020", "density" => 125],
["date" => "12-02-2020", "density" => 112],
["date" => "06-02-2020", "density" => 122],
["date" => "05-02-2020", "density" => 118],
["date" => "04-02-2020", "density" => 119],
["date" => "03-02-2020", "density" => 119],
];
foreach(getPairs($array) as [$a, $b]) {
// We must check if $b is empty to account for end of the array
if ($b) {
$diff = $a["density"] - $b["density"];
} else { // Otherwise there are no more elements to diff against
$diff = 0;
}
if ($diff > 0) {
$diff = "+$diff";
}
echo "{$a['date']} - {$a['density']} - Diff is $diff from Previous Date<br><br>";
}
This should give you the desired output.
14-02-2020 - 110 - Diff is -4 from Previous Date 13-02-2020 - 114 - Diff is +2 from Previous Date 12-02-2020 - 112 - Diff is +2 from Previous Date 11-02-2020 - 110 - Diff is +5 from Previous Date 10-02-2020 - 105 - Diff is -20 from Previous Date 07-02-2020 - 125 - Diff is +13 from Previous Date 12-02-2020 - 112 - Diff is -10 from Previous Date 06-02-2020 - 122 - Diff is +4 from Previous Date 05-02-2020 - 118 - Diff is -1 from Previous Date 04-02-2020 - 119 - Diff is 0 from Previous Date 03-02-2020 - 119 - Diff is 0 from Previous Date
N.B.
Technically speaking you aren't actually getting the difference between the current date and the previous date, here. Rather, you are getting the difference between the current date and the next date. If you were getting the deltas between previous dates you wouldn't need the information from the next date at all. Instead you'd just use a temporary variable to store the last
densityvalue and diff against that with each iteration. However, I've tailored my answer to suit your expected result.
Otherwise the expected output would actually be:
14-02-2020 - 110 - Diff is +110 from Previous Date 13-02-2020 - 114 - Diff is +4 from Previous Date 12-02-2020 - 112 - Diff is -2 from Previous Date 11-02-2020 - 110 - Diff is -2 from Previous Date 10-02-2020 - 105 - Diff is -5 from Previous Date 07-02-2020 - 125 - Diff is +20 from Previous Date 12-02-2020 - 112 - Diff is -13 from Previous Date 06-02-2020 - 122 - Diff is +10 from Previous Date 05-02-2020 - 118 - Diff is -4 from Previous Date 04-02-2020 - 119 - Diff is +1 from Previous Date 03-02-2020 - 119 - Diff is 0 from Previous Date
And to achieve that you don't need the generator implementation I used at all. You can simply do this with a regular foreach loop like so:
$lastDiff = 0; // Initialize the diff to 0
foreach($array as $a) {
$date = $a['date'];
$density = $a['density'];
$diff = $a["density"] - $lastDiff; // diff against the last known value
if ($diff > 0) {
$diff = "+$diff";
}
echo "$date - $density - Diff is $diff from Previous Date<br><br>";
$lastDiff = $a["density"]; // store the last known density here for the next iteration
}