calculation returning wrong value on last array iteration

[xenomorf]

hi all,

i am currently doing some statistic analysis using PHP. Data are retrieved from database. I have a bit of problem with my function

 

 public function upperControlLimit(array $data , $target, $sd, $positiveShiftToDetect = .5)
  {
$k = $positiveShiftToDetect * $sd;

$n = count($data);

for($i=0; $i<$n; $i++)
{
  if(!empty($data[$i-1]))
  {
    $upper[] = max(0,max(0, $data[$i-1] - $target - $k) + $data[$i] - $target - $k);
  }
  else $upper[] = max(0, $data[$i] - $target - $k);
}

return $upper;
}

below are the data i am using to check the function

 

 

Array
(
    [0] => 0.175
    [1] => 0.152
    [2] => 0.15
    [3] => 0.207
    [4] => 0.136
    [5] => 0.212
    [6] => 0.166
    [7] => 0.141
    [8] => 0.157
    [9] => 0.197
    [10] => 0.172
    [11] => 0.183
    [12] => 0.166
    [13] => 0.164
    [14] => 0.141
    [15] => 0.186
    [16] => 0.127
    [17] => 0.149
    [18] => 0.155
    [19] => 0.21
    [20] => 0.197
    [21] => 0.191
    [22] => 0.211
    [23] => 0.158
    [24] => 0.201
)

 

 

other specific data

$target = 0.16
$sd = 0.027925531914894

my function return this value

 

 

Array
(
[0] => 0.0010372340425532
[1] => 0
[2] => 0
[3] => 0.033037234042553
[4] => 0
[5] => 0.038037234042553
[6] => 0.030074468085106
[7] => 0
[8] => 0
[9] => 0.023037234042553
[10] => 0.021074468085106
[11] => 0.0090372340425532
[12] => 0.0010744680851064
[13] => 0
[14] => 0
[15] => 0.012037234042553
[16] => 0
[17] => 0
[18] => 0
[19] => 0.036037234042553
[20] => 0.059074468085106
[21] => 0.040074468085106
[22] => 0.054074468085106
[23] => 0.021074468085106
[24] => 0.027037234042553
)

 

 

the data

 

 

+--------+-------+-------+--------+
| Sample |   X   |  SH   |   SL   |
+--------+-------+-------+--------+
|      1 | 0.175 | 0.001 |      0 |
|      2 | 0.152 |     0 |      0 |
|      3 |  0.15 |     0 |      0 |
|      4 | 0.207 | 0.033 |      0 |
|      5 | 0.136 |     0 |  -0.01 |
|      6 | 0.212 | 0.038 |      0 |
|      7 | 0.166 |  0.03 |      0 |
|      8 | 0.141 |     0 | -0.005 |
|      9 | 0.157 |     0 |      0 |
|     10 | 0.197 | 0.023 |      0 |
|     11 | 0.172 | 0.021 |      0 |
|     12 | 0.183 |  0.03 |      0 |
|     13 | 0.166 | 0.022 |      0 |
|     14 | 0.164 | 0.012 |      0 |
|     15 | 0.141 |     0 | -0.005 |
|     16 | 0.186 | 0.012 |      0 |
|     17 | 0.127 |     0 | -0.019 |
|     18 | 0.149 |     0 | -0.016 |
|     19 | 0.155 |     0 | -0.007 |
|     20 |  0.21 | 0.036 |      0 |
|     21 | 0.197 | 0.059 |      0 |
|     22 | 0.191 | 0.076 |      0 |
|     23 | 0.211 | 0.113 |      0 |
|     24 | 0.158 | 0.097 |      0 |
|     25 | 0.201 | 0.124 |      0 |
+--------+-------+-------+--------+

 

 

i am using the SH column to check my calculation.

 

my problem is, my function returning a wrong calculation starting from array[21] to [24] from my function.

 

the formula and the data are taken from here: https://www.spcforexcel.com/knowledge/variable-control-charts/keeping-process-target-cusum-charts

[xenomorf]

seems like i myself made the function much more complicated than it should be

 

i've updated my function

  public function upperControlLimit(array $data , $target, $sd, $positiveShiftToDetect = .5)
  {
    $k = $positiveShiftToDetect * $sd;

    $n = count($data);

    //for SH(1), SH(i-1) is zero (0)
    $upper[] = max(0,$data[0]-$target-$k);

    for($i=1; $i<$n; $i++)
    {
      $upper[] = max(0,$upper[$i-1]+$data[$i]-$target-$k);
    }

    return $upper;
  }

now all calculation are correct!!

[Psycho]

Don't use a for() loop for an array with an arbitrary value to indicate the index - use a foreach() loop.

 

making some other changes this should be even simpler

 

  public function upperControlLimit(array $dataAry, $target, $sd, $positiveShiftToDetect = .5)
  {
    $k = $positiveShiftToDetect * $sd;
 
    $upperAry = array();
    $upperVal = 0;
 
    foreach($dataAry as $data)
    {
      $upperVal = max(0, $upperVal+$data-$target-$k);
      $upperAry[] = $upperVal;
    }
 
    return $upperAry;
  }