$data2 = $data2 + $data; Does not work

[popunonkok]

Hi

 

Im plotting a graph using JPGraph.

 

The first plotted line uses $data as its source. It works like a charm, I collect the data from a MySql database.

 

However, now I want to plot a new line. I want that line to use the same data but I want to add each value to the value before and so on.

 

This is my code that dosent work.

while ($row = mysql_fetch_array($result)) {
$data[] = $row['EF0000000DBBDD1D']*6/1000;
$data1[] = strtotime($row['DATETIME']);
$data2[] = $data2 + $data;
}

echo "-----------------------------";
print_r ($data);
print_r ($data2);

 

The result Im looking for is this:

 

$data =   2, 2, 2, 2, 2, 2, 2, 2, 2.....
$data2 = 2, 4, 6, 8, 10, 12, 14.......

 

I thought that this should be easy but I just cant seem to find the answer.

 

Any help is appreciated.

 

[btherl]

That should be $data2[$index] + $data[$index-1], which is difficult because your index is not explicit.  I would change it to this:

 

$index =0 ;
while ($row = mysql_fetch_array($result)) {
$data[$index] = $row['EF0000000DBBDD1D']*6/1000;
$data1[$index] = strtotime($row['DATETIME']);
$data2[$index] = ($index == 0 ? $data[$index] : $data2[$index-1] + $data[$index]);
$index++;
}

 

The conditional says "If index is 0 (ie it's the first data point), just put it straight in.  Otherwise, add it to the previous data point from $data2 array"

[Psycho]

EDIT: btherl beat me to it, but I will post this anyway since my solution is much more efficient (no offense btherl)

 

Um, yeah. If you notice the code you put in the post title is NOT the same as the code you posted in the description. There is a huge difference - mainly that $data2 is an array!

$data2[] = $data2 + $data;

 

How can you create a new value by adding an array to a numeric value? You need to think about what you are really trying to accomplish in simple terms and then translate it into code. Here is how I would state the problem: "I want to generate a value on each iteration which is the sum of the current iteration value and all previous iteration values".

 

There is a very easy solution to your problem. Since $data contains the current iteration value and all previous iteration values, you just need the sum of all the values in that array!:

 

while ($row = mysql_fetch_array($result))
{
    $data[]  = $row['EF0000000DBBDD1D']*6/1000;
    $data1[] = strtotime($row['DATETIME']);
    $data2[] = array_sum($data);
}

[btherl]

I'll agree that's that's a simpler solution mjdamato, but it's O(n^2) rather than O(n).  So it depends what you mean by "efficient"   Unless there's 1000s of data points it probably doesn't matter, and I would go with your simpler solution.

[Psycho]

I'll agree that's that's a simpler solution mjdamato, but it's O(n^2) rather than O(n).  So it depends what you mean by "efficient"   Unless there's 1000s of data points it probably doesn't matter, and I would go with your simpler solution.

 

I see what you are saying. However, if there are a large number of records, then I wouldn't want to run a ternary operator on each iteration (they are less efficient than normal IF statements). Instead, I would probably use a tracking variable. E.g.

$total = 0;
while ($row = mysql_fetch_array($result))
{
    $value = $row['EF0000000DBBDD1D']*6/1000;
    $total =+ $value;
    $data[]  = $value;
    $data1[] = strtotime($row['DATETIME']);
    $data2[] = $total;
}