Building an array gives PHP Undefined Offset: 0

Question

I can probably find a way to work around this problem I am having, but would really like to understand what language construct is causing it.

The logic is as follows, I want to build up a Session variable as the user clicks on a route on Google Maps that is an array of waypoints. The POST packet is a JSON object:

?waypoint={"waypoint":[{"lat":"23.3","long":"145"}]}

The code that accepts the POST and interprets is:

    <?php
     session_start ();
    //session_destroy();
    //die;
    //$waypoints[] = array();  <<== uncomment this and the behaviour changes
    if (isset ( $_GET ['waypoint'] )) {
        $waypoint =  $_GET ['waypoint'];
        $waypoint = json_decode($waypoint,true);
        print_r($waypoint['waypoint']); <<<========output 1
        if (isset ( $_SESSION ['waypoints'] )) {
            $waypoints = $_SESSION ['waypoints'];
            $waypoints [] = $waypoint['waypoint'];
            $_SESSION ['waypoints']=$waypoints;
        } else {
            $_SESSION ['waypoints'] = $waypoint['waypoint']; 
            $waypoints[] = $waypoint['waypoint'];
        }
    }

    foreach( $_SESSION ['waypoints'] as $var) {
        var_dump($var);    <<<<====== Output 2
        echo($var[0]['lat']); <<<<==== Here is the problem.

    };
    ?>

Here is the problem. The first time through, Output 1 and 2 give the outputs below and the correct value for lat is output = 23.3

  **Output 1:** Array ( [0] => Array ( [lat] => 23.3 [long] => 145 ) )
  -------------------------------------------------------
  **Output 2:**
  array (size=1)
  0 => 
    array (size=2)
      'lat' => string '23.3' (length=4)
      'long' => string '145' (length=3)

The second call, produces an error Notice: Undefined offset: 0 and subsequent calls are fine, that is the array builds up as expected. Now if I uncomment the statement waypoints[] = array(); It bombs on the first pass, yet all subsequent runs are okay.

Is anyone able to interpret this behaviour?

Thanks

Can we assume you do actually have a session_start() in this code that you have not shown us? — RiggsFolly
– RiggsFolly, Commented Sep 25, 2016 at 14:32

RiggsFolly · Accepted Answer · 2016-09-25 14:46:23Z

2

You seem to be making life a little more complex than it need to be.

session_start();

if (isset ( $_GET ['waypoint'] )) {

    $waypoint = json_decode($_GET ['waypoint'],true);
    print_r($waypoint['waypoint']); <<<========output 1

    // as whatever happens we are just adding a waypoint
    // to an array of waypoints in the SESSION variable
    // this one line will create the $_SESSION['waypoints'][]
    // or add a new [] to it.

    $_SESSION['waypoints'][] = $waypoint;
}

// Just as belt and brace check, 
// assuming someone could try to pass this code some rubbish
// like xxx.php&hacker=yes we ought to check before attempting to
// use the $_SESSION['waypoints'] that one exists

if ( isset($_SESSION['waypoints'] ) {
    foreach( $_SESSION['waypoints'] as $var) {
        // $var is now a waypoint array so it is not $var[0]['lat']
        echo sprintf( 'Latitude = %s - Longitude = %s', 
                            $var['lat'], 
                            $var['long']
                    );
    }
}
?>

edited Sep 25, 2016 at 14:46

answered Sep 25, 2016 at 14:40

RiggsFolly

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1 Comment

user6743474 Over a year ago

Thanks for this - especially the comment regarding "belt and brace check" I will recast my code.

IzzEps · Accepted Answer · 2016-09-25 14:42:32Z

0

The Issue

your issue is in the first line of the else condition that reads $_SESSION ['waypoints'] = $waypoint['waypoint'];.

The Fix

If you change it to $_SESSION ['waypoints'][] = $waypoint['waypoint']; then you should resolve the errors you are getting.

The Explanation

What you are looking for in echo($var[0]['lat']); is for the values of each waypoint to contain an array which they do when they are set in your if ($waypoints [] = $waypoint['waypoint'];) condition but not in your else condition ($_SESSION ['waypoints'] = $waypoint['waypoint'];)

answered Sep 25, 2016 at 14:42

IzzEps

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user6743474 Over a year ago

Thankyou. Yes that fixed it. I agree with RiggsFolly that it is more complex than it needs to be, but it grew as I tried to get to the bottom of the problem.

jakub wrona · Accepted Answer · 2016-09-25 14:41:47Z

0

else {
    $_SESSION ['waypoints'][] = $waypoint['waypoint']; 
}

answered Sep 25, 2016 at 14:41

jakub wrona

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