strange behaviour ternary operator

Question

Coming from C# I must do a project in PHP.

I am using this code:

$transport = 'T';

$vehicle = (
 ( $transport == 'B' ) ? 'bus' :
 ( $transport == 'A' ) ? 'airplane' :
 ( $transport == 'T' ) ? 'train' :
 ( $transport == 'C' ) ? 'car' :
 ( $transport == 'H' ) ? 'horse' :
 'feet' );

echo $vehicle;

I would expect it to print train, but I get horse. Codepad example: http://codepad.org/rWllfrht

Who can explain this strange behaviour?

Answer 1

Not seeing any explanation about why your code is broken in the other answers, so here is a quick run-down.

The problem here is made more obvious is you add brackets to make the implicit order of evaluation more explicit.

Here's a trimmed down version of your code, which still produces the incorrect result of "horse":

 $t = 'T';

 ( $t == 'T' ) ? 'train' : 
 ( $t == 'C' ) ? 'car' : 
 ( $t == 'H' ) ? 'horse' : 'feet';

First, lets unroll it:

( $t == 'T' ) ? 'train' : ( $t == 'C' ) ? 'car' : ( $t == 'H' ) ? 'horse' : 'feet'; 

Next, I'll add explicit parenthesis where there are already implicit ones:

((($t == 'T') ? 'train' : ($t == 'C')) ? 'car' : ($t == 'H')) ? 'horse' : 'feet';

Next, we can resolve your comparisons:

((true ? 'train' : false) ? 'car' : false) ? 'horse' : 'feet';

You should start to see why this is broken. The first ternary evaluates true ? 'train' : 'false' to 'train':

('train' ? 'car' : false) ? 'horse' : 'feet';

Because 'train' is true when cast to a boolean, the result is now 'car':

'car' ? 'horse' : 'feet';

Again, because a non-empty string is "true", the result is now 'horse'. So, the first time a true comes up in your horrible nested case statement, the result will cascade through all remaining statements, throwing out the previous value for the "true" branch of the next operator.

The solution is to avoid this code. It is an attempt to be far, far too clever, and the result is a broken, unreadable mess. There is absolutely no reason to use it. Choose a switch statement, it's purpose built for exactly what you're trying to do.

Answer 2

This doesn't work as expected due to a bug in the PHP language grammar, as seen at: http://en.wikipedia.org/wiki/%3F:#PHP

Here's a simple version that DOES work:

$transport = 'T';

$vehicle = (
 ( $transport == 'B' ? 'bus' :
 ( $transport == 'A' ? 'airplane' :
 ( $transport == 'T'  ? 'train' :
 ( $transport == 'C'  ? 'car' :
 ( $transport == 'H'  ? 'horse' :
 'feet' ))))));

echo $vehicle;

But as everyone else said, I agree this isn't the best way to do this. You could use a switch case, if else if, or associative array and be a lot more readable.

Answer 3

This is a sort of "working-as-intended-even-though-it's-clearly-wrong" behaviour of PHP. It doesn't associate that way, so while this code works in most other languages, it will fail in PHP. Lesson? Learn to use parenthesis over unusual association paradigms. Lesson Two? Ternary isn't a magic bullet, while it can be nice and compact, it should only be used when it's readable. IMHO nested ternary statements are just ugly.

Answer 4

I'm not sure exactly why you chose to use this form of syntax, as mentioned in a comment, this would be a nightmare to debug... A switch case might be a better choice here -

$vehicle = '';
switch($transport){
  case 'B' :
    $vehicle = 'bus';
  break;
  case 'A' :
    $vehicle = 'airplane';
  break;
  ...
  default:
   // undefined cases
  break;
}

References -

• switch statement

Answer 5

Learn to love paranthesis if you want to do something like this:

$vehicle =     ( ( $transport == 'B' ) ? 'bus' : 
                    (( $transport == 'A' ) ? 'airplane' :
                       (( $transport == 'T' ) ? 'train' :
                         (( $transport == 'C' ) ? 'car' :
                           (( $transport == 'H' ) ? 'horse' :'feet')))) );

Each right hand side of the ternary, needs to be clearly contained because of PHP's order of operations for ternary http://php.net/manual/en/language.operators.comparison.php.

By the way, from that page they explicitly recommend against stacking them like this...

Note: It is recommended that you avoid "stacking" ternary expressions. PHP's behaviour when using more than one ternary operator within a single statement is non-obvious:

Answer 6

I won't advise you to use such code but for educational purposes it should be

$transport = 'T';
$vehicle = (
        ($transport == 'B') ? 'bus' : 
            (($transport == 'A') ? 'airplane' : 
                (($transport == 'T') ? 'train' : 
                        (($transport == 'C') ? 'car' : 
                                (($transport == 'H') ? 'horse' : 'feet'))))
        );

echo $vehicle;

A better code should be

$transport = 'T';
switch ($transport) {
    case 'A' :
        $vehicle = 'airplane';
        break;
    case 'B' :
        $vehicle = 'bus';
        break;
    case 'C' :
        $vehicle = 'car';
        break;
    case 'H' :
        $vehicle = 'horse';
        break;
    case 'T' :
        $vehicle = 'train';
        break;
    default :
        $vehicle = 'teleportation';
        break;
}

echo $vehicle;

Or better still:

$transport = 'T';
$array = array('A'=>'airplane','B'=>"bus","C"=>"car","H"=>"horse","T"=>"train");
echo isset($array[$transport]) ? $array[$transport] : null;

Or, use a database:

 SELECT name FROM transpotationTable WHERE someKey = '$transport'