Ternary operator and string concatenation quirk?

问题

Hi I just want to know why this code yields (at least for me) an incorrect result.

Well, probably i\'m in fault here

$description = \'Paper: \' . ($paperType == \'bond\') ? \'Bond\' : \'Other\';

I was guessing that if paperType equals \'Bond\' then description is \'Paper: Bond\' and if paperType is not equals to \'Bond\' then description is \'Paper: Other\'.

But when I run this code the results are description is either \'Bond\' or \'Other\' and left me wondering where the string \'Paper: \' went???

回答1:

$description = 'Paper: ' . ($paperType == 'bond' ? 'Bond' : 'Other');

Try adding parentheses so the string is concatenated to a string in the right order.

回答2:

It is related with operator precedence. You have to do the following:

$description = 'Paper: ' . (($paperType == 'bond') ? 'Bond' : 'Other');

回答3:

I think everyone gave the solution, I would like to contribute the reason for the unexpected result.

First of all here you can check the origin, and how the operators are evaluated (left, right, associative, etc).

http://php.net/manual/fa/language.operators.precedence.php

Now if we analyze your sentence.

$ paperType = 'bond';
$ description = 'Paper:'. ($ paperType == 'bond')? 'Bond': 'Other';

1) We review the table and find that the parentheses are evaluated first, then the '.' (concatenation) is evaluated and at the end the ternary operator '?', therefore we could associate this as follows:

// evaluate the parenthesis ... ($ paperType == 'bond')
$ description = ('Paper:'. 1)? 'Bond': 'Other';
//result
$ description = 'Paper: 1'? 'Bond': 'Other';

2) We now have the ternary operator, we know that a string is evaluated "true"

// php documentation When converting to boolean, the following values are considered FALSE:

... the empty string, and the string "0"