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From Henrik Martin <hen...@netgate.net>
Subject What am I missing with this Elvis operator expression?
Date Mon, 11 Apr 2016 22:41:59 GMT
Greetings. I ran into this little surprise with the Elvis operator 
today. I guess it must boil down to my misunderstanding of how the 
operator work. I have a method that tries to calculate a value based on 
an input. If the input value is set to -1, I'd like to change it to a 
positive value (which is calculated, but in the test I just hard coded 
it to 10). In the process of doing so, I ran into a surprise. Here's a 
Spock test case that illustrates it:

import spock.lang.Specification

class ElvisTest extends Specification {
   def "test"() {
     setup:
     def sleepFor = -1
     sleepFor = sleepFor > 0 ?: 10
     println sleepFor
     sleepFor = 972
     sleepFor = sleepFor > 0 ?: 10
     println sleepFor
   }
}
When sleepFor is initialized to -1, the operator works as I expected it 
and my println statement prints 910. If I initialize sleepFor to some 
other value like the one in the test, all of a sudden the result of the 
Elvis operator is true instead of an integer. I naïvely thought that ?: 
would yield the numeric value of the sleepFor variable in the left hand 
expression instead of the boolean result of it (as in sleepFor > 0). 
What am I missing? In this case, I could always use the Java style of:

sleepFor = sleepFor > 0 ? sleepFor : 10

but the whole point of the Elvis operator is brevity, and this behavior 
was a small surprise to me. Thanks,

-H


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