Programming Forums > Application Development > Java

namsu · Jan 7th, 2008, 10:56 AM

I am unsure of what the ? : is doing in Java. Ive searched in Google I cannot find anything, I have forgotten which book I read it out of but heres an example of some code that involves it:

(Toggle Plain Text)

61    public void setGrossSales( double sales )
62    {
63       grossSales = ( sales < 0.0 ) ? 0.0 : sales;
64    }

Sane · Jan 7th, 2008, 11:35 AM

A question mark is known as a "conditional operator". Essentially, it's a one line if statement.

http://www.google.ca/search?hl=en&q=...operator&meta=

It evaluates the statement before the question mark. If it's true, the statement inherits the value of the left side of the colon. If the statement is false, it inherits the value on the right side of the colon.

(Toggle Plain Text)

statement that will be true or false ? value on left side of colon if true : value on right side of colon if false

Is equal to:

(Toggle Plain Text)

if (statement that will be true or false)
    use value on left side
else
    use value on right side

So the code you posted:

(Toggle Plain Text)

public void setGrossSales( double sales )
{
    grossSales = ( sales < 0.0 ) ? 0.0 : sales;
}

Is equivilent to:

(Toggle Plain Text)

public void setGrossSales( double sales )
{
    if (sales < 0.0)
        grossSales = 0.0;
    else
        grossSales = sales;
}

It's an easy way to prevent the number from being negative, in one line.

namsu · Jan 7th, 2008, 11:37 AM

Quote:

Originally Posted by Sane

A question mark is known as a "conditional operator". Essentially, it's a one line if statement.

http://www.google.ca/search?hl=en&q=...operator&meta=

It evaluates the statement before the question mark. If it's true, the statement inherits the value of the left side of the colon. If the statement is false, it inherits the value on the right side of the colon.

(Toggle Plain Text)

statement that will be true or false ? value on left side of colon if true : value on right side of colon if false

Is equal to:

(Toggle Plain Text)

if (statement that will be true or false) is (true)
    use value on left side
else
    use value on right side

Thanks

mrynit · Jan 7th, 2008, 3:01 PM

@namsu you don't have to quote his entire post to say thank you.

I hardly ever see the ? operator. Using if or if else looks better as in more human readable than ?. It's style to me.

Sane · Jan 7th, 2008, 3:30 PM

Well there are instances where it becomes necessary in order to prevent repeating code, or adding new variables in to the scenario. I'll make something up to demonstrate this...

Quote:

The function "isPrime(x)" returns 1 if x is prime, 0 if it is composite.

Using this function, output whether or not n is a prime number in the following format:

"The number 'n' is a prime number."
/ or /
"The number 'n' is a composite number."

E.G.

"The number '4' is a composite number."
"The number '6079' is a prime number."

So what would be the best way to code it? With if/else statements:

(Switching to C Code now)

 C Syntax (Toggle Plain Text) 
if (isPrime(n))
    printf("The number '%d' is a prime number.", n);
else
    printf("The number '%d' is a composite number.", n);
if (isPrime(n))
    printf("The number '%d' is a prime number.", n);
else
    printf("The number '%d' is a composite number.", n);

With a conditional operator:

 C Syntax (Toggle Plain Text) 
printf("The number '%d' is a %s number.", n, 
       isPrime(n) ? "prime" : "composite");
printf("The number '%d' is a %s number.", n, 
       isPrime(n) ? "prime" : "composite");

Each has their drawbacks, but personally I'd go with the latter, for sake of not repeating code.
It's easier to change the format of the output, and the input specifications.

namsu · Jan 7th, 2008, 3:30 PM

Quote:

Originally Posted by mrynit

@namsu you don't have to quote his entire post to say thank you.

I hardly ever see the ? operator. Using if or if else looks better as in more human readable than ?. It's style to me.

Yeah sorry I will just say Thanks and post next time.

Jan 7th, 2008, 10:56 AM	#1
namsu Newbie Join Date: Jan 2008 Posts: 15 Rep Power: 0	What does ? : do? I am unsure of what the ? : is doing in Java. Ive searched in Google I cannot find anything, I have forgotten which book I read it out of but heres an example of some code that involves it: (Toggle Plain Text) 61 public void setGrossSales( double sales ) 62 { 63 grossSales = ( sales < 0.0 ) ? 0.0 : sales; 64 } 61 public void setGrossSales( double sales ) 62 { 63 grossSales = ( sales < 0.0 ) ? 0.0 : sales; 64 }

Jan 7th, 2008, 11:35 AM	#2
Sane Programming Guru Join Date: Apr 2005 Posts: 1,826 Rep Power: 5	Re: What does ? : do? A question mark is known as a "conditional operator". Essentially, it's a one line `if` statement. http://www.google.ca/search?hl=en&q=...operator&meta= It evaluates the statement before the question mark. If it's true, the statement inherits the value of the left side of the colon. If the statement is false, it inherits the value on the right side of the colon. (Toggle Plain Text) statement that will be true or false ? value on left side of colon if true : value on right side of colon if false statement that will be true or false ? value on left side of colon if true : value on right side of colon if false Is equal to: (Toggle Plain Text) if (statement that will be true or false) use value on left side else use value on right side if (statement that will be true or false) use value on left side else use value on right side So the code you posted: (Toggle Plain Text) public void setGrossSales( double sales ) { grossSales = ( sales < 0.0 ) ? 0.0 : sales; } public void setGrossSales( double sales ) { grossSales = ( sales < 0.0 ) ? 0.0 : sales; } Is equivilent to: (Toggle Plain Text) public void setGrossSales( double sales ) { if (sales < 0.0) grossSales = 0.0; else grossSales = sales; } public void setGrossSales( double sales ) { if (sales < 0.0) grossSales = 0.0; else grossSales = sales; } It's an easy way to prevent the number from being negative, in one line.

Jan 7th, 2008, 3:01 PM	#4
mrynit Hobbyist Programmer Join Date: Mar 2006 Location: WA, USA Posts: 332 Rep Power: 3	Re: What does ? : do? @namsu you don't have to quote his entire post to say thank you. I hardly ever see the ? operator. Using if or if else looks better as in more human readable than ?. It's style to me. __________________ i dont know much about programming but i try to help

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Programming Forums > Application Development > Java

What does ? : do?