What is a Question Mark "?" and Colon ":" Operator Used for?

Question

Two questions about using a question mark "?" and colon ":" operator within the parentheses of a print function: What do they do? Also, does anyone know the standard term for them or where I can find more information on their use? I've read that they are similar to an 'if' 'else' statement.

int row = 10;
int column;
while (row >= 1)
{
    column = 1;
    while(column <= 10)
    {
        System.out.print(row % 2 == 1 ? "<" : "\r>");
        ++column;
    }
    --row;
    System.out.println();
}

Solution 1

This is the ternary conditional operator, which can be used anywhere, not just the print statement. It's sometimes just called "the ternary operator", but it's not the only ternary operator, just the most common one.

Here's a good example from Wikipedia demonstrating how it works:

A traditional if-else construct in C, Java and JavaScript is written:

if (a > b) {
    result = x;
} else {
    result = y;
}

This can be rewritten as the following statement:

result = a > b ? x : y;

Basically it takes the form:

boolean statement ? true result : false result;

So if the boolean statement is true, you get the first part, and if it's false you get the second one.

Try these if that still doesn't make sense:

System.out.println(true ? "true!" : "false.");
System.out.println(false ? "true!" : "false.");

Solution 2

Thats an if/else statement equilavent to

if(row % 2 == 1){
  System.out.print("<");
}else{
  System.out.print("\r>");
}

Solution 3

a=1;
b=2;

x=3;
y=4;

answer = a > b ? x : y;

answer=4 since the condition is false it takes y value.

A question mark (?)
. The value to use if the condition is true

A colon (:)
. The value to use if the condition is false

Solution 4

Also just though I'd post the answer to another related question I had,

a = x ? : y;

Is equivalent to:

a = x ? x : y;

If x is false or null then the value of y is taken.

Solution 5

Maybe It can be perfect example for Android, For example:

void setWaitScreen(boolean set) {
    findViewById(R.id.screen_main).setVisibility(
            set ? View.GONE : View.VISIBLE);
    findViewById(R.id.screen_wait).setVisibility(
            set ? View.VISIBLE : View.GONE);
}

View more solutions

Comments

[Reinstate Monica -- notmaynard]

What language? Not in Java.

[Nikola Yovchev]

That can be but not in java.

[moo moo]

I am working in Java and yes this does work.

[uyuyuy99]

Sorry, but this does not work in Java. You sure you're not thinking about, say, PHP?

[Sheepy]

Ternary operator refers to any operator with three parameters, thus this is a ternary operator but not the ternary operator. Major languages (C#, Java, PHP) consider it a conditional operator, and call it the ?: operator. Occasionally (JavaScript) it is called the conditional operator.

[Brendan Long]

@Sheepy Thanks, I updated the answer.

[Cemafor]

Groovy has the "Elvis operator" (?:) that behaves like this, similar to C#'s null coalescing operator (??).

[Cleb]

Is there any difference regarding performance? I personally find the classical version far easier to read - especially when x and y are again function calls.

[Brendan Long]

@Cleb Theoretically there's no difference, but compilers may optimize the two statements differently (making different assumptions about branch prediction). Most people don't need to worry about that level of optimization, and if you do, you probably want to use attributes like __builtin_expect anyway.

[Cody]

To me, your answer was the only one that made sense based on how you laid it out. So thank you and I have republished your answer with a citation. garrett.ms/2019/07/24/…

[Menai Ala Eddine - Aladdin]

Is it possible to call function after 7 or 8 ? for example if a>7 type 7 then execute other function.

[user unknown]

Do you mean "print 7" with "type 7"? Why don't you try it out? The Java jargon uses the expression method, not functions - in other contexts there is the distinction of function and procedure, dependent on the existence of a return type, which is required in a ternary expression. Afaik, you can call a function which, after printing, returns a value.