What is ? mark in Julia?

General Usage
1

e.g. what does it do here:
pow2(x, n) = n <= 0 ? 1 : x*pow2(x, n-1)
or here
minmax(xx, y) = (y < xx) ? (y, xx) : (xx, y)

2

If you use the help mode on the REPL by typing ? you can see the following documentation.

help?> ?
search: ? ?:

  a ? b : c

  Short form for conditionals; read "if a, evaluate b otherwise evaluate c". Also known as the ternary operator (https://en.wikipedia.org/wiki/%3F:).

  This syntax is equivalent to if a; b else c end, but is often used to emphasize the value b-or-c which is being used as part of a larger expression, rather
  than the side effects that evaluating b or c may have.

  See the manual section on control flow for more details.

  Examples
  ≡≡≡≡≡≡≡≡≡≡

  julia> x = 1; y = 2;
  
  julia> x > y ? println("x is larger") : println("y is larger")
  y is larger

So basically

pow2(x, n) = n <= 0 ? 1 : x*pow2(x, n-1)

is the same as:

function pow2(x,n)
    if n<=0
        return 1
    else
        return x*pow2(x,n-1)
    end
end
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3

This is eqivalent to

function minmax(xx, y)
    if y < xx
        return (y, xx)
    else
        return (xx, y)
    end 
end
6 Likes
4

I love the syntax because it matches how I think about conditional assignment.

In my head “to choose x, ask yourself ‘is y greater than 0?’ if so x should be y, otherwise it should be 0.”

In code:

x = y > 0 ? y : 0
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5

Since it wasn’t mentioned earlier, this is known as the ternary conditional operator and is common across most languages: Ternary conditional operator - Wikipedia

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