The Ternary Operator for Compact Conditionals

Julia programs often need to make decisions and execute different blocks of code based on conditions, typically using if-elseif-else statements. For straightforward choices where you simply need to select one of two values, Julia offers a more compact syntax: the ternary operator. It is a helpful tool for writing concise conditional expressions.

The ternary operator, sometimes called the conditional operator, provides a shorthand way to express a simple if-else choice that results in a value. Its structure is quite intuitive:

condition ? expression_if_true : expression_if_false

Let's break this down:

• condition: This is an expression that evaluates to either true or false.
• ?: This symbol separates the condition from the possible outcomes. You can think of it as asking "is this condition true?"
• expression_if_true: If the condition evaluates to true, this is the expression whose value will be used.
• :: This symbol separates the true-outcome expression from the false-outcome expression.
• expression_if_false: If the condition evaluates to false, this is the expression whose value will be used.

The entire ternary operation is itself an expression, meaning it evaluates to a single value, either the result of expression_if_true or expression_if_false.

Consider a common scenario: assigning a value to a variable based on a condition. Suppose we want to determine a discount rate.

is_member = true
discount_rate = is_member ? 0.10 : 0.05 # 10% for members, 5% for non-members

println("Your discount rate is: $discount_rate")
# Output: Your discount rate is: 0.1

Without the ternary operator, you would write this using an if-else statement:

is_member = true
local discount_rate # Declare variable to be accessible after if-block

if is_member
    discount_rate = 0.10
else
    discount_rate = 0.05
end

println("Your discount rate is: $discount_rate")
# Output: Your discount rate is: 0.1

As you can see, the ternary operator makes the assignment more compact for this simple case.

Here's another example. Let's classify a number as "positive" or "non-positive":

num = -5
classification = num > 0 ? "positive" : "non-positive"

println("The number $num is $classification.")
# Output: The number -5 is non-positive.

The diagram below shows the flow of logic in a ternary operation:

This diagram illustrates the decision path. Based on the condition's outcome, one of two expressions is evaluated to produce the final result.

Because the ternary operator forms an expression, you can use it directly where a value is expected, such as inside a println call or as part of a larger calculation:

score = 75
println("Your grade is: $(score >= 50 ? "Pass" : "Fail")")
# Output: Your grade is: Pass

age = 17
ticket_price = 10.0 + (age < 18 ? 0.0 : 5.0) # Children under 18 get a $5 discount from base $15
println("Ticket price: $$ticket_price")
# Output: Ticket price: $10.0

Readability and Appropriate Use

While the ternary operator can make your code more concise, it's important to use it judiciously. Its main advantage is for simple, clear-cut conditional assignments. If the condition or the expressions become complex, a standard if-else statement is often more readable.

Avoid nesting ternary operators, as this can quickly lead to code that is difficult to understand:

# Less readable example (avoid this style)
# result = x > 0 ? (y > 0 ? "Both positive" : "x positive, y not") : "x not positive"

In such cases, an if-else structure is much clearer:

local result # Using local for illustration within a block
x = 1
y = -1
if x > 0
    if y > 0
        result = "Both positive"
    else
        result = "x positive, y not"
    end
else
    result = "x not positive"
end
println(result)
# Output: x positive, y not

Ternary Operator and Julia's if-else Expressions

It's worth noting that in Julia, if-else blocks are also expressions, meaning they evaluate to the value of the executed branch. For example:

x = 10
message = if x > 5
              "Greater than 5"
          else
              "5 or less"
          end
println(message) # Output: Greater than 5

The ternary operator condition ? true_expr : false_expr is essentially a more compact syntax for the specific if-else expression: if condition true_expr else false_expr end. For simple two-way branches, the ternary operator can be more succinct and often preferred for its brevity.

In summary, the ternary operator is a valuable addition to your Julia toolkit for writing short and expressive conditional assignments. Use it when it enhances clarity and conciseness, but revert to standard if-else statements for more complex logic to ensure your code remains easy to read and maintain.