Booleans and Comparisons

Comparisons produce true or false (type Bool):

x = 7
println(x == 7, " ", x != 7, " ", x < 10, " ", x >= 8)
println(1 < x < 10)
println(0 ≤ x ≤ 5)
println(!(x > 5))
println(x > 5 && x < 10, " ", x < 5 || x == 7)
println("apple" < "banana", " ", "Zoo" < "apple")
println(1 == 1.0, " ", 1 === 1.0)

true false true false
true
false
false
true true
true true
true false

• != or ≠ for "not equal"; ≤ and ≥ also work (type \le and \ge + Tab).
• Comparisons chain like in math: 1 < x < 10.
• && (and), || (or), ! (not).
• Strings compare alphabetically by character code, so uppercase letters come before lowercase.
• == compares values; === checks that two things are identical (same type and same bits, or the same object).

Short-circuit evaluation

&& and || stop as soon as the answer is known — which is often used as a compact "if":

function check_age(age)
    age < 0 && return "invalid"
    age >= 18 || return "minor"
    return "adult"
end

println(check_age(-3), " ", check_age(12), " ", check_age(30))

invalid minor adult

Read cond && action as "if cond, then action" and cond || action as "unless cond, action".

Only Bool is a condition

Unlike Python or JavaScript, Julia doesn't treat 0, "", or an empty list as false — conditions must be real Bool values:

items = Int[]
try
    if length(items)
        println("has items")
    end
catch err
    println(typeof(err))
end
println(isempty(items))

TypeError
true

Write the test you mean: isempty(items), length(items) > 0, x != 0. (try/catch catches the error here so the program can keep going; Level 2 covers it.)