if-else-if-else

if-else-if-else is the most fundamental decision-making tool in programming. Its logic is very intuitive — if the condition is true, execute some code; otherwise, check the next condition; if none are true, go the default path.

Basic Syntax

In the syntax specification, the definitions of if expression and if statement are identical:

if Expr Block ('else' 'if' Expr Block)* ('else' Block)?

Translated into everyday language: starts with if, followed by a conditional expression and a code block, then zero to many else if condition code block, and finally an optional else code block.

The simplest form — only if:

if temperature > 30 {
    print("It's hot, turn on the AC")
}

With else:

if is_raining {
    print("Bring an umbrella")
} else {
    print("No need for an umbrella")
}

Multiple conditions with else if:

score = 85

if score >= 90 {
    print("Excellent")
} else if score >= 80 {
    print("Good")
} else if score >= 60 {
    print("Pass")
} else {
    print("Needs improvement")
}

if is an Expression

This is one of the most important features of YaoXiang's control flow: if can be used as an expression to compute a value.

// if expression: the value from each branch is assigned to result
result = if x > 0 {
    "positive"
} else if x < 0 {
    "negative"
} else {
    "zero"
}
// result is now one of "positive", "negative", or "zero"

When if is used as an expression, the return value types from all branches must be consistent:

score = 88

// All branches return String, types are consistent, no problem
grade = if score >= 90 {
    "A"
} else if score >= 80 {
    "B"
} else if score >= 60 {
    "C"
} else {
    "D"
}
print(grade)  // "B"

In the code block of each branch, the value of the last expression is the return value of that branch. You can also use return to explicitly return, but in branches it's usually sufficient to just write the expression.

// Writing the expression directly — recommended
category = if age < 18 { "minor" } else { "adult" }

// Can also explicitly return — same effect
category = if age < 18 {
    return "minor"
} else {
    return "adult"
}

If you only use if for conditional logic without needing a value, it's just a regular statement — fully compatible with the expression form.

Nested if

You can write if inside if to handle multi-level conditional logic:

age = 25
has_ticket = true

if age >= 18 {
    if has_ticket {
        print("Welcome")
    } else {
        print("Please purchase a ticket first")
    }
} else {
    print("Minors must be accompanied by a guardian")
}

When dealing with nested expressions, YaoXiang doesn't have the "dangling else" ambiguity found in C — each else always belongs to the nearest unmatched if.

Combining Conditions with Boolean Operators

You can use and, or, and not to combine multiple conditions:

username = "admin"
password = "123456"

// and: both conditions must be true
if username == "admin" and password == "123456" {
    print("Login successful")
}

// or: either condition can be true
if role == "admin" or role == "moderator" {
    print("Has admin privileges")
}

// not: negation
if not is_banned {
    print("Allowed to post")
}

// Combined usage
if (age >= 18 and age <= 60) or is_vip {
    print("Eligible to participate")
}

In terms of operator precedence, not is higher than and, and and is higher than or. When in doubt, add parentheses to make your intent clearer.

Summary

Key Point	Description
Basic Structure	if condition { ... } else if condition { ... } else { ... }
else if	YaoXiang uses else if for multi-way branching
Expression	if can return a value, all branches must have consistent types
Branch Return	The last expression in a branch block is the return value
Nesting	You can write if inside if, no dangling else ambiguity
Boolean Ops	and, or, not to combine conditions

In the next chapter you will learn about for loops — the standard way to iterate over collections and ranges.