if/else Expressions
if and else let your program pick a different path depending on whether something is true or false.if condition {
// body
} else if condition2 {
// body
} else {
// body
}
let name = if condition { value1 } else { value2 };Basic if/else
An if expression evaluates a boolean condition and runs the associated block only when it is true; an optional else block runs otherwise. No parentheses are needed around the condition, unlike C or Java.
Example: Basic if/else
// Entry point: execution of the program starts here
fn main() {
// Declare an immutable variable `number`, set to `7`
let number = 7;
// Check whether `number > 5`
if number > 5 {
// Print "Greater than five" to the console (with a trailing newline)
println!("Greater than five");
// Otherwise, run this branch
} else {
// Print "Five or less" to the console (with a trailing newline)
println!("Five or less");
}
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else if Chains
Multiple conditions can be chained with else if, checked in order from top to bottom. The first matching branch runs, and the rest are skipped.
Example: else if Chains
// Entry point: execution of the program starts here
fn main() {
// Declare an immutable variable `grade`, set to `85`
let grade = 85;
// Check whether `grade >= 90`
if grade >= 90 {
// Print "A" to the console (with a trailing newline)
println!("A");
// Otherwise, if `grade >= 80`, run this branch
} else if grade >= 80 {
// Print "B" to the console (with a trailing newline)
println!("B");
// Otherwise, run this branch
} else {
// Print "C or below" to the console (with a trailing newline)
println!("C or below");
}
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if as an Expression
Because if is an expression, not just a statement, it can produce a value directly, which you can assign to a variable. Both branches must return values of the same type.
5 and 6 -- they are the tail expressions of each block.Example: if as an Expression
// Entry point: execution of the program starts here
fn main() {
// Declare an immutable variable `condition`, set to `true`
let condition = true;
// Declare an immutable variable `number`, set to `if condition { 5 } else { 6 }`
let number = if condition { 5 } else { 6 };
// Print "number is {}" to the console, substituting in `number` (with a trailing newline)
println!("number is {}", number);
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Condition Must Be a Bool
Rust requires the if condition to be an actual bool value; it will not implicitly treat a nonzero integer as true, unlike C. This prevents an entire category of bugs from mistyped conditions.
Example: Condition Must Be a Bool
// Entry point: execution of the program starts here
fn main() {
// Declare an immutable variable `count`, set to `3`
let count = 3;
// Check whether `count != 0`
if count != 0 {
// Print "count is non-zero: {}" to the console, substituting in `count` (with a trailing newline)
println!("count is non-zero: {}", count);
}
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- Wrapping the condition in parentheses like
if (x > 5), which is unnecessary and unidiomatic in Rust. - Forgetting that
ifis an expression and can return a value, then writing overly verbose code with extralet mutvariables. - Mixing types between the
ifandelsebranches when usingifas an expression, which fails to compile.
if/else if/elsebranch on boolean conditions; no parentheses are required around the condition.ifis an expression in Rust, so it can produce a value that is assigned directly to a variable.- All branches of an
ifused as an expression must evaluate to the same type. - The condition must be a
bool-- Rust does not implicitly convert numbers to booleans.
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