Rust Control Flow: if, match and Loops
Control flow decides which code runs and how many times.
Operators First
Rust uses the usual operators: + - * / % for maths, == != > < >= <= for comparison, and these for logic:
&&(and): both sides true||(or): at least one side true!(not): reverses true and false
Dividing two integers drops the decimal: 7 / 2 is 3.
if and else
The condition must be a bool. Rust will not treat a number as true or false.
Example
fn main() {
let marks = 58;
if marks >= 80 {
println!("Grade A");
} else if marks >= 50 {
println!("Pass");
} else {
println!("Fail");
}
}Pass
if as an Expression
if gives back a value, so you can use it on the right side of let. Both branches must have the same type.
Example
fn main() {
let age = 16;
let ticket = if age < 18 { 200 } else { 500 };
println!("Ticket price: Rs {}", ticket);
}Ticket price: Rs 200
match
match compares a value against patterns. It must cover every possible case, so the compiler warns you if you forget one. _ matches anything.
Example
fn main() {
let day = 5;
let name = match day {
1 => "Monday",
2 => "Tuesday",
3 => "Wednesday",
4 => "Thursday",
5 => "Friday",
6 | 7 => "Weekend",
_ => "Invalid",
};
println!("{}", name);
let marks = 73;
match marks {
90..=100 => println!("Excellent"),
70..=89 => println!("Good"),
0..=69 => println!("Keep trying"),
_ => println!("Invalid marks"),
}
}Friday Good
In a match arm, 6 | 7 means "6 or 7", and 70..=89 means every number from 70 to 89.
loop
loop repeats forever until you break. A break can also return a value:
Example
fn main() {
let mut savings = 0;
let mut months = 0;
let result = loop {
savings += 4000;
months += 1;
if savings >= 15000 {
break months;
}
};
println!("Goal reached in {} months", result);
}Goal reached in 4 months
while
while runs as long as the condition is true:
Example
fn main() {
let mut count = 3;
while count > 0 {
println!("{}...", count);
count -= 1;
}
println!("Go!");
}3... 2... 1... Go!
for and Ranges
for goes through a range or a collection. It is the loop you will use most.
Example
fn main() {
for i in 1..4 {
print!("{} ", i);
}
println!();
for i in (1..=5).rev() {
print!("{} ", i);
}
println!();
let fruits = ["Mango", "Apple", "Banana"];
for fruit in fruits {
println!("I like {}", fruit);
}
for (i, fruit) in fruits.iter().enumerate() {
if i == 1 {
continue;
}
println!("{}: {}", i, fruit);
}
}1 2 3 5 4 3 2 1 I like Mango I like Apple I like Banana 0: Mango 2: Banana
| Range | Values |
|---|---|
1..4 | 1, 2, 3 (end not included) |
1..=4 | 1, 2, 3, 4 (end included) |
(1..=4).rev() | 4, 3, 2, 1 |
Exercise
Exercise
What does the range 0..3 produce in a for loop?