Control Flow
The ability to run some code depending on whether a condition is true and to
run some code repeatedly while a condition is true are basic building blocks
in most programming languages. The most common constructs that let you control
the flow of execution of Rust code are if expressions and loops.
if Expressions
An if expression allows you to branch your code depending on conditions. You
provide a condition and then state, “If this condition is met, run this block
of code. If the condition is not met, do not run this block of code.”
Create a new project called branches in your projects directory to explore
the if expression. In the src/main.rs file, input the following:
Filename: src/main.rs
fn main() { let number = 3; if number < 5 { println!("condition was true"); } else { println!("condition was false"); } }
All if expressions start with the keyword if, followed by a condition. In
this case, the condition checks whether or not the variable number has a
value less than 5. We place the block of code to execute if the condition is
true immediately after the condition inside curly brackets. Blocks of code
associated with the conditions in if expressions are sometimes called arms,
just like the arms in match expressions that we discussed in the “Comparing
the Guess to the Secret Number” section of Chapter 2.
Optionally, we can also include an else expression, which we chose to do
here, to give the program an alternative block of code to execute should the
condition evaluate to false. If you don’t provide an else expression and
the condition is false, the program will just skip the if block and move on
to the next bit of code.
Try running this code; you should see the following output:
$ cargo run
Compiling branches v0.1.0 (file:///projects/branches)
Finished dev [unoptimized + debuginfo] target(s) in 0.31s
Running `target/debug/branches`
condition was true
Let’s try changing the value of number to a value that makes the condition
false to see what happens:
fn main() {
let number = 7;
if number < 5 {
println!("condition was true");
} else {
println!("condition was false");
}
}Run the program again, and look at the output:
$ cargo run
Compiling branches v0.1.0 (file:///projects/branches)
Finished dev [unoptimized + debuginfo] target(s) in 0.31s
Running `target/debug/branches`
condition was false
It’s also worth noting that the condition in this code must be a bool. If
the condition isn’t a bool, we’ll get an error. For example, try running the
following code:
Filename: src/main.rs
fn main() {
let number = 3;
if number {
println!("number was three");
}
}The if condition evaluates to a value of 3 this time, and Rust throws an
error:
$ cargo run
Compiling branches v0.1.0 (file:///projects/branches)
error[E0308]: mismatched types
--> src/main.rs:4:8
|
4 | if number {
| ^^^^^^ expected `bool`, found integer
For more information about this error, try `rustc --explain E0308`.
error: could not compile `branches` due to previous error
The error indicates that Rust expected a bool but got an integer. Unlike
languages such as Ruby and JavaScript, Rust will not automatically try to
convert non-Boolean types to a Boolean. You must be explicit and always provide
if with a Boolean as its condition. If we want the if code block to run
only when a number is not equal to 0, for example, we can change the if
expression to the following:
Filename: src/main.rs
fn main() { let number = 3; if number != 0 { println!("number was something other than zero"); } }
Running this code will print number was something other than zero.
Handling Multiple Conditions with else if
You can use multiple conditions by combining if and else in an else if
expression. For example:
Filename: src/main.rs
fn main() { let number = 6; if number % 4 == 0 { println!("number is divisible by 4"); } else if number % 3 == 0 { println!("number is divisible by 3"); } else if number % 2 == 0 { println!("number is divisible by 2"); } else { println!("number is not divisible by 4, 3, or 2"); } }
This program has four possible paths it can take. After running it, you should see the following output:
$ cargo run
Compiling branches v0.1.0 (file:///projects/branches)
Finished dev [unoptimized + debuginfo] target(s) in 0.31s
Running `target/debug/branches`
number is divisible by 3
When this program executes, it checks each if expression in turn and executes
the first body for which the condition evaluates to true. Note that even
though 6 is divisible by 2, we don’t see the output number is divisible by 2,
nor do we see the number is not divisible by 4, 3, or 2 text from the else
block. That’s because Rust only executes the block for the first true
condition, and once it finds one, it doesn’t even check the rest.
Using too many else if expressions can clutter your code, so if you have more
than one, you might want to refactor your code. Chapter 6 describes a powerful
Rust branching construct called match for these cases.
Using if in a let Statement
Because if is an expression, we can use it on the right side of a let
statement to assign the outcome to a variable, as in Listing 3-2.
Filename: src/main.rs
fn main() { let condition = true; let number = if condition { 5 } else { 6 }; println!("The value of number is: {number}"); }
Listing 3-2: Assigning the result of an if expression
to a variable
The number variable will be bound to a value based on the outcome of the if
expression. Run this code to see what happens:
$ cargo run
Compiling branches v0.1.0 (file:///projects/branches)
Finished dev [unoptimized + debuginfo] target(s) in 0.30s
Running `target/debug/branches`
The value of number is: 5
Remember that blocks of code evaluate to the last expression in them, and
numbers by themselves are also expressions. In this case, the value of the
whole if expression depends on which block of code executes. This means the
values that have the potential to be results from each arm of the if must be
the same type; in Listing 3-2, the results of both the if arm and the else
arm were i32 integers. If the types are mismatched, as in the following
example, we’ll get an error:
Filename: src/main.rs
fn main() {
let condition = true;
let number = if condition { 5 } else { "six" };
println!("The value of number is: {number}");
}When we try to compile this code, we’ll get an error. The if and else arms
have value types that are incompatible, and Rust indicates exactly where to
find the problem in the program:
$ cargo run
Compiling branches v0.1.0 (file:///projects/branches)
error[E0308]: `if` and `else` have incompatible types
--> src/main.rs:4:44
|
4 | let number = if condition { 5 } else { "six" };
| - ^^^^^ expected integer, found `&str`
| |
| expected because of this
For more information about this error, try `rustc --explain E0308`.
error: could not compile `branches` due to previous error
The expression in the if block evaluates to an integer, and the expression in
the else block evaluates to a string. This won’t work because variables must
have a single type, and Rust needs to know at compile time what type the
number variable is, definitively. Knowing the type of number lets the
compiler verify the type is valid everywhere we use number. Rust wouldn’t be
able to do that if the type of number was only determined at runtime; the
compiler would be more complex and would make fewer guarantees about the code
if it had to keep track of multiple hypothetical types for any variable.
Repetition with Loops
It’s often useful to execute a block of code more than once. For this task, Rust provides several loops, which will run through the code inside the loop body to the end and then start immediately back at the beginning. To experiment with loops, let’s make a new project called loops.
Rust has three kinds of loops: loop, while, and for. Let’s try each one.
Repeating Code with loop
The loop keyword tells Rust to execute a block of code over and over again
forever or until you explicitly tell it to stop.
As an example, change the src/main.rs file in your loops directory to look like this:
Filename: src/main.rs
fn main() {
loop {
println!("again!");
}
}When we run this program, we’ll see again! printed over and over continuously
until we stop the program manually. Most terminals support the keyboard
shortcut ctrl-c to interrupt a program that is
stuck in a continual loop. Give it a try:
$ cargo run
Compiling loops v0.1.0 (file:///projects/loops)
Finished dev [unoptimized + debuginfo] target(s) in 0.29s
Running `target/debug/loops`
again!
again!
again!
again!
^Cagain!
The symbol ^C represents where you pressed ctrl-c. You may or may not see the word again!
printed after the ^C, depending on where the code was in the loop when it
received the interrupt signal.
Fortunately, Rust also provides a way to break out of a loop using code. You
can place the break keyword within the loop to tell the program when to stop
executing the loop. Recall that we did this in the guessing game in the
“Quitting After a Correct Guess” section of Chapter 2 to exit the program when the user won the game by
guessing the correct number.
We also used continue in the guessing game, which in a loop tells the program
to skip over any remaining code in this iteration of the loop and go to the
next iteration.
Returning Values from Loops
One of the uses of a loop is to retry an operation you know might fail, such
as checking whether a thread has completed its job. You might also need to pass
the result of that operation out of the loop to the rest of your code. To do
this, you can add the value you want returned after the break expression you
use to stop the loop; that value will be returned out of the loop so you can
use it, as shown here:
fn main() { let mut counter = 0; let result = loop { counter += 1; if counter == 10 { break counter * 2; } }; println!("The result is {result}"); }
Before the loop, we declare a variable named counter and initialize it to
0. Then we declare a variable named result to hold the value returned from
the loop. On every iteration of the loop, we add 1 to the counter variable,
and then check whether the counter is equal to 10. When it is, we use the
break keyword with the value counter * 2. After the loop, we use a
semicolon to end the statement that assigns the value to result. Finally, we
print the value in result, which in this case is 20.
Loop Labels to Disambiguate Between Multiple Loops
If you have loops within loops, break and continue apply to the innermost
loop at that point. You can optionally specify a loop label on a loop that
you can then use with break or continue to specify that those keywords
apply to the labeled loop instead of the innermost loop. Loop labels must begin
with a single quote. Here’s an example with two nested loops:
fn main() { let mut count = 0; 'counting_up: loop { println!("count = {count}"); let mut remaining = 10; loop { println!("remaining = {remaining}"); if remaining == 9 { break; } if count == 2 { break 'counting_up; } remaining -= 1; } count += 1; } println!("End count = {count}"); }
The outer loop has the label 'counting_up, and it will count up from 0 to 2.
The inner loop without a label counts down from 10 to 9. The first break that
doesn’t specify a label will exit the inner loop only. The break 'counting_up; statement will exit the outer loop. This code prints:
$ cargo run
Compiling loops v0.1.0 (file:///projects/loops)
Finished dev [unoptimized + debuginfo] target(s) in 0.58s
Running `target/debug/loops`
count = 0
remaining = 10
remaining = 9
count = 1
remaining = 10
remaining = 9
count = 2
remaining = 10
End count = 2
Conditional Loops with while
A program will often need to evaluate a condition within a loop. While the
condition is true, the loop runs. When the condition ceases to be true, the
program calls break, stopping the loop. It’s possible to implement behavior
like this using a combination of loop, if, else, and break; you could
try that now in a program, if you’d like. However, this pattern is so common
that Rust has a built-in language construct for it, called a while loop. In
Listing 3-3, we use while to loop the program three times, counting down each
time, and then, after the loop, print a message and exit.
Filename: src/main.rs
fn main() { let mut number = 3; while number != 0 { println!("{number}!"); number -= 1; } println!("LIFTOFF!!!"); }
Listing 3-3: Using a while loop to run code while a
condition holds true
This construct eliminates a lot of nesting that would be necessary if you used
loop, if, else, and break, and it’s clearer. While a condition
evaluates to true, the code runs; otherwise, it exits the loop.
Looping Through a Collection with for
You can choose to use the while construct to loop over the elements of a
collection, such as an array. For example, the loop in Listing 3-4 prints each
element in the array a.
Filename: src/main.rs
fn main() { let a = [10, 20, 30, 40, 50]; let mut index = 0; while index < 5 { println!("the value is: {}", a[index]); index += 1; } }
Listing 3-4: Looping through each element of a collection
using a while loop
Here, the code counts up through the elements in the array. It starts at index
0, and then loops until it reaches the final index in the array (that is,
when index < 5 is no longer true). Running this code will print every
element in the array:
$ cargo run
Compiling loops v0.1.0 (file:///projects/loops)
Finished dev [unoptimized + debuginfo] target(s) in 0.32s
Running `target/debug/loops`
the value is: 10
the value is: 20
the value is: 30
the value is: 40
the value is: 50
All five array values appear in the terminal, as expected. Even though index
will reach a value of 5 at some point, the loop stops executing before trying
to fetch a sixth value from the array.
However, this approach is error prone; we could cause the program to panic if
the index value or test condition is incorrect. For example, if you changed the
definition of the a array to have four elements but forgot to update the
condition to while index < 4, the code would panic. It’s also slow, because
the compiler adds runtime code to perform the conditional check of whether the
index is within the bounds of the array on every iteration through the loop.
As a more concise alternative, you can use a for loop and execute some code
for each item in a collection. A for loop looks like the code in Listing 3-5.
Filename: src/main.rs
fn main() { let a = [10, 20, 30, 40, 50]; for element in a { println!("the value is: {element}"); } }
Listing 3-5: Looping through each element of a collection
using a for loop
When we run this code, we’ll see the same output as in Listing 3-4. More importantly, we’ve now increased the safety of the code and eliminated the chance of bugs that might result from going beyond the end of the array or not going far enough and missing some items.
Using the for loop, you wouldn’t need to remember to change any other code if
you changed the number of values in the array, as you would with the method
used in Listing 3-4.
The safety and conciseness of for loops make them the most commonly used loop
construct in Rust. Even in situations in which you want to run some code a
certain number of times, as in the countdown example that used a while loop
in Listing 3-3, most Rustaceans would use a for loop. The way to do that
would be to use a Range, provided by the standard library, which generates
all numbers in sequence starting from one number and ending before another
number.
Here’s what the countdown would look like using a for loop and another method
we’ve not yet talked about, rev, to reverse the range:
Filename: src/main.rs
fn main() { for number in (1..4).rev() { println!("{number}!"); } println!("LIFTOFF!!!"); }
This code is a bit nicer, isn’t it?
Summary
You made it! This was a sizable chapter: you learned about variables, scalar
and compound data types, functions, comments, if expressions, and loops! To
practice with the concepts discussed in this chapter, try building programs to
do the following:
- Convert temperatures between Fahrenheit and Celsius.
- Generate the nth Fibonacci number.
- Print the lyrics to the Christmas carol “The Twelve Days of Christmas,” taking advantage of the repetition in the song.
When you’re ready to move on, we’ll talk about a concept in Rust that doesn’t commonly exist in other programming languages: ownership.