Control Flow
Deciding whether or not to run some code depending on if a condition is true or
deciding 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 us to branch our code depending on conditions. We
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, which is followed by a
condition. In this case, the condition checks whether or not the variable
number has a value less than 5. The block of code we want to execute if the
condition is true is placed immediately after the condition inside curly
braces. 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)
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:
let number = 7;
Run the program again, and look at the output:
$ cargo run
Compiling branches v0.1.0 (file:///projects/branches)
Running `target/debug/branches`
condition was false
It’s also worth noting that the condition in this code must be a bool. To
see what happens if the condition isn’t a bool, 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:
error[E0308]: mismatched types
--> src/main.rs:4:8
|
4 | if number {
| ^^^^^^ expected bool, found integral variable
|
= note: expected type `bool`
found type `{integer}`
The error indicates that Rust expected a bool but got an integer. Rust will
not automatically try to convert non-boolean types to a boolean, unlike
languages such as Ruby and JavaScript. 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.
Multiple Conditions with else if
We can have 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)
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 holds 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. The
reason is that Rust will only execute the block for the first true condition,
and once it finds one, it won’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, for instance in Listing 3-4:
Filename: src/main.rs
fn main() {
let condition = true;
let number = if condition {
5
} else {
6
};
println!("The value of number is: {}", number);
}
Listing 3-4: 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)
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 3-4, the results of both the if arm and the else
arm were i32 integers. But what happens if the types are mismatched, as in
the following example?
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 run 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:
error[E0308]: if and else have incompatible types
--> src/main.rs:4:18
|
4 | let number = if condition {
| __________________^
5 | | 5
6 | | } else {
7 | | "six"
8 | | };
| |_____^ expected integral variable, found reference
|
= note: expected type `{integer}`
found type `&'static str`
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. Rust needs to know at compile time what type the number
variable is, definitively, so it can verify at compile time that its 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. A loop runs through the code inside the loop body to the end and then starts 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 a keyboard shortcut,
ctrl-C, to halt a program that is stuck in a
continual loop. Give it a try:
$ cargo run
Compiling loops v0.1.0 (file:///projects/loops)
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 halt signal.
Fortunately, Rust provides another, more reliable way to break out of a loop.
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.
Conditional Loops with while
It’s often useful for a program to evaluate a condition within a loop. While
the condition is true, the loop runs. When the condition ceases to be true, you
call break, stopping the loop. This loop type could be implemented 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, and it’s called a while loop. The following example uses while: the
program loops three times, counting down each time. Then, after the loop, it
prints another message and exits:
Filename: src/main.rs
fn main() {
let mut number = 3;
while number != 0 {
println!("{}!", number);
number = number - 1;
}
println!("LIFTOFF!!!");
}
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 holds
true, the code runs; otherwise, it exits the loop.
Looping Through a Collection with for
You could use the while construct to loop over the elements of a collection,
such as an array. For example:
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 = index + 1;
}
}
Listing 3-5: 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 out every
element in the array:
$ cargo run
Compiling loops v0.1.0 (file:///projects/loops)
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.
But this approach is error prone; we could cause the program to panic if the index length is incorrect. It’s also slow, because the compiler adds runtime code to perform the conditional check on every element on every iteration through the loop.
As a more efficient alternative, you can use a for loop and execute some code
for each item in a collection. A for loop looks like this:
Filename: src/main.rs
fn main() {
let a = [10, 20, 30, 40, 50];
for element in a.iter() {
println!("the value is: {}", element);
}
}
Listing 3-6: 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 3-5. 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.
For example, in the code in Listing 3-5, if you removed an item from the a
array but forgot to update the condition to while index < 4, the code would
panic. Using the for loop, you don’t need to remember to change any other
code if you changed the number of values in the array.
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 3-5, most Rustaceans would use a for loop. The way to do that
would be to use a Range, which is a type provided by the standard library
that 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! That was a sizable chapter: you learned about variables, scalar
and if expressions, and loops! If you want 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.