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< h2 id = "control-flow" > < a class = "header" href = "#control-flow" > Control flow< / a > < / h2 >
< p > < strong > See this chapter on YouTube: < a href = "https://youtu.be/UAymDOpv_us" > Part 1< / a > and < a href = "https://youtu.be/eqysTfiiQZs" > Part 2< / a > < / strong > < / p >
< p > Control flow means telling your code what to do in different situations. The simplest control flow is < code > if< / code > .< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number = 5;
if my_number == 7 {
println!(" It's seven" );
}
}
< / code > < / pre > < / pre >
< p > Also note that you use < code > ==< / code > and not < code > =< / code > . < code > ==< / code > is to compare, < code > =< / code > is to < em > assign< / em > (to give a value). Also note that we wrote < code > if my_number == 7< / code > and not < code > if (my_number == 7)< / code > . You don't need brackets with < code > if< / code > in Rust.< / p >
< p > < code > else if< / code > and < code > else< / code > give you more control:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number = 5;
if my_number == 7 {
println!(" It's seven" );
} else if my_number == 6 {
println!(" It's six" )
} else {
println!(" It's a different number" )
}
}
< / code > < / pre > < / pre >
< p > This prints < code > It's a different number< / code > because it's not equal to 7 or 6.< / p >
< p > You can add more conditions with < code > & & < / code > (and) and < code > ||< / code > (or).< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number = 5;
if my_number % 2 == 1 & & my_number > 0 { // % 2 means the number that remains after diving by two
println!(" It's a positive odd number" );
} else if my_number == 6 {
println!(" It's six" )
} else {
println!(" It's a different number" )
}
}
< / code > < / pre > < / pre >
< p > This prints < code > It's a positive odd number< / code > because when you divide it by 2 you have a remainder of 1, and it's greater than 0.< / p >
< p > You can see that too much < code > if< / code > , < code > else< / code > , and < code > else if< / code > can be difficult to read. In this case you can use < code > match< / code > instead, which looks much cleaner. But you must match for every possible result. For example, this will not work:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number: u8 = 5;
match my_number {
0 => println!(" it's zero" ),
1 => println!(" it's one" ),
2 => println!(" it's two" ),
// ⚠️
}
}
< / code > < / pre > < / pre >
< p > The compiler says:< / p >
< pre > < code class = "language-text" > error[E0004]: non-exhaustive patterns: `3u8..=std::u8::MAX` not covered
--> src\main.rs:3:11
|
3 | match my_number {
| ^^^^^^^^^ pattern `3u8..=std::u8::MAX` not covered
< / code > < / pre >
< p > This means " you told me about 0 to 2, but < code > u8< / code > s can go up to 255. What about 3? What about 4? What about 5?" And so on. So you can add < code > _< / code > which means " anything else" .< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number: u8 = 5;
match my_number {
0 => println!(" it's zero" ),
1 => println!(" it's one" ),
2 => println!(" it's two" ),
_ => println!(" It's some other number" ),
}
}
< / code > < / pre > < / pre >
< p > That prints < code > It's some other number< / code > .< / p >
< p > Remember this for match:< / p >
< ul >
< li > You write < code > match< / code > and then make a < code > {}< / code > code block.< / li >
< li > Write the < em > pattern< / em > on the left and use a < code > => < / code > fat arrow to say what to do when it matches.< / li >
< li > Each line is called an " arm" .< / li >
< li > Put a comma between the arms (not a semicolon).< / li >
< / ul >
< p > You can declare a value with a match:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number = 5;
let second_number = match my_number {
0 => 0,
5 => 10,
_ => 2,
};
}
< / code > < / pre > < / pre >
< p > < code > second_number< / code > will be 10. Do you see the semicolon at the end? That is because, after the match is over, we actually told the compiler this: < code > let second_number = 10;< / code > < / p >
< p > You can match on more complicated things too. You use a tuple to do it.< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let sky = " cloudy" ;
let temperature = " warm" ;
match (sky, temperature) {
(" cloudy" , " cold" ) => println!(" It's dark and unpleasant today" ),
(" clear" , " warm" ) => println!(" It's a nice day" ),
(" cloudy" , " warm" ) => println!(" It's dark but not bad" ),
_ => println!(" Not sure what the weather is." ),
}
}
< / code > < / pre > < / pre >
< p > This prints < code > It's dark but not bad< / code > because it matches " cloudy" and " warm" for < code > sky< / code > and < code > temperature< / code > .< / p >
< p > You can even put < code > if< / code > inside of < code > match< / code > . This is called a " match guard" :< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let children = 5;
let married = true;
match (children, married) {
(children, married) if married == false => println!(" Not married with {} children" , children),
(children, married) if children == 0 & & married == true => println!(" Married but no children" ),
_ => println!(" Married? {}. Number of children: {}." , married, children),
}
}
< / code > < / pre > < / pre >
< p > This will print < code > Married? true. Number of children: 5.< / code > < / p >
< p > You can use _ as many times as you want in a match. In this match on colours, we have three but only check one at a time.< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn match_colours(rbg: (i32, i32, i32)) {
match rbg {
(r, _, _) if r < 10 => println!(" Not much red" ),
(_, b, _) if b < 10 => println!(" Not much blue" ),
(_, _, g) if g < 10 => println!(" Not much green" ),
_ => println!(" Each colour has at least 10" ),
}
}
fn main() {
let first = (200, 0, 0);
let second = (50, 50, 50);
let third = (200, 50, 0);
match_colours(first);
match_colours(second);
match_colours(third);
}
< / code > < / pre > < / pre >
< p > This prints:< / p >
< pre > < code class = "language-text" > Not much blue
Each colour has at least 10
Not much green
< / code > < / pre >
< p > This also shows how < code > match< / code > statements work, because in the first example it only printed < code > Not much blue< / code > . But < code > first< / code > also has not much green. A < code > match< / code > statement always stops when it finds a match, and doesn't check the rest. This is a good example of code that compiles well but is not the code you want.< / p >
< p > You can make a really big < code > match< / code > statement to fix it, but it is probably better to use a < code > for< / code > loop. We will talk about loops soon.< / p >
< p > A match has to return the same type. So you can't do this:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number = 10;
let some_variable = match my_number {
10 => 8,
_ => " Not ten" , // ⚠️
};
}
< / code > < / pre > < / pre >
< p > The compiler tells you that:< / p >
< pre > < code class = "language-text" > error[E0308]: `match` arms have incompatible types
--> src\main.rs:17:14
|
15 | let some_variable = match my_number {
| _________________________-
16 | | 10 => 8,
| | - this is found to be of type `{integer}`
17 | | _ => " Not ten" ,
| | ^^^^^^^^^ expected integer, found `& str`
18 | | };
| |_____- `match` arms have incompatible types
< / code > < / pre >
< p > This will also not work, for the same reason:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let some_variable = if my_number == 10 { 8 } else { " something else " }; // ⚠️
}
< / code > < / pre > < / pre >
< p > But this works, because it's not a < code > match< / code > so you have a different < code > let< / code > statement each time:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number = 10;
if my_number == 10 {
let some_variable = 8;
} else {
let some_variable = " Something else" ;
}
}
< / code > < / pre > < / pre >
< p > You can also use < code > @< / code > to give a name to the value of a < code > match< / code > expression, and then you can use it. In this example we match an < code > i32< / code > input in a function. If it's 4 or 13 we want to use that number in a < code > println!< / code > statement. Otherwise, we don't need to use it.< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn match_number(input: i32) {
match input {
number @ 4 => println!(" {} is an unlucky number in China (sounds close to 死)!" , number),
number @ 13 => println!(" {} is unlucky in North America, lucky in Italy! In bocca al lupo!" , number),
_ => println!(" Looks like a normal number" ),
}
}
fn main() {
match_number(50);
match_number(13);
match_number(4);
}
< / code > < / pre > < / pre >
< p > This prints:< / p >
< pre > < code class = "language-text" > Looks like a normal number
13 is unlucky in North America, lucky in Italy! In bocca al lupo!
4 is an unlucky number in China (sounds close to 死)!
< / code > < / pre >
< / main >
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