2020-12-25 14:16:02 +00:00
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< h2 > < a class = "header" href = "#printing-hello-world" id = "printing-hello-world" > Printing 'hello, world!'< / a > < / h2 >
2021-02-05 07:42:46 +00:00
< p > < strong > See this chapter on YouTube: < a href = "https://youtu.be/yYlPHRl2geQ" > Video 1< / a > , < a href = "https://youtu.be/DTCSfBJJZb8" > Video 2< / a > < / strong > < / p >
2020-12-25 14:16:02 +00:00
< p > When you start a new Rust program, it always has this code:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
println!(" Hello, world!" );
}
< / code > < / pre > < / pre >
< ul >
< li > < code > fn< / code > means function,< / li >
< li > < code > main< / code > is the function that starts the program,< / li >
< li > < code > ()< / code > means that we didn't give the function any variables to start.< / li >
< / ul >
< p > < code > {}< / code > is called a < strong > code block< / strong > . This is the space where code lives.< / p >
< p > < code > println!< / code > is a < strong > macro< / strong > that prints to the console. A < strong > macro< / strong > is like a function that writes code for you. Macros have a < code > !< / code > after them. We will learn about making macros later. For now, remember that < code > !< / code > means that it is a macro.< / p >
< p > To learn about the < code > ;< / code > , we will create another function. First, in < code > main< / code > we will print a number 8:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
println!(" Hello, world number {}!" , 8);
}
< / code > < / pre > < / pre >
< p > The < code > {}< / code > in < code > println!< / code > means " put the variable inside here" . This prints < code > Hello, world number 8!< / code > .< / p >
< p > We can put more in, just like we did before:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
println!(" Hello, worlds number {} and {}!" , 8, 9);
}
< / code > < / pre > < / pre >
< p > This prints < code > Hello, worlds number 8 and 9!< / code > .< / p >
< p > Now let's create the function.< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn number() -> i32 {
8
}
fn main() {
println!(" Hello, world number {}!" , number());
}
< / code > < / pre > < / pre >
< p > This also prints < code > Hello, world number 8!< / code > . When Rust looks at < code > number()< / code > it sees a function. This function:< / p >
< ul >
< li > Does not take anything (because it has < code > ()< / code > )< / li >
< li > Returns an < code > i32< / code > . The < code > -> < / code > (called a " skinny arrow" ) shows what the function returns.< / li >
< / ul >
< p > Inside the function is just < code > 8< / code > . Because there is no < code > ;< / code > , this is the value it returns. If it had a < code > ;< / code > , it would not return anything (it would return a < code > ()< / code > ). Rust will not compile this if it has a < code > ;< / code > , because the return is < code > i32< / code > and < code > ;< / code > returns < code > ()< / code > , not < code > i32< / code > :< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
println!(" Hello, world number {}" , number());
}
fn number() -> i32 {
8; // ⚠️
}
< / code > < / pre > < / pre >
< pre > < code class = "language-text" > 5 | fn number() -> i32 {
| ------ ^^^ expected `i32`, found `()`
| |
| implicitly returns `()` as its body has no tail or `return` expression
6 | 8;
| - help: consider removing this semicolon
< / code > < / pre >
< p > This means " you told me that < code > number()< / code > returns an < code > i32< / code > , but you added a < code > ;< / code > so it doesn't return anything" . So the compiler suggests removing the semicolon.< / p >
< p > You can also write < code > return 8;< / code > but in Rust it is normal to just remove the < code > ;< / code > to < code > return< / code > .< / p >
< p > When you want to give variables to a function, put them inside the < code > ()< / code > . You have to give them a name and write the type.< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn multiply(number_one: i32, number_two: i32) { // Two i32s will enter the function. We will call them number_one and number_two.
let result = number_one * number_two;
println!(" {} times {} is {}" , number_one, number_two, result);
}
fn main() {
multiply(8, 9); // We can give the numbers directly
let some_number = 10; // Or we can declare two variables
let some_other_number = 2;
multiply(some_number, some_other_number); // and put them in the function
}
< / code > < / pre > < / pre >
< p > We can also return an < code > i32< / code > . Just take out the semicolon at the end:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn multiply(number_one: i32, number_two: i32) -> i32 {
let result = number_one * number_two;
println!(" {} times {} is {}" , number_one, number_two, result);
result // this is the i32 that we return
}
fn main() {
let multiply_result = multiply(8, 9); // We used multiply() to print and to give the result to multiply_result
}
< / code > < / pre > < / pre >
< h3 > < a class = "header" href = "#declaring-variables-and-code-blocks" id = "declaring-variables-and-code-blocks" > Declaring variables and code blocks< / a > < / h3 >
< p > Use < code > let< / code > to declare a variable (declare a variable = tell Rust to make a variable).< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number = 8;
println!(" Hello, number {}" , my_number);
}
< / code > < / pre > < / pre >
< p > Variables start and end inside a code block < code > {}< / code > . In this example, < code > my_number< / code > ends before we call < code > println!< / code > , because it is inside its own code block.< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
{
let my_number = 8; // my_number starts here
// my_number ends here!
}
println!(" Hello, number {}" , my_number); // ⚠️ there is no my_number and
// println!() can't find it
}
< / code > < / pre > < / pre >
< p > You can use a code block to return a value:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number = {
let second_number = 8;
second_number + 9 // No semicolon, so the code block returns 8 + 9.
// It works just like a function
};
println!(" My number is: {}" , my_number);
}
< / code > < / pre > < / pre >
< p > If you add a semicolon inside the block, it will return < code > ()< / code > (nothing):< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number = {
let second_number = 8; // declare second_number,
second_number + 9; // add 9 to second_number
// but we didn't return it!
// second_number dies now
};
println!(" My number is: {:?}" , my_number); // my_number is ()
}
< / code > < / pre > < / pre >
< p > So why did we write < code > {:?}< / code > and not < code > {}< / code > ? We will talk about that now.< / p >
< / main >
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