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2021-02-24 06:13:19 +00:00
< h2 id = "the-dbg-macro-and-inspect" > < a class = "header" href = "#the-dbg-macro-and-inspect" > The dbg! macro and .inspect< / a > < / h2 >
2020-12-25 14:16:02 +00:00
< p > < code > dbg!< / code > is a very useful macro that prints quick information. It is a good alternative to < code > println!< / code > because it is faster to type and gives more information:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number = 8;
dbg!(my_number);
}
< / code > < / pre > < / pre >
< p > This prints < code > [src\main.rs:4] my_number = 8< / code > .< / p >
< p > But actually, you can put < code > dbg!< / code > in many other places, and even wrap code in it. Look at this code for example:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let mut my_number = 9;
my_number += 10;
let new_vec = vec![8, 9, 10];
let double_vec = new_vec.iter().map(|x| x * 2).collect::< Vec< i32> > ();
}
< / code > < / pre > < / pre >
< p > This code creates a new mutable number and changes it. Then it creates a vec, and uses < code > iter< / code > and < code > map< / code > and < code > collect< / code > to create a new vec. We can put < code > dbg!< / code > almost everywhere in this code. < code > dbg!< / code > asks the compiler: " What are you doing at this moment?" and tells you.< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let mut my_number = dbg!(9);
dbg!(my_number += 10);
let new_vec = dbg!(vec![8, 9, 10]);
let double_vec = dbg!(new_vec.iter().map(|x| x * 2).collect::< Vec< i32> > ());
dbg!(double_vec);
}
< / code > < / pre > < / pre >
< p > So this prints:< / p >
< pre > < code class = "language-text" > [src\main.rs:3] 9 = 9
< / code > < / pre >
< p > and:< / p >
< pre > < code class = "language-text" > [src\main.rs:4] my_number += 10 = ()
< / code > < / pre >
< p > and:< / p >
< pre > < code class = "language-text" > [src\main.rs:6] vec![8, 9, 10] = [
8,
9,
10,
]
< / code > < / pre >
< p > and this one, which even shows you the value of the expression:< / p >
< pre > < code class = "language-text" > [src\main.rs:8] new_vec.iter().map(|x| x * 2).collect::< Vec< i32> > () = [
16,
18,
20,
]
< / code > < / pre >
< p > and:< / p >
< pre > < code class = "language-text" > [src\main.rs:10] double_vec = [
16,
18,
20,
]
< / code > < / pre >
< p > < code > .inspect< / code > is a bit similar to < code > dbg!< / code > but you use it like < code > map< / code > in an iterator. It gives you the iterator item and you can print it or do whatever you want. For example, let's look at our < code > double_vec< / code > again.< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let new_vec = vec![8, 9, 10];
let double_vec = new_vec
.iter()
.map(|x| x * 2)
.collect::< Vec< i32> > ();
}
< / code > < / pre > < / pre >
< p > We want to know more information about what the code is doing. So we add < code > inspect()< / code > in two places:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let new_vec = vec![8, 9, 10];
let double_vec = new_vec
.iter()
.inspect(|first_item| println!(" The item is: {}" , first_item))
.map(|x| x * 2)
.inspect(|next_item| println!(" Then it is: {}" , next_item))
.collect::< Vec< i32> > ();
}
< / code > < / pre > < / pre >
< p > This prints:< / p >
< pre > < code class = "language-text" > The item is: 8
Then it is: 16
The item is: 9
Then it is: 18
The item is: 10
Then it is: 20
< / code > < / pre >
< p > And because < code > .inspect< / code > takes a closure, we can write as much as we want:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let new_vec = vec![8, 9, 10];
let double_vec = new_vec
.iter()
.inspect(|first_item| {
println!(" The item is: {}" , first_item);
match **first_item % 2 { // first item is a & & i32 so we use **
0 => println!(" It is even." ),
_ => println!(" It is odd." ),
}
println!(" In binary it is {:b}." , first_item);
})
.map(|x| x * 2)
.collect::< Vec< i32> > ();
}
< / code > < / pre > < / pre >
< p > This prints:< / p >
< pre > < code class = "language-text" > The item is: 8
It is even.
In binary it is 1000.
The item is: 9
It is odd.
In binary it is 1001.
The item is: 10
It is even.
In binary it is 1010.
< / code > < / pre >
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