[Rust] Place to store values

Danh Võ — Thu, 22 Dec 2022 10:09:14 +0000

Like other programing languages, to hold data we need some things called constant and variable.

CONSTANT

We could not change value of a constant. Good choice to store value that is not changed over time.

const SECONDS_OF_ONE_MINUTE: i32 = 60;
const SECONDS_OF_ONE_HOUR: i32 = SECONDS_OF_ONE_MINUTE * 60;
const SECONDS_OF_ONE_DAY: i32 = SECONDS_OF_ONE_HOUR * 24;

VARIABLE

1) Immutable variable (default and favor by Rust):
After assigning a value to a variable by keyword let, we could not re-assign it. For example:

let var = 1;
println!("value: {var}");
var = 2;  // Will cause error at compile time.
println!("value: {var}");

At this point, it is quite similar to constant, right?
But there are 2 things different between immutable variable and constant:

We can use variable shadowing to change variable value. Just repeat the let keyword. But there is a small thing behind the scene. Rust creates another new variable with a new memory slot and with a same name. Not the original one. IMO, its functionality seems to run contrary to its name (immutable but changeable by somehow)

let var = 1;
println!("value: {var}");
let var = 2;  // Is OK
println!("value: {var}");

Mutable was born to serve this purpose.

2) Mutable variable:
Just put mut keyword after let:

let mut name = "Unknown";
name = "Danh";

Easy huh?

Reference: https://doc.rust-lang.org/stable/book/ch03-01-variables-and-mutability.html

First taste on Rust!

Danh Võ — Thu, 15 Dec 2022 14:27:20 +0000

I have just finished the Chapter 1 of The Rust Programming Language book. It is quite easy to follow. Below are several things to take away from what I have just learnt.

Rust is a compiled language

Similar to C/C++, Rust needs an entry point to run the whole program (main function) and a compiler to transform human readable text into binary file. To compile Rust source code, we use rustc:

danh@friday:~/workspace/self-study/hello-rust/src$ cat hell_world.rs
fn main() {
    println!("Hello world!");
}
danh@friday:~/workspace/self-study/hello-rust/src$ rustc hell_world.rs
danh@friday:~/workspace/self-study/hello-rust/src$ ll
total 3956
drwxrwxr-x 2 danh danh    4096 Dec 15 17:16 ./
drwxrwxr-x 5 danh danh    4096 Dec 15 17:16 ../
-rwxrwxr-x 1 danh danh 4036560 Dec 15 17:08 hell_world*
-rw-rw-r-- 1 danh danh      44 Dec 15 17:08 hell_world.rs
danh@friday:~/workspace/self-study/hello-rust/src$ ./hell_world
Hello world!

As Rust says, its brilliant compiler will handle and caution us if there is any vulnerable code sections and suggest a hint during compile time. Great!

Make the life easy by using Cargo

Rust provides cargo to create, build, run and release our Rust project in a convenient way. Over than that, cargo help us in package dependency management (similar to npm in Javascript world).

To create new project:

danh@friday:~/workspace/self-study$ cargo new hello-world
     Created binary (application) `hello-world` package
danh@friday:~/workspace/self-study$ ll hello-world/
total 24
drwxrwxr-x 4 danh danh 4096 Dec 15 17:26 ./
drwxrwxr-x 7 danh danh 4096 Dec 15 17:26 ../
-rw-rw-r-- 1 danh danh  180 Dec 15 17:26 Cargo.toml
drwxrwxr-x 6 danh danh 4096 Dec 15 17:26 .git/
-rw-rw-r-- 1 danh danh    8 Dec 15 17:26 .gitignore
drwxrwxr-x 2 danh danh 4096 Dec 15 17:26 src/
danh@friday:~/workspace/self-study$

There are 2 major new things:

Cargo.toml: Cargo config file which contains the current Rust version information and package dependencies.
src: directory contains Rust source code.

To build project:

danh@friday:~/workspace/self-study/hello-world$ cargo build
   Compiling hello-world v0.1.0 (/home/danh/workspace/self-study/hello-world)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31s
danh@friday:~/workspace/self-study/hello-world$ ll target/debug/hello-world
-rwxrwxr-x 2 danh danh 4046280 Dec 15 17:33 target/debug/hello-world

Note: The binary file is located at ./target/debug/ so we can call it directly from there.

To build & run in one command:

danh@friday:~/workspace/self-study/hello-world$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s
     Running `target/debug/hello-world`
Hello, world!

To build for production grade binary:

danh@friday:~/workspace/self-study/hello-world$ cargo build --release
   Compiling hello-world v0.1.0 (/home/danh/workspace/self-study/hello-world)
    Finished release [optimized] target(s) in 0.15s
danh@friday:~/workspace/self-study/hello-world$ ll target/
total 24
drwxrwxr-x 4 danh danh 4096 Dec 15 17:38 ./
drwxrwxr-x 5 danh danh 4096 Dec 15 17:33 ../
-rw-rw-r-- 1 danh danh  177 Dec 15 17:33 CACHEDIR.TAG
drwxrwxr-x 7 danh danh 4096 Dec 15 17:33 debug/
drwxrwxr-x 7 danh danh 4096 Dec 15 17:38 release/
-rw-rw-r-- 1 danh danh 1138 Dec 15 17:33 .rustc_info.json