Program structure in Rust - Step-by-Step Execution

Concept Flow - Program structure in Rust

Start

↓

fn main()

↓

Statements inside main

↓

Program ends

Rust programs start with the main function, which runs statements inside it, then ends.

Execution Sample

Rust

fn main() {
    println!("Hello, world!");
}

This program prints 'Hello, world!' to the screen.

Execution Table

Step	Action	Code Line	Output
1	Program starts	fn main() {
2	Enter main function	println!("Hello, world!");
3	Print to screen	println!("Hello, world!");	Hello, world!
4	Exit main function	}
5	Program ends

💡 Program ends after main function finishes.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
No variables	-	-	-	-

Key Moments - 3 Insights

Why does Rust need a main function?

Rust programs always start running from the main function, as shown in step 2 of the execution table.

What does println! do inside main?

println! prints text to the screen, which you can see in step 3 where 'Hello, world!' is output.

Can code run outside main in Rust?

No, Rust runs code inside functions like main; the program ends after main finishes (step 5).

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 3?

A The program prints 'Hello, world!'

B The program starts

C The program ends

D The program enters main

💡 Hint

Check the Output column at step 3 in the execution table.

At which step does the program exit the main function?

A Step 2

B Step 5

C Step 4

D Step 3

💡 Hint

Look at the Action column for 'Exit main function' in the execution table.

If we add another println! after the first, how would the output change?

A It would print nothing

B It would print two lines of text

C It would cause an error

D It would print only the first line

💡 Hint

Think about how println! works inside main as shown in the execution table.

Concept Snapshot

Rust programs start with fn main() {
Inside main, statements run in order
Use println! to print text
Program ends after main finishes
No code runs outside functions

Full Transcript

This visual trace shows how a Rust program runs starting from the main function. The program begins at step 1, enters main at step 2, executes the println! statement at step 3 which prints 'Hello, world!' to the screen, then exits main at step 4 and ends at step 5. Variables are not used here. Key points include that Rust always starts running from main, println! prints text, and the program ends after main finishes. The quiz questions check understanding of these steps and outputs.