Constants in Rust - Step-by-Step Execution

Concept Flow - Constants

Declare constant with 'const'

↓

Assign fixed value

↓

Use constant in code

↓

Value cannot change

↓

Program runs

Constants are declared once with 'const', assigned a fixed value, and used without changing throughout the program.

Execution Sample

Rust

const MAX_POINTS: u32 = 100_000;

fn main() {
    println!("Max points: {}", MAX_POINTS);
}

This code declares a constant MAX_POINTS and prints its value.

Execution Table

Step	Action	Value/Result	Notes
1	Declare constant MAX_POINTS	MAX_POINTS = 100_000	Constant set once, cannot change
2	Enter main function	Start main	Program begins execution
3	Print MAX_POINTS	Output: Max points: 100000	Uses constant value
4	End program	Program ends	No changes to MAX_POINTS

💡 Program ends after printing constant value; constants cannot be changed.

Variable Tracker

Variable	Start	After 1	After 2	Final
MAX_POINTS	undefined	100000	100000	100000

Key Moments - 2 Insights

Why can't we change the value of MAX_POINTS after declaring it?

Because MAX_POINTS is declared with 'const', it is immutable and fixed at compile time, as shown in execution_table step 1 where it is set once and never changes.

Can constants be declared inside functions?

Yes, constants can be declared inside functions, but they are then scoped to that function. They remain immutable and fixed at compile time.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of MAX_POINTS at step 3?

A undefined

B 0

C 100000

D Cannot be determined

💡 Hint

Refer to execution_table row 3 where MAX_POINTS is printed with value 100000.

At which step does the program print the constant value?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Check execution_table row 3 for the print action.

If we try to assign a new value to MAX_POINTS after step 1, what would happen?

A Compile-time error

B The program runs normally

C Runtime error

D Value changes successfully

💡 Hint

Constants declared with 'const' cannot be changed; see key_moments about immutability.

Concept Snapshot

Constants in Rust:
- Declared with 'const' keyword
- Must have explicit type
- Assigned fixed value at compile time
- Immutable: value cannot change
- Accessible globally or in module scope

Full Transcript

This visual execution shows how constants work in Rust. First, a constant MAX_POINTS is declared with the 'const' keyword and assigned the value 100000. This value is fixed and cannot be changed later. When the program runs, it enters the main function and prints the constant's value. The constant remains unchanged throughout the program. Constants have explicit types. Trying to change a constant causes a compile-time error.