Formatting output in Rust - Step-by-Step Execution

Concept Flow - Formatting output

Start

↓

Prepare data

↓

Choose format string

↓

Apply formatting

↓

Print or store output

↓

End

This flow shows how Rust takes data, applies a format string, and produces formatted output.

Execution Sample

Rust

fn main() {
    let name = "Alice";
    let age = 30;
    println!("Name: {}, Age: {}", name, age);
}

This code prints a formatted string with a name and age using placeholders.

Execution Table

Step	Action	Format String	Values	Result
1	Start main function
2	Assign variable name			name = "Alice"
3	Assign variable age			age = 30
4	Call println! macro	Name: {}, Age: {}	name="Alice", age=30	Prints: Name: Alice, Age: 30
5	End main function

💡 Program ends after printing formatted output.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
name	uninitialized	"Alice"	"Alice"	"Alice"
age	uninitialized	uninitialized	30	30

Key Moments - 2 Insights

Why do we use curly braces {} in the format string?

Curly braces {} are placeholders that tell Rust where to insert the values, as shown in step 4 of the execution_table.

What happens if the number of placeholders doesn't match the number of values?

Rust will give a compile-time error because the println! macro expects the number of placeholders to match the number of values, as implied in step 4.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what is printed?

A Name: Alice, Age: 30

B Name: {}, Age: {}

C Alice 30

D Name: name, Age: age

💡 Hint

Check the Result column in step 4 of the execution_table.

According to variable_tracker, what is the value of 'age' after step 3?

A uninitialized

B 30

C "Alice"

D 0

💡 Hint

Look at the 'age' row under 'After Step 3' in variable_tracker.

If we add another placeholder {} in the format string but do not add a value, what will happen?

A The program prints an extra empty space

B The program ignores the extra placeholder

C Rust gives a compile-time error

D The program crashes at runtime

💡 Hint

Refer to key_moments about placeholder and value count matching.

Concept Snapshot

Rust formatting output uses curly braces {} as placeholders.
Use println!("text {}, {}", val1, val2) to print formatted text.
The number of placeholders must match the number of values.
Rust checks this at compile time to avoid errors.
This helps create readable and dynamic output.

Full Transcript

This example shows how Rust formats output using the println! macro. First, variables name and age are assigned values. Then, the println! macro is called with a format string containing curly braces {} as placeholders. The values of name and age are inserted into these placeholders in order. The program prints the final string: "Name: Alice, Age: 30". If the number of placeholders and values do not match, Rust will give a compile-time error. This ensures safe and clear formatting of output.