Output using println macro in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust code using println macro?

Consider the following Rust code snippet. What will it print when run?

Rust

fn main() {
    let x = 5;
    println!("The value of x is: {}", x);
}

A x = 5

B The value of x is: x

C The value of x is: {}

D The value of x is: 5

💡 Hint

Look at how the placeholder {} is replaced by the variable x.

The println! macro replaces {} with the value of x, which is 5, so it prints exactly: The value of x is: 5

❓ Predict Output

intermediate

What does this println macro print with multiple placeholders?

What is the output of this Rust code?

Rust

fn main() {
    let a = 3;
    let b = 7;
    println!("Sum of {} and {} is {}", a, b, a + b);
}

A Sum of a and b is a + b

B Sum of 3 and 7 is 10

C Sum of {} and {} is {}

D Sum of 3 and 7 is a + b

💡 Hint

Each {} is replaced by the corresponding argument in order.

The placeholders {} are replaced by a, b, and a+b respectively, so the output is: Sum of 3 and 7 is 10

❓ Predict Output

advanced

What is the output of this Rust code with named arguments in println?

Look at this Rust code using named arguments in println! macro. What will it print?

Rust

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

A Name: {person}, Age: {years}

B Name: name, Age: age

C Name: Alice, Age: 30

D Name: Alice Age: 30

💡 Hint

Named arguments replace placeholders with matching names.

The placeholders {person} and {years} are replaced by the values of name and age variables, so it prints: Name: Alice, Age: 30

❓ Predict Output

advanced

What error does this Rust code produce when using println macro incorrectly?

What error will this Rust code produce?

Rust

fn main() {
    let x = 10;
    println!("Value is: {}", );
}

A error: expected expression, found `)`

B Value is: 10

C Value is: {}

D error: mismatched types

💡 Hint

Check if all placeholders have matching arguments.

The println! macro expects an argument for the {} placeholder, but none is provided, causing a syntax error: expected expression, found `)`

🧠 Conceptual

expert

How many items are printed by this Rust code using println! in a loop?

How many lines will this Rust program print?

Rust

fn main() {
    for i in 1..=5 {
        println!("Number: {}", i);
    }
}

A 5

B 4

C 6

D 1

💡 Hint

The range 1..=5 includes numbers from 1 to 5 inclusive.

The loop runs from 1 to 5 inclusive, so it prints 5 lines, one for each number.

Practice

1. What does the println! macro do in Rust?

easy

A. Creates a new variable

B. Prints text or values to the screen with a new line

C. Reads input from the user

D. Stops the program execution

Solution

1. Step 1: Understand the purpose of println!

   The println! macro is used to show messages or values on the screen, adding a new line after printing.

2. Step 2: Compare with other options

   Reading input, creating variables, or stopping the program are not tasks of println!.

3. Final Answer:

   Prints text or values to the screen with a new line -> Option B

4. Quick Check:

   println! prints output [OK]

Hint: Remember: println! always prints with a new line [OK]

Common Mistakes:

• Confusing println! with input functions
• Thinking println! creates variables
• Assuming println! stops the program

2. Which of the following is the correct syntax to print the number 5 using println! in Rust?

easy

A. println!("{}", 5);

B. println!(5);

C. println!("number: 5");

D. println!("5");

Solution

1. Step 1: Understand how to print values with placeholders

   To print a number using println! with a placeholder, use {} inside the string and pass the value after the comma.

2. Step 2: Check each option

   println!("5"); prints the string "5" literally, println!(5); is invalid syntax, println!("number: 5"); prints a fixed string without using a placeholder. println!("{}", 5); correctly uses {} to print the number 5.

3. Final Answer:

   println!("{}", 5); -> Option A

4. Quick Check:

   Use curly braces {} for values [OK]

Hint: Use curly braces {} to insert values inside println! [OK]

Common Mistakes:

• Trying to print numbers without quotes or placeholders
• Forgetting the comma between string and value
• Using invalid syntax without quotes

3. What will be the output of this Rust code?

let x = 10;
println!("Value: {}", x);

medium

A. Value: 10

B. Value: x

C. Value: {}

D. 10

Solution

1. Step 1: Identify the variable and placeholder

   The variable x holds the value 10. The println! uses {} to insert the value of x.

2. Step 2: Understand the output format

   The output will replace {} with 10, printing "Value: 10" exactly.

3. Final Answer:

   Value: 10 -> Option A

4. Quick Check:

   Placeholder {} shows variable value [OK]

Hint: Curly braces {} show variable values in output [OK]

Common Mistakes:

• Printing the variable name instead of its value
• Printing the placeholder literally
• Omitting the variable in println!

4. Identify the error in this Rust code:

let name = "Alice";
println!("Hello, name!");

medium

A. String quotes are incorrect

B. Variable name is not declared

C. println! macro is misspelled

D. Missing curly braces to print variable value

Solution

1. Step 1: Check how the variable is used in println!

   The code prints the string "Hello, name!" literally because it lacks curly braces to insert the variable name.

2. Step 2: Identify the fix

   To print the value of name, use {} inside the string and pass name after the comma.

3. Final Answer:

   Missing curly braces to print variable value -> Option D

4. Quick Check:

   Use {} to insert variables in println! [OK]

Hint: Use {} and comma to print variables, not plain text [OK]

Common Mistakes:

• Forgetting curly braces around variable
• Not passing variable after the string
• Assuming variable prints without placeholders

5. You want to print the message: "Temperature is 23°C" using println!. Which code correctly prints this message including the degree symbol?

hard

A. println!("Temperature is 23C");

B. println!("Temperature is 23\degree C");

C. println!("Temperature is 23\u{00B0}C");

D. println!("Temperature is 23\xB0C");

Solution

1. Step 1: Understand how to print special Unicode characters

   The degree symbol (°) can be printed using Unicode escape \u{00B0} in Rust strings.

2. Step 2: Check each option for correctness

   println!("Temperature is 23\u{00B0}C"); uses the correct Unicode escape. println!("Temperature is 23C"); does not include the degree symbol. println!("Temperature is 23\degree C"); uses an invalid escape. println!("Temperature is 23\xB0C"); uses a hex escape which is incorrect for Unicode in Rust strings.

3. Final Answer:

   println!("Temperature is 23\u{00B0}C"); -> Option C

4. Quick Check:

   Use \u{XXXX} for Unicode chars in Rust strings [OK]

Hint: Use \u{XXXX} for special Unicode characters in strings [OK]

Common Mistakes:

• Using invalid escape sequences
• Assuming special characters print without escapes
• Confusing hex and Unicode escapes