For loop in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a for loop in Rust?

A for loop in Rust is a control flow statement that repeats a block of code for each item in a collection or range.

beginner

How do you write a for loop to print numbers 1 to 5 in Rust?

You use a range with the syntax:

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

This prints numbers from 1 to 5 inclusive.

beginner

What does the range 1..5 mean in a Rust for loop?

The range 1..5 means numbers from 1 up to but not including 5, so it includes 1, 2, 3, and 4.

intermediate

Can you use a for loop to iterate over elements in a vector in Rust?

Yes, you can loop over a vector like this: <pre>let v = vec![10, 20, 30];
for val in &v {
    println!("{}", val);
}</pre> This prints each element in the vector.

beginner

What happens if you use for i in 0..=3 in Rust?

The loop runs with i = 0, 1, 2, 3 because ..= means inclusive range including the last number.

What does the Rust for loop for i in 1..4 do?

A Loops with i = 1, 2, 3

B Loops with i = 1, 2, 3, 4

C Loops with i = 0, 1, 2, 3

D Loops with i = 4 only

The range 1..4 includes 1, 2, and 3 but excludes 4.

How do you loop over all elements in a vector v in Rust?

A for val in v { ... }

B for val in &v { ... }

C for val in v[] { ... }

D for val in v() { ... }

Using &v borrows the vector and lets you iterate over its elements safely.

What symbol is used for an inclusive range in Rust?

A ..=

B ...

C ..

D =>

The inclusive range uses ..= to include the last number.

Which of these is a valid for loop in Rust?

A for i = 0 to 5 { println!("{}", i); }

B for (i=0; i<5; i++) { println!("{}", i); }

C for i in 0..5 { println!("{}", i); }

D loop i in 0..5 { println!("{}", i); }

Rust uses for i in range syntax, not C-style for loops.

What will this code print?

for x in 3..=5 {
    println!("{}", x);
}

A 5 only

B 3 4

C 4 5

D 3 4 5

The inclusive range 3..=5 includes 3, 4, and 5.

Explain how a for loop works in Rust and give an example using a range.

Think about how you count numbers one by one.

Describe how to loop over elements in a vector in Rust and why borrowing is important.

Remember Rust’s ownership rules.

Practice

1. What does a for loop do in Rust?

easy

A. Declares a variable

B. Defines a new function

C. Repeats actions for each item in a collection

D. Stops the program immediately

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop runs code repeatedly for each item in a list or range.

2. Step 2: Match the description to the options

   Only Repeats actions for each item in a collection describes repeating actions over items in a collection.

3. Final Answer:

   Repeats actions for each item in a collection -> Option C

4. Quick Check:

   For loop = repeat over items [OK]

Hint: For loops repeat actions over collections [OK]

Common Mistakes:

• Confusing for loop with function definition
• Thinking for loop declares variables
• Believing for loop stops program

2. Which of these is the correct syntax for a for loop iterating over numbers 1 to 3 in Rust?

easy

A. for i = 1 to 3 { println!("{}", i); }

B. for i in 1..=3 { println!("{}", i); }

C. for (i in 1..3) println!("{}", i);

D. for i in range(1,3) { println!("{}", i); }

Solution

1. Step 1: Recall Rust for loop syntax

   Rust uses for variable in range { } with ranges like 1..=3 to include 3.

2. Step 2: Check each option

   for i in 1..=3 { println!("{}", i); } uses correct Rust syntax with inclusive range. Others use invalid syntax or functions not in Rust.

3. Final Answer:

   for i in 1..=3 { println!("{}", i); } -> Option B

4. Quick Check:

   Rust for loop uses in and ranges [OK]

Hint: Use 'for var in start..=end' for inclusive ranges [OK]

Common Mistakes:

• Using 'to' instead of 'in' keyword
• Using parentheses around loop variable
• Using Python or other language range syntax

3. What is the output of this Rust code?

let mut sum = 0;
for i in 1..4 {
    sum += i;
}
println!("{}", sum);

medium

A. 6

B. 10

C. 9

D. 7

Solution

1. Step 1: Identify the loop range and iterations

   The loop runs for i = 1, 2, 3 because 1..4 excludes 4.

2. Step 2: Calculate the sum of iterations

   sum = 0 + 1 + 2 + 3 = 6

3. Final Answer:

   6 -> Option A

4. Quick Check:

   1+2+3 = 6 [OK]

Hint: Remember range excludes the end number [OK]

Common Mistakes:

• Including 4 in the sum
• Starting sum at 1 instead of 0
• Confusing inclusive and exclusive ranges

4. Find the error in this Rust for loop code:

for i in 0..5
    println!("{}", i);
}

medium

A. Variable i is not declared

B. Range syntax is incorrect

C. println! macro is used incorrectly

D. Missing curly braces around loop body

Solution

1. Step 1: Check loop syntax

   Rust requires curly braces { } around the loop body if it has more than one statement or to be explicit.

2. Step 2: Identify the missing brace

   The code has a closing brace '}' without an opening brace '{' before println!().

3. Final Answer:

   Missing curly braces around loop body -> Option D

4. Quick Check:

   For loop body needs braces [OK]

Hint: Always use braces {} for loop bodies [OK]

Common Mistakes:

• Forgetting opening brace
• Assuming range syntax is wrong
• Thinking variable needs declaration outside loop

5. How can you use a for loop in Rust to print only even numbers from 2 to 10 inclusive?

hard

A. for i in 2..=10 { if i % 2 == 0 { println!("{}", i); } }

B. for i in 2..10 { if i % 2 = 0 { println!("{}", i); } }

C. for i in 2..=10 { if i / 2 == 0 { println!("{}", i); } }

D. for i in 2..=10 { if i % 2 != 0 { println!("{}", i); } }

Solution

1. Step 1: Understand the range and condition

   We want numbers from 2 to 10 inclusive, so use 2..=10. To print even numbers, check if remainder when divided by 2 is zero.

2. Step 2: Check each option's condition

   for i in 2..=10 { if i % 2 == 0 { println!("{}", i); } } correctly uses i % 2 == 0. for i in 2..10 { if i % 2 = 0 { println!("{}", i); } } uses assignment (=) instead of comparison (==). for i in 2..=10 { if i / 2 == 0 { println!("{}", i); } } uses division instead of modulo. for i in 2..=10 { if i % 2 != 0 { println!("{}", i); } } prints odd numbers.

3. Final Answer:

   for i in 2..=10 { if i % 2 == 0 { println!("{}", i); } } -> Option A

4. Quick Check:

   Use modulo == 0 for even check [OK]

Hint: Use 'if i % 2 == 0' inside for loop [OK]

Common Mistakes:

• Using '=' instead of '==' in condition
• Using division instead of modulo
• Printing odd numbers by mistake