Iterating over collections in Rust - Mini Project: Build & Apply

Iterating over collections

📖 Scenario: You are managing a small fruit store. You have a list of fruits and their prices. You want to go through each fruit and see its price.

🎯 Goal: Learn how to iterate over a collection in Rust using a for loop to access each item.

📋 What You'll Learn

Create a vector of fruit names

Create a variable to count the number of fruits

Use a for loop to iterate over the vector

Print each fruit name during iteration

💡 Why This Matters

🌍 Real World

Iterating over collections is common when you want to process lists of data, like items in a store, user inputs, or sensor readings.

💼 Career

Knowing how to loop over collections is a basic skill for software developers, data analysts, and anyone working with programming languages.

1

Create a vector of fruits

Create a vector called fruits with these exact string values: "Apple", "Banana", "Cherry", "Date".

Rust

# Create a vector called fruits with the exact values
# Your code here

Hint

Use vec![...] to create a vector with the given fruit names.

2

Create a counter variable

Add a variable called count and set it to 0 to count the number of fruits.

Rust

fn main() {
    let fruits = vec!["Apple", "Banana", "Cherry", "Date"];
    // Create a variable called count and set it to 0
    # Your code here
}

Hint

Use let mut count = 0; to create a variable that can change.

3

Iterate over the fruits vector

Use a for loop with the variable fruit to iterate over fruits.iter(). Inside the loop, increase count by 1.

Rust

fn main() {
    let fruits = vec!["Apple", "Banana", "Cherry", "Date"];
    let mut count = 0;
    // Use a for loop with fruit to iterate over fruits.iter()
    // Increase count by 1 inside the loop
    # Your code here
}

Hint

Use for fruit in fruits.iter() to loop over each fruit.

4

Print each fruit and the total count

Inside the for loop, add a println! statement to print the fruit name. After the loop, print the total number of fruits using println! with the variable count.

Rust

fn main() {
    let fruits = vec!["Apple", "Banana", "Cherry", "Date"];
    let mut count = 0;
    for fruit in fruits.iter() {
        // Print the fruit name
        # Your code here
        count += 1;
    }
    // Print the total count of fruits
    # Your code here
}

Hint

Use println!("Fruit: {}", fruit); inside the loop and println!("Total fruits: {}", count); after the loop.

Practice

1. What does the following Rust code do?

let numbers = [1, 2, 3];
for num in numbers {
    println!("{}", num);
}

easy

A. Causes a compile error because arrays can't be iterated

B. Prints each number in the array on a new line

C. Prints the whole array as one line

D. Prints the length of the array

Solution

1. Step 1: Understand the for loop over an array

   The for loop visits each element in the array named 'numbers'.

2. Step 2: Action inside the loop

   Each element is printed on its own line using println! macro.

3. Final Answer:

   Prints each number in the array on a new line -> Option B

4. Quick Check:

   for loop over array prints elements one by one [OK]

Hint: For loops visit each item in order [OK]

Common Mistakes:

• Thinking arrays can't be iterated
• Expecting the whole array printed at once
• Confusing print! with println!

2. Which of these is the correct syntax to iterate over a vector named v in Rust?

easy

A. for i in v { println!("{}", i); }

B. for i to v { println!("{}", i); }

C. foreach i in v { println!("{}", i); }

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

Solution

1. Step 1: Recall Rust for loop syntax

   Rust uses 'for variable in collection' to iterate over items.

2. Step 2: Check each option

   for i in v { println!("{}", i); } matches correct syntax; others use invalid keywords.

3. Final Answer:

   for i in v { println!("{}", i); } -> Option A

4. Quick Check:

   Rust uses 'for ... in ...' loops [OK]

Hint: Remember: for variable in collection [OK]

Common Mistakes:

• Using 'to' instead of 'in'
• Using 'foreach' which is not Rust syntax
• Using 'loop' incorrectly

3. What is the output of this Rust code?

let v = vec![10, 20, 30];
let mut sum = 0;
for &num in &v {
    sum += num;
}
println!("{}", sum);

medium

A. 60

B. 303020

C. Error due to borrowing

D. 0

Solution

1. Step 1: Understand iteration with references

   The loop uses '&num in &v' to borrow each element by reference and dereference it with '&num'.

2. Step 2: Calculate sum of elements

   sum starts at 0, then adds 10, 20, and 30, resulting in 60.

3. Final Answer:

   60 -> Option A

4. Quick Check:

   Sum of 10+20+30 = 60 [OK]

Hint: Sum elements by iterating with references [OK]

Common Mistakes:

• Confusing references and values
• Expecting concatenation instead of sum
• Thinking borrowing causes error here

4. Identify the error in this Rust code snippet:

let arr = [1, 2, 3];
for num in arr {
    num += 1;
    println!("{}", num);
}

medium

A. Loop variable 'num' must be a reference

B. Syntax error: missing semicolon after println!

C. Array cannot be iterated directly

D. Cannot modify 'num' because it is an immutable copy

Solution

1. Step 1: Check mutability of loop variable

   In Rust, loop variables are immutable by default; 'num' is a copy of array element.

2. Step 2: Understand why 'num += 1' fails

   Trying to change 'num' causes error because it is not declared mutable.

3. Final Answer:

   Cannot modify 'num' because it is an immutable copy -> Option D

4. Quick Check:

   Loop variables are immutable unless declared mutable [OK]

Hint: Loop vars are immutable unless 'mut' used [OK]

Common Mistakes:

• Thinking semicolon missing causes error
• Believing arrays can't be iterated
• Assuming loop vars are references by default

5. You have a vector of strings: let words = vec!["apple", "banana", "cherry"];. How do you create a new vector containing the lengths of each word using iteration?

hard

A. let lengths = words.map(|w| w.len());

B. let lengths = for w in words { w.len() };

C. let lengths: Vec<usize> = words.iter().map(|w| w.len()).collect();

D. let lengths = words.iter().for_each(|w| w.len());

Solution

1. Step 1: Use iterator and map to transform elements

   words.iter() creates an iterator over references to strings; map applies a function to each element.

2. Step 2: Collect results into a new vector

   collect() gathers the mapped lengths into a Vec<usize>.

3. Final Answer:

   let lengths: Vec<usize> = words.iter().map(|w| w.len()).collect(); -> Option C

4. Quick Check:

   Use iter().map(...).collect() to transform collections [OK]

Hint: Use iter().map(...).collect() to build new collections [OK]

Common Mistakes:

• Trying to use for loop as expression
• Using map directly on vector without iter()
• Using for_each which returns () not a collection