Recursion on Arrays and Strings in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to recursively sum all elements in the array.

DSA Rust

fn sum_array(arr: &[i32], index: usize) -> i32 {
    if index == arr.len() {
        return 0;
    }
    arr[index] + [1](arr, index + 1)
}

Drag options to blanks, or click blank then click option'

Asum_array

Bsum

Carray_sum

DsumArr

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong function name causes a compile error.

Forgetting to increment the index leads to infinite recursion.

The function calls itself recursively with the next index to sum all elements.

2fill in blank

medium

Complete the code to check if a string is a palindrome using recursion.

DSA Rust

fn is_palindrome(s: &str) -> bool {
    if s.len() <= 1 {
        return true;
    }
    let bytes = s.as_bytes();
    if bytes[0] != bytes[s.len() - 1] {
        return false;
    }
    is_palindrome(&s[1..[1]])
}

Drag options to blanks, or click blank then click option'

As.len() - 3

Bs.len()

Cs.len() - 1

Ds.len() - 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using s.len() includes the last character again causing infinite recursion.

Using s.len() - 2 removes too many characters.

We remove the first and last characters each time, so the slice ends at length minus one.

3fill in blank

hard

Fix the error in the recursive function that counts occurrences of a target in an array.

DSA Rust

fn count_occurrences(arr: &[i32], target: i32, index: usize) -> i32 {
    if index == arr.len() {
        return 0;
    }
    let count = if arr[index] == target { 1 } else { 0 };
    count + [1](arr, target, index + 1)
}

Drag options to blanks, or click blank then click option'

Acount_occurrences

BcountOccurrence

Ccount_occur

Doccurrences_count

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different function name causes a compile error.

Forgetting to increment index causes infinite recursion.

The function must call itself by its exact name to recurse correctly.

4fill in blank

hard

Fill both blanks to recursively reverse a string slice.

DSA Rust

fn reverse_string(s: &str) -> String {
    if s.is_empty() {
        return String::new();
    }
    let last_char = &s[s.len() - 1..s.len()];
    let rest = &s[0..[1]];
    format!("{}{}", last_char, [2](rest))
}

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As.len() - 1

Bs.len()

Creverse_string

Dreverse

Attempts:

3 left

💡 Hint

Common Mistakes

Using s.len() includes the last character again causing incorrect slicing.

Using wrong function name causes compile error.

We take the substring excluding the last character and recursively reverse it using the function name.

5fill in blank

hard

Fill the blanks to build a dictionary of word lengths for words longer than 3 characters.

DSA Rust

fn word_lengths(words: &[&str]) -> std::collections::HashMap<String, usize> {
    let mut map = std::collections::HashMap::new();
    for &word in words {
        if word.len() > 3 {
            map.insert([1].to_string(), [2]);
        }
    }
    map
}

// Usage example:
// let words = ["apple", "bat", "carrot"];
// let result = word_lengths(&words);

Drag options to blanks, or click blank then click option'

Aword

Bword.len()

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable for key or value.

Not converting the word to a string for the key.

The key is the word as a string, and the value is the length of the word.

Practice

1. What does recursion on arrays typically involve when processing elements?

easy

A. Using loops to iterate over the array elements

B. Calling the same function with a smaller part of the array until a base case is reached

C. Sorting the array before processing

D. Creating a new array for each element

Solution

1. Step 1: Understand recursion on arrays

   Recursion on arrays means the function calls itself with a smaller portion of the array each time.

2. Step 2: Identify the base case

   The recursion stops when the array is empty or has one element (base case).

3. Final Answer:

   Calling the same function with a smaller part of the array until a base case is reached -> Option B

4. Quick Check:

   Recursion = smaller input calls [OK]

Hint: Recursion breaks problem into smaller parts until simple case [OK]

Common Mistakes:

• Confusing recursion with loops
• Not defining a base case
• Trying to process the whole array at once

2. Which of the following is the correct Rust function signature for a recursive function that takes a slice of integers?

easy

A. fn recurse(arr: &mut [i32]) -> i32

B. fn recurse(arr: Vec<i32>) -> i32

C. fn recurse(arr: i32) -> &[i32]

D. fn recurse(arr: &[i32]) -> i32

Solution

1. Step 1: Understand Rust slice parameter

   A recursive function processing an array usually takes a slice reference like &[i32] to avoid ownership issues.

2. Step 2: Check return type

   Returning i32 is common if the function calculates a value like sum or count.

3. Final Answer:

   fn recurse(arr: &[i32]) -> i32 -> Option D

4. Quick Check:

   Slice input with reference = fn recurse(arr: &[i32]) -> i32 [OK]

Hint: Use &[i32] for read-only array slices in recursion [OK]

Common Mistakes:

• Using Vec<i32> causes ownership moves
• Returning a reference to local data
• Using mutable slice when not needed

3. What is the output of this Rust recursive function when called with arr = &[1, 2, 3]?

fn sum(arr: &[i32]) -> i32 {
    if arr.is_empty() {
        0
    } else {
        arr[0] + sum(&arr[1..])
    }
}

println!("{}", sum(arr));

medium

A. 0

B. 3

C. 6

D. Compilation error

Solution

1. Step 1: Trace recursive calls

   sum(&[1,2,3]) = 1 + sum(&[2,3]) sum(&[2,3]) = 2 + sum(&[3]) sum(&[3]) = 3 + sum(&[]) sum(&[]) = 0 (base case)

2. Step 2: Calculate total sum

   sum = 1 + 2 + 3 + 0 = 6

3. Final Answer:

   6 -> Option C

4. Quick Check:

   1+2+3 = 6 [OK]

Hint: Sum adds first element plus recursive sum of rest [OK]

Common Mistakes:

• Forgetting base case returns 0
• Adding wrong slice range
• Confusing output with length

4. Identify the error in this recursive Rust function that tries to count occurrences of a number in an array:

fn count_occurrences(arr: &[i32], target: i32) -> i32 {
    if arr.len() == 0 {
        0
    } else if arr[0] == target {
        1 + count_occurrences(&arr[1..], target)
    } else {
        count_occurrences(arr, target)
    }
}

medium

A. Infinite recursion because the else branch calls with the same array slice

B. Missing base case for empty array

C. Incorrect comparison operator in if condition

D. Function should return bool instead of i32

Solution

1. Step 1: Analyze recursive calls

   In the else branch, the function calls itself with the same array slice, causing infinite recursion.

2. Step 2: Identify fix

   The else branch should call with &arr[1..] to move forward and avoid infinite loop.

3. Final Answer:

   Infinite recursion because the else branch calls with the same array slice -> Option A

4. Quick Check:

   Same slice call causes infinite recursion [OK]

Hint: Always reduce array size in recursive calls to avoid infinite loops [OK]

Common Mistakes:

• Not reducing slice in else branch
• Confusing base case condition
• Returning wrong type

5. Given a string slice s: &str, which recursive Rust function correctly reverses the string?

hard

A. fn reverse(s: &str) -> String {
if s.len() == 0 {
String::new()
} else {
format!("{}{}", reverse(&s[1..]), &s[0..1])
}
}

B. fn reverse(s: &str) -> String {
if s.len() == 1 {
s.to_string()
} else {
format!("{}{}", &s[0..1], reverse(&s[1..]))
}
}

C. fn reverse(s: &str) -> String {
if s.is_empty() {
String::new()
} else {
format!("{}{}", &s[0..1], reverse(&s[1..]))
}
}

D. }
}
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{ esle }
)(wen::gnirtS
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Solution

1. Step 1: Understand string slicing and recursion

   To reverse, recursively reverse substring from index 1 onward, then append first character.

2. Step 2: Check base case and concatenation order

   Base case is empty string returning empty String. Concatenate reversed tail + first char.

3. Step 3: Compare options

   fn reverse(s: &str) -> String { if s.len() == 0 { String::new() } else { format!("{}{}", reverse(&s[1..]), &s[0..1]) } } matches correct base case and concatenation order.

4. Final Answer:

   fn reverse(s: &str) -> String { if s.len() == 0 { String::new() } else { format!("{}{}", reverse(&s[1..]), &s[0..1]) } } -> Option A

5. Quick Check:

   Reverse tail + first char = correct recursion [OK]

Hint: Reverse substring then add first char at end for string reversal [OK]

Common Mistakes:

• Appending first char before recursive call
• Wrong base case condition
• Using incorrect string slices