Frequency Counter Pattern Using Hash Map in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new empty hash map for counting frequencies.

DSA Rust

let mut freq_map = std::collections::HashMap::[1]();

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

Aclear

Binsert

Cnew

Dwith_capacity

Attempts:

3 left

💡 Hint

Common Mistakes

Using insert instead of new to create the map.

Trying to use clear on a map that doesn't exist yet.

We use HashMap::new() to create an empty hash map for frequency counting.

2fill in blank

medium

Complete the code to increase the count of a character in the frequency map.

DSA Rust

let count = freq_map.entry(ch).or_insert([1]);
*count += 1;

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

A-1

B1

Cch

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Starting count at 1 instead of 0 causes off-by-one errors.

Using the character itself as the count.

We start counting from zero, so or_insert(0) sets initial count to zero.

3fill in blank

hard

Fix the error in the code to check if two strings have the same character frequencies.

DSA Rust

if freq_map1 == [1] {
    println!("Strings have the same frequency");
}

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

Afreq_map2

Bfreq_map1

Cfreq_map3

Dfreq_map

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing freq_map1 with itself instead of freq_map2.

Using an undefined variable for comparison.

We compare freq_map1 with freq_map2 to check if frequencies match.

4fill in blank

hard

Fill both blanks to count frequencies of characters in a string.

DSA Rust

for ch in [1].chars() {
    let count = freq_map.entry(ch).or_insert([2]);
    *count += 1;
}

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

Ainput_string

B0

C1

Dchars

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'chars' as the string variable instead of calling chars() method.

Starting counts at 1 instead of 0.

We iterate over input_string characters and start counts at zero.

5fill in blank

hard

Fill all three blanks to create a frequency map with characters as keys and counts as values.

DSA Rust

let freq_map: std::collections::HashMap<[1], [2]> = input_string.chars().fold(std::collections::HashMap::new(), |mut acc, ch| {
    *acc.entry(ch).or_insert([3]) += 1;
    acc
});

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

Achar

Bi32

C0

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using String instead of char for keys.

Starting counts at 1 instead of 0.

Keys are chars, values are i32 counts starting at zero.

Practice

1. What is the main purpose of using a hash map in the Frequency Counter Pattern?

easy

A. To remove duplicate items from the collection

B. To sort the items in ascending order

C. To count how many times each item appears in a collection

D. To reverse the order of items in the collection

Solution

1. Step 1: Understand the role of a hash map

   A hash map stores key-value pairs, allowing quick lookup of counts for each item.

2. Step 2: Connect to frequency counting

   Using the hash map, we track how many times each item appears by updating counts.

3. Final Answer:

   To count how many times each item appears in a collection -> Option C

4. Quick Check:

   Frequency counting = Counting items [OK]

Hint: Hash map keys track items, values track counts [OK]

Common Mistakes:

• Thinking hash map sorts items
• Confusing frequency counting with removing duplicates
• Assuming hash map reverses order

2. Which of the following Rust code snippets correctly initializes an empty hash map for frequency counting?

easy

A. let mut freq = Vec::with_capacity(10);

B. let freq = Vec::new();

C. let freq = HashSet::new();

D. let mut freq = HashMap::new();

Solution

1. Step 1: Identify the correct data structure for frequency counting

   Frequency counting requires a hash map to store keys and counts.

2. Step 2: Check Rust syntax for hash map initialization

   In Rust, HashMap::new() creates an empty hash map; mut allows modification.

3. Final Answer:

   let mut freq = HashMap::new(); -> Option D

4. Quick Check:

   HashMap::new() initializes empty map [OK]

Hint: Use HashMap::new() with mut for frequency map [OK]

Common Mistakes:

• Using Vec or HashSet instead of HashMap
• Forgetting mut keyword to modify map
• Confusing capacity with initialization

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

use std::collections::HashMap;

fn main() {
    let items = vec!["apple", "banana", "apple", "orange", "banana", "apple"];
    let mut freq = HashMap::new();

    for item in items {
        let count = freq.entry(item).or_insert(0);
        *count += 1;
    }

    println!("{:?}", freq);
}

medium

A. {"apple": 1, "banana": 1, "orange": 1}

B. {"apple": 3, "banana": 2, "orange": 1}

C. {"apple": 2, "banana": 2, "orange": 2}

D. Compilation error due to missing mut

Solution

1. Step 1: Count occurrences of each fruit

   "apple" appears 3 times, "banana" 2 times, "orange" 1 time in the vector.

2. Step 2: Check how counts are updated in the hash map

   Each item increments its count correctly using entry and or_insert.

3. Final Answer:

   {"apple": 3, "banana": 2, "orange": 1} -> Option B

4. Quick Check:

   Count each fruit correctly = {"apple":3,"banana":2,"orange":1} [OK]

Hint: Count each item incrementally with entry().or_insert(0) [OK]

Common Mistakes:

• Assuming counts start at 1 instead of 0
• Forgetting to dereference count to increment
• Confusing output format or order

4. Identify the error in this Rust code snippet for frequency counting:

use std::collections::HashMap;

fn main() {
    let items = vec!["cat", "dog", "cat"];
    let mut freq = HashMap::new();

    for item in items {
        let count = freq.entry(item).or_insert(1);
        *count += 1;
    }

    println!("{:?}", freq);
}

medium

A. or_insert should start with 0, not 1

B. Missing mut keyword for freq

C. Cannot use entry on a vector

D. Dereferencing count is not needed

Solution

1. Step 1: Understand or_insert initial value

   or_insert(1) sets initial count to 1, but counting starts from zero.

2. Step 2: Effect of starting at 1 on counts

   Starting at 1 causes counts to be off by one; correct is or_insert(0).

3. Final Answer:

   or_insert should start with 0, not 1 -> Option A

4. Quick Check:

   Frequency count starts at zero [OK]

Hint: Start counts at 0 with or_insert(0) [OK]

Common Mistakes:

• Starting count at 1 inflates frequencies
• Forgetting mut on freq (not error here)
• Misunderstanding dereference necessity

5. Given a list of words, how would you use the Frequency Counter Pattern with a hash map in Rust to find all words that appear exactly twice? Choose the best approach.

hard

A. Count frequencies with a hash map, then collect keys with value 2 into a vector

B. Sort the list and pick every second word

C. Use a hash set to store words and print those that appear twice

D. Count frequencies but ignore words appearing more than once

Solution

1. Step 1: Use hash map to count all word frequencies

   Traverse the list, increment counts for each word in the hash map.

2. Step 2: Filter keys with frequency exactly two

   Iterate over the hash map entries and collect words with count == 2.

3. Final Answer:

   Count frequencies with a hash map, then collect keys with value 2 into a vector -> Option A

4. Quick Check:

   Frequency count + filter for 2 = correct approach [OK]

Hint: Count all, then filter keys with count 2 [OK]

Common Mistakes:

• Sorting does not guarantee frequency counts
• Hash set cannot count duplicates
• Ignoring words with frequency > 1 loses needed data