Count Words with Given Prefix in DSA Rust - Build from Scratch

Count Words with Given Prefix

📖 Scenario: You are building a simple tool to help a bookstore find how many book titles start with a certain prefix. This helps the store quickly find popular categories.

🎯 Goal: Create a Rust program that counts how many words in a list start with a given prefix.

📋 What You'll Learn

Create a vector of strings called words with exact values

Create a string variable called prefix with an exact value

Use a for loop to count how many words start with prefix

Print the count as the final output

💡 Why This Matters

🌍 Real World

Counting words with a prefix is useful in search engines, autocomplete features, and filtering lists quickly.

💼 Career

This skill helps in text processing, search optimization, and building efficient user interfaces.

1

Create the list of words

Create a vector of strings called words with these exact entries: "apple", "app", "application", "banana", "apricot".

DSA Rust

# Create a vector called words with the given strings
# Your code here

Hint

Use vec![...] to create a vector of string slices.

2

Set the prefix to search

Create a string variable called prefix and set it to "app".

DSA Rust

fn main() {
    let words = vec!["apple", "app", "application", "banana", "apricot"];
    // Create a string variable called prefix and set it to "app"
    // Your code here
}

Hint

Use let prefix = "app"; to create the prefix variable.

3

Count words starting with the prefix

Create a mutable integer variable called count and set it to 0. Use a for loop with variable word to iterate over words.iter(). Inside the loop, use word.starts_with(prefix) to check if the word starts with prefix. If yes, increase count by 1.

DSA Rust

fn main() {
    let words = vec!["apple", "app", "application", "banana", "apricot"];
    let prefix = "app";
    // Create a mutable count variable set to 0
    // Use a for loop to iterate over words.iter()
    // Inside the loop, check if word starts with prefix
    // If yes, increment count
    // Your code here
}

Hint

Use let mut count = 0; and a for loop with words.iter(). Use starts_with method on each word.

4

Print the count

Print the value of count using println!.

DSA Rust

fn main() {
    let words = vec!["apple", "app", "application", "banana", "apricot"];
    let prefix = "app";
    let mut count = 0;
    for word in words.iter() {
        if word.starts_with(prefix) {
            count += 1;
        }
    }
    // Print the count
    // Your code here
}

Hint

Use println!("{}", count); to print the count.

Practice

1. What does the problem "Count Words with Given Prefix" ask you to do?

easy

A. Find the longest word in a list

B. Count how many words in a list start with a specific prefix

C. Count the total number of characters in all words

D. Sort the words alphabetically

Solution

1. Step 1: Understand the problem statement

   The problem asks to find how many words start with a given prefix, not about length or sorting.

2. Step 2: Identify the key task

   Counting words that begin with the prefix is the main task.

3. Final Answer:

   Count how many words in a list start with a specific prefix -> Option B

4. Quick Check:

   Count words starting with prefix = Count how many words in a list start with a specific prefix [OK]

Hint: Focus on words starting with prefix, not full word or sorting [OK]

Common Mistakes:

• Confusing prefix with substring anywhere in word
• Counting total characters instead of words
• Sorting words instead of counting

2. Which of the following Rust code snippets correctly checks if a word starts with a given prefix prefix?

easy

A. word.starts_with(prefix)

B. word.contains(prefix)

C. word.ends_with(prefix)

D. word == prefix

Solution

1. Step 1: Recall Rust string methods

   Rust's starts_with method checks if a string begins with a given substring.

2. Step 2: Compare options

   contains checks anywhere, ends_with checks suffix, and == checks full equality, so only starts_with fits.

3. Final Answer:

   word.starts_with(prefix) -> Option A

4. Quick Check:

   Use starts_with() to check prefix [OK]

Hint: Use starts_with() method to check prefix in Rust strings [OK]

Common Mistakes:

• Using contains() which matches anywhere, not just prefix
• Using ends_with() which checks suffix
• Comparing full equality instead of prefix

3. What is the output of this Rust code?

let words = vec!["apple", "app", "application", "banana"];
let prefix = "app";
let count = words.iter().filter(|w| w.starts_with(prefix)).count();
println!("{}", count);

medium

A. 3

B. 2

C. 1

D. 4

Solution

1. Step 1: Identify words starting with "app"

   Words are "apple", "app", "application", and "banana". The first three start with "app".

2. Step 2: Count matching words

   Count is 3 because "apple", "app", and "application" start with "app".

3. Final Answer:

   3 -> Option A

4. Quick Check:

   3 words start with "app" [OK]

Hint: Count words where starts_with(prefix) is true [OK]

Common Mistakes:

• Counting only exact matches
• Including words that don't start with prefix
• Off-by-one counting errors

4. The following Rust code tries to count words starting with a prefix but gives wrong output. What is the bug?

let words = vec!["cat", "car", "dog", "scare"];
let prefix = "car";
let count = words.iter().filter(|w| w.contains(prefix)).count();
println!("{}", count);

medium

A. Missing semicolon after count declaration

B. Prefix variable is not declared correctly

C. The vector words is empty

D. Using contains() instead of starts_with() causes wrong matches

Solution

1. Step 1: Understand difference between contains() and starts_with()

   contains() checks if prefix appears anywhere, not just at start.

2. Step 2: Identify incorrect matches

   "scare" contains "car" but does not start with "car", so contains() wrongly counts it.

3. Final Answer:

   Using contains() instead of starts_with() causes wrong matches -> Option D

4. Quick Check:

   Use starts_with() to count prefix matches [OK]

Hint: Replace contains() with starts_with() to fix prefix counting [OK]

Common Mistakes:

• Confusing contains() with starts_with()
• Assuming contains() only matches prefix
• Ignoring method differences

5. Given a list of words ["test", "tester", "testing", "toast", "team"] and prefix "te", which Rust code correctly counts words starting with the prefix and ignores case sensitivity?

hard

A. words.iter().filter(|w| w.contains(prefix)).count()

B. words.iter().filter(|w| w.starts_with(prefix)).count()

C. words.iter().filter(|w| w.to_lowercase().starts_with(&prefix.to_lowercase())).count()

D. words.iter().filter(|w| w.ends_with(prefix)).count()

Solution

1. Step 1: Understand case sensitivity in Rust strings

   Rust's starts_with is case sensitive, so we convert both word and prefix to lowercase.

2. Step 2: Check each option for case-insensitive prefix check

   words.iter().filter(|w| w.to_lowercase().starts_with(&prefix.to_lowercase())).count() converts both to lowercase before checking prefix, ensuring case-insensitive match. Others do not handle case.

3. Final Answer:

   words.iter().filter(|w| w.to_lowercase().starts_with(&prefix.to_lowercase())).count() -> Option C

4. Quick Check:

   Convert both strings to lowercase before starts_with() [OK]

Hint: Lowercase both word and prefix before starts_with() for case-insensitive [OK]

Common Mistakes:

• Ignoring case sensitivity in starts_with()
• Using contains() which is wrong for prefix
• Using ends_with() instead of starts_with()