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

Count Words with Given Prefix

📖 Scenario: You are building a simple search feature for a book app. Users type a few letters, and you want to count how many book titles start with those letters.

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

📋 What You'll Learn

Create a list of words with exact given titles

Create a variable for the prefix to search

Use a for loop to count words starting with the prefix

Print the final count

💡 Why This Matters

🌍 Real World

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

💼 Career

This skill helps in building efficient search and filter functions in software development.

1

Create the list of book titles

Create a list called book_titles with these exact strings: 'apple', 'application', 'banana', 'appetite', 'apricot'

DSA Python

# Create the list book_titles with the given words
# Your code here

Hint

Use square brackets [] to create a list and separate words with commas.

2

Set the prefix to search

Create a variable called prefix and set it to the string 'app'

DSA Python

book_titles = ['apple', 'application', 'banana', 'appetite', 'apricot']
# Create the prefix variable with value 'app'
# Your code here

Hint

Use = to assign the string 'app' to the variable prefix.

3

Count words starting with the prefix

Create a variable called count and set it to 0. Then use a for loop with variable word to iterate over book_titles. Inside the loop, use an if statement to check if word.startswith(prefix). If yes, increase count by 1.

DSA Python

book_titles = ['apple', 'application', 'banana', 'appetite', 'apricot']
prefix = 'app'
# Initialize count to 0
# Use a for loop to check each word
# Increase count if word starts with prefix
# Your code here

Hint

Remember to use startswith() method to check the prefix.

4

Print the count result

Write a print statement to display the value of count

DSA Python

book_titles = ['apple', 'application', 'banana', 'appetite', 'apricot']
prefix = 'app'
count = 0
for word in book_titles:
    if word.startswith(prefix):
        count += 1
# Print the count
# Your code here

Hint

The output should be the number of words starting with 'app'.

Practice

1. What does the function countWordsStartingWith(words, prefix) do when given a list of words and a prefix?

easy

A. Counts how many words contain the prefix anywhere inside

B. Returns the first word that starts with the prefix

C. Counts how many words in the list start with the given prefix

D. Removes all words that start with the prefix from the list

Solution

1. Step 1: Understand the function purpose

   The function is designed to count words that begin with a specific prefix, not just contain it anywhere.

2. Step 2: Compare options with function behavior

   Only Counts how many words in the list start with the given prefix correctly describes counting words starting with the prefix. Others describe different behaviors.

3. Final Answer:

   Counts how many words in the list start with the given prefix -> Option C

4. Quick Check:

   Counting words starting with prefix = Counts how many words in the list start with the given prefix [OK]

Hint: Focus on 'start with' meaning prefix at word beginning [OK]

Common Mistakes:

• Confusing 'start with' and 'contain' prefix
• Thinking function returns words instead of count
• Assuming it removes words instead of counting

2. Which of the following Python code snippets correctly counts words starting with a prefix p in a list words?

easy

A. sum(word.startswith(p) for word in words)

B. sum(word.startswith(p) == 'True' for word in words)

C. sum(word.startswith(p) is 'True' for word in words)

D. sum(word.startswith(p) = True for word in words)

Solution

1. Step 1: Check syntax correctness

   sum(word.startswith(p) = True for word in words) uses '=' inside sum which is invalid syntax for comparison.

2. Step 2: Understand boolean summation

   Options B and C incorrectly compare boolean to string 'True' (always False). Only sum(word.startswith(p) for word in words) correctly sums booleans directly (True counts as 1).

3. Final Answer:

   sum(word.startswith(p) for word in words) -> Option A

4. Quick Check:

   Use sum with boolean directly for counting [OK]

Hint: Use sum with boolean expressions directly, no need for == or is [OK]

Common Mistakes:

• Using '=' instead of '==' for comparison
• Comparing boolean to string 'True'
• Confusing syntax causing errors

3. What is the output of the following code?

words = ['apple', 'app', 'application', 'banana', 'apply']
prefix = 'app'
count = sum(word.startswith(prefix) for word in words)
print(count)

medium

A. 2

B. 3

C. 5

D. 4

Solution

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

   Words starting with 'app' are 'apple', 'app', 'application', and 'apply' -- total 4 words.

2. Step 2: Calculate sum of True values

   Each word starting with prefix counts as 1, so sum is 4.

3. Final Answer:

   4 -> Option D

4. Quick Check:

   Count words starting with 'app' = 4 [OK]

Hint: Count words starting exactly with prefix, not just containing it [OK]

Common Mistakes:

• Counting words that contain prefix anywhere
• Missing one word that starts with prefix
• Confusing prefix length with word length

4. Find the error in this code that tries to count words starting with prefix 'pre':

def count_prefix(words, prefix):
    count = 0
    for word in words:
        if word.startswith == prefix:
            count += 1
    return count

words = ['prefix', 'preach', 'prevent', 'post']
prefix = 'pre'
print(count_prefix(words, prefix))

medium

A. Using 'word.startswith == prefix' instead of 'word.startswith(prefix)'

B. Missing return statement in function

C. Incorrect variable name 'count' used

D. Loop should be 'for prefix in words' instead of 'for word in words'

Solution

1. Step 1: Identify incorrect method usage

   The code uses 'word.startswith == prefix' which compares method to string, incorrect syntax.

2. Step 2: Correct method call

   It should be 'word.startswith(prefix)' to check if word starts with prefix.

3. Final Answer:

   Using 'word.startswith == prefix' instead of 'word.startswith(prefix)' -> Option A

4. Quick Check:

   Method call needs parentheses with argument [OK]

Hint: Remember to call methods with parentheses and arguments [OK]

Common Mistakes:

• Comparing method itself instead of calling it
• Forgetting parentheses after method name
• Mixing variable names in loop

5. Given a list of words, how would you count words starting with any prefix from a list of prefixes prefixes? Choose the correct code snippet.

words = ['cat', 'car', 'dog', 'cart', 'cow']
prefixes = ['ca', 'do', 'co']

hard

A. sum(word.startswith(prefixes[0]) or word.startswith(prefixes[1]) for word in words)

B. sum(any(word.startswith(p) for p in prefixes) for word in words)

C. sum(word.startswith(prefixes) for word in words)

D. sum(word.startswith(p) and p in prefixes for word in words)

Solution

1. Step 1: Understand requirement for multiple prefixes

   We need to check if each word starts with any prefix in the prefixes list.

2. Step 2: Analyze options for correct logic

   sum(any(word.startswith(p) for p in prefixes) for word in words) uses any() to correctly check if word starts with any prefix. sum(word.startswith(prefixes[0]) or word.startswith(prefixes[1]) for word in words) only checks first two prefixes (misses 'cow' with 'co'). sum(word.startswith(prefixes) for word in words) passes list to startswith (TypeError). sum(word.startswith(p) and p in prefixes for word in words) uses unbound p and incorrect and logic.

3. Final Answer:

   sum(any(word.startswith(p) for p in prefixes) for word in words) -> Option B

4. Quick Check:

   Use any() to check multiple prefixes per word [OK]

Hint: Use any() inside sum() to check multiple prefixes per word [OK]

Common Mistakes:

• Passing list directly to startswith instead of iterating
• Using 'and' instead of 'or' logic
• Hardcoding prefix checks instead of looping