Character Frequency Counting in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of character frequency count using dictionary

What is the output of the following Python code that counts character frequency in a string?

DSA Python

text = "hello"
freq = {}
for ch in text:
    freq[ch] = freq.get(ch, 0) + 1
print(freq)

A {"h": 1, "e": 1, "l": 2, "o": 1}

B {"h": 1, "e": 1, "l": 1, "o": 1}

C {"h": 1, "e": 1, "l": 3, "o": 1}

D {"h": 0, "e": 1, "l": 2, "o": 1}

💡 Hint

Count how many times each letter appears in the word 'hello'.

The code loops through each character in the string 'hello'. It adds 1 to the count for each character in the dictionary. The letter 'l' appears twice, so its count is 2. All other letters appear once.

🧠 Conceptual

intermediate

Understanding character frequency counting with default dictionary

Which Python data structure is best suited to count character frequencies without manually checking if a key exists?

A set

B collections.defaultdict(int)

C list

D tuple

💡 Hint

Think about a dictionary that automatically gives zero for missing keys.

defaultdict(int) automatically initializes missing keys with 0, making counting easier without manual checks.

🔧 Debug

advanced

Identify the error in character frequency counting code

What error does the following code produce when counting characters in 'test'?

DSA Python

text = "test"
freq = {}
for ch in text:
    freq[ch] += 1
print(freq)

A KeyError

B SyntaxError

C TypeError

D No error, outputs {'t': 2, 'e': 1, 's': 1}

💡 Hint

Check what happens when you try to add 1 to a key that does not exist yet.

The dictionary freq is empty initially. Trying to do freq[ch] += 1 without initializing freq[ch] causes a KeyError because the key does not exist.

❓ Predict Output

advanced

Output of character frequency count using Counter

What is the output of this Python code using collections.Counter to count characters in 'banana'?

DSA Python

from collections import Counter
text = "banana"
freq = Counter(text)
print(dict(freq))

A {"b": 1, "a": 2, "n": 3}

B {"b": 1, "a": 1, "n": 1}

C {"b": 2, "a": 3, "n": 1}

D {"b": 1, "a": 3, "n": 2}

💡 Hint

Count how many times each letter appears in 'banana'.

Counter counts each character's occurrences. 'a' appears 3 times, 'n' 2 times, and 'b' once.

🚀 Application

expert

Find the character with the highest frequency

Given the string 'abracadabra', which character has the highest frequency and what is its count?

DSA Python

text = "abracadabra"
freq = {}
for ch in text:
    freq[ch] = freq.get(ch, 0) + 1
max_char = max(freq, key=freq.get)
max_count = freq[max_char]
print(max_char, max_count)

A c 1

B b 2

C a 5

D r 2

💡 Hint

Count how many times each letter appears and find the one with the largest count.

The letter 'a' appears 5 times, which is more than any other character in 'abracadabra'.