Count and Say Problem in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the first term of the count-and-say sequence.

DSA Python

def count_and_say(n):
    result = [1]
    return result

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

A"1"

B1

C"0"

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using integer 1 instead of string "1".

Starting with "0" or 0 which is incorrect.

The count-and-say sequence starts with the string "1" as the first term.

2fill in blank

medium

Complete the code to iterate from 1 to n-1 to build the sequence.

DSA Python

for i in range([1], n):
    pass

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

An

B0

C1

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Starting loop from 0 which repeats the first term.

Starting from n which skips all iterations.

The loop starts from 1 because the first term is already initialized.

3fill in blank

hard

Fix the error in the inner loop that counts consecutive characters.

DSA Python

count = 1
for j in range(1, len(result)):
    if result[j] == result[j-1]:
        count [1] 1
    else:
        count = 1

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

A*=

B-=

C=

D+=

Attempts:

3 left

💡 Hint

Common Mistakes

Using = which resets count instead of increasing.

Using -= which decreases count incorrectly.

We increase count by 1 when consecutive characters match, so use +=.

4fill in blank

hard

Fill both blanks to append count and character to the next sequence string.

DSA Python

next_seq += str([1]) + [2]

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

Acount

Bresult[j-1]

Cresult[j]

D"count"

Attempts:

3 left

💡 Hint

Common Mistakes

Appending the current character instead of previous.

Appending count without converting to string.

Append the count as string and the previous character counted.

5fill in blank

hard

Fill all three blanks to complete the count-and-say function logic.

DSA Python

def count_and_say(n):
    result = [1]
    for i in range([2], n):
        next_seq = ""
        count = 1
        for j in range(1, len(result)):
            if result[j] == result[j-1]:
                count [3] 1
            else:
                next_seq += str(count) + result[j-1]
                count = 1
        next_seq += str(count) + result[-1]
        result = next_seq
    return result

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

A"1"

B1

C+=

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using integer 1 instead of string "1" for result.

Starting loop from 0 instead of 1.

Using = instead of += to increment count.

The function starts with "1" as string, loops from 1 to n, and increments count with +=.

Practice

1. What does the "Count and Say" sequence describe in simple terms?

easy

A. It sorts the digits of the previous term in ascending order.

B. It describes the count of digits followed by the digit itself in the previous term.

C. It reverses the digits of the previous term.

D. It adds 1 to each digit of the previous term.

Solution

1. Step 1: Understand the sequence rule

   The sequence reads the previous term by counting consecutive digits and saying the count followed by the digit.

2. Step 2: Apply the rule to a term

   For example, "1211" is read as "one 1, one 2, two 1s" which becomes "111221".

3. Final Answer:

   It describes the count of digits followed by the digit itself in the previous term. -> Option B

4. Quick Check:

   Count and Say = Count digits + Say digit [OK]

Hint: Remember: count digits, then say digit [OK]

Common Mistakes:

• Thinking it sorts digits instead of counting
• Assuming it reverses the sequence
• Adding numbers instead of counting

2. Which of the following is the correct Python syntax to start the Count and Say sequence with the first term as "1"?

easy

A. sequence = "1"

B. sequence = [1]

C. sequence = 1

D. sequence = (1)

Solution

1. Step 1: Identify the data type for sequence terms

   The terms are strings representing digits, so the first term should be a string "1".

2. Step 2: Check each option's data type

   sequence = "1" assigns a string "1", which is correct. sequence = [1] is a list, C is an integer, D is an integer in parentheses.

3. Final Answer:

   sequence = "1" -> Option A

4. Quick Check:

   Sequence term must be string "1" [OK]

Hint: Use string for digits, not int or list [OK]

Common Mistakes:

• Using integer 1 instead of string "1"
• Using list instead of string
• Using parentheses which do not create string

3. What is the output of the following Python code snippet for the 4th term of the Count and Say sequence?

def count_and_say(n):
    result = "1"
    for _ in range(n - 1):
        temp = ""
        i = 0
        while i < len(result):
            count = 1
            while i + 1 < len(result) and result[i] == result[i + 1]:
                i += 1
                count += 1
            temp += str(count) + result[i]
            i += 1
        result = temp
    return result

print(count_and_say(4))

medium

A. 1211

B. 111221

C. 21

D. 312211

Solution

1. Step 1: Trace the sequence terms up to n=4

   Term 1: "1"; Term 2: "11" (one 1); Term 3: "21" (two 1s); Term 4: "1211" (one 2, one 1).

2. Step 2: Confirm output of count_and_say(4)

   The function returns "1211" for n=4 as reasoned above.

3. Final Answer:

   312211 -> Option D

4. Quick Check:

   4th term = 312211 [OK]

Hint: Write terms stepwise: 1, 11, 21, 1211, 111221, 312211 [OK]

Common Mistakes:

• Confusing term 3 and 4 outputs
• Miscounting repeated digits
• Mixing digits order in output

4. The following code attempts to generate the Count and Say sequence but has a bug. What is the error?

def count_and_say_bug(n):
    result = "1"
    for _ in range(n):
        temp = ""
        i = 0
        while i < len(result):
            count = 1
            while i + 1 < len(result) and result[i] == result[i + 1]:
                count += 1
            temp += str(count) + result[i]
            i += 1
        result = temp
    return result

print(count_and_say_bug(4))

medium

A. Returns empty string always

B. Syntax error because of missing colon

C. Infinite loop due to missing i increment inside inner while loop

D. Index out of range error

Solution

1. Step 1: Analyze the inner while loop

   The inner while loop increments count but does not increment i, causing infinite loop when consecutive digits match.

2. Step 2: Identify missing increment

   To fix, add i += 1 inside the inner while loop to move forward.

3. Final Answer:

   Infinite loop due to missing i increment inside inner while loop -> Option C

4. Quick Check:

   Missing i++ in inner loop causes infinite loop [OK]

Hint: Check all loops increment their counters properly [OK]

Common Mistakes:

• Forgetting to increment loop variable inside nested loops
• Assuming syntax error without checking logic
• Ignoring infinite loop possibility

5. Given the Count and Say sequence starting with "1", what is the 6th term in the sequence?

hard

A. 312211

B. 13112221

C. 1113213211

D. 1211

Solution

1. Step 1: List terms up to 6th

   Term 1: "1"; Term 2: "11"; Term 3: "21"; Term 4: "1211"; Term 5: "111221"; Term 6: "312211".

2. Step 2: Confirm 6th term matches option

   312211 "312211" matches the 6th term as derived.

3. Final Answer:

   312211 -> Option A

4. Quick Check:

   6th term = 312211 [OK]

Hint: Write terms stepwise to avoid confusion [OK]

Common Mistakes:

• Mixing up 5th and 6th terms
• Skipping terms or misreading counts
• Choosing longer terms incorrectly