Count and Say Problem in DSA C - Build from Scratch

Count and Say Problem

📖 Scenario: Imagine you are working with a simple number game where each number describes the previous number's digits. This is called the "Count and Say" sequence. For example, starting with "1", the next number says "one 1" which is "11", then "two 1s" which is "21", and so on.

🎯 Goal: You will write a program in C that generates the nth term of the Count and Say sequence. You will start with the first term "1" and build up to the desired term by describing the previous term.

📋 What You'll Learn

Create a string variable to hold the current term starting with "1"

Create an integer variable n to represent which term to generate

Write a loop to generate terms from 2 to n by reading the previous term and building the next

Print the final term after generating it

💡 Why This Matters

🌍 Real World

The Count and Say sequence is a simple example of run-length encoding, a technique used in data compression.

💼 Career

Understanding string manipulation and loops in C is essential for many programming jobs, especially those involving low-level data processing.

1

Setup initial term and target term

Create a string variable called current initialized to "1" and an integer variable called n set to 5.

DSA C

char current[100] = "1";
int n = 5;
// Your code here

Hint

Use a character array to hold the string "1" and an integer variable for n.

2

Add a temporary buffer for building next term

Create a character array called next with size 100 to store the next term during generation.

DSA C

char current[100] = "1";
int n = 5;
char next[100];
// Your code here

Hint

This buffer will hold the new term while you build it from the old term.

3

Generate Count and Say terms up to n

Write a for loop from i = 2 to i <= n. Inside, use variables count, j, and k to read current and build next by counting consecutive digits. After building next, copy it back to current.

DSA C

char current[100] = "1";
int n = 5;
char next[100];

// Your code here

Hint

Use nested loops to count repeated digits and build the next term. Then copy next back to current.

4

Print the final Count and Say term

Write a printf statement to print the string current.

DSA C

char current[100] = "1";
int n = 5;
char next[100];

for (int i = 2; i <= n; i++) {
    int j = 0, k = 0;
    while (current[j] != '\0') {
        char digit = current[j];
        int count = 1;
        while (current[j + 1] == digit) {
            count++;
            j++;
        }
        next[k++] = count + '0';
        next[k++] = digit;
        j++;
    }
    next[k] = '\0';
    int m = 0;
    while (next[m] != '\0') {
        current[m] = next[m];
        m++;
    }
    current[m] = '\0';
}

// Your code here

Hint

The 5th term of the Count and Say sequence starting with "1" is "111221".

Practice

1. What is the output of the first term in the Count and Say sequence?

easy

A. "1"

B. "11"

C. "21"

D. "1211"

Solution

1. Step 1: Understand the first term definition

   The first term in the Count and Say sequence is always "1" by definition.

2. Step 2: Confirm the output

   Since the first term is "1", the output is exactly "1".

3. Final Answer:

   "1" -> Option A

4. Quick Check:

   First term = "1" [OK]

Hint: First term is always "1" in Count and Say [OK]

Common Mistakes:

• Confusing first term with second term
• Thinking sequence starts with "11"
• Mixing up output format

2. Which of the following is the correct syntax to generate the next term from a given term char* term in C for the Count and Say problem?

easy

A. Use a loop to count consecutive characters and build a new string

B. Directly print the term without processing

C. Use recursion without counting characters

D. Sort the characters in the term before counting

Solution

1. Step 1: Understand the Count and Say logic

   The next term is generated by counting consecutive characters and describing them.

2. Step 2: Identify correct approach

   Using a loop to count consecutive characters and build a new string matches the problem's logic.

3. Final Answer:

   Use a loop to count consecutive characters and build a new string -> Option A

4. Quick Check:

   Loop + count consecutive chars = correct syntax [OK]

Hint: Count consecutive chars with loop to build next term [OK]

Common Mistakes:

• Skipping counting step
• Trying to sort characters
• Using recursion without counting

3. What is the output of the Count and Say sequence at term 4?

medium

A. "21"

B. "111221"

C. "1211"

D. "3112"

Solution

1. Step 1: Generate terms up to 4

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

2. Step 2: Confirm term 4 output

   The fourth term is "1211" describing term 3.

3. Final Answer:

   "1211" -> Option C

4. Quick Check:

   Term 4 = "1211" [OK]

Hint: Build terms stepwise: count then say [OK]

Common Mistakes:

• Skipping terms and guessing output
• Mixing term 3 and 4
• Incorrect counting of digits

4. Given the following C code snippet for Count and Say, what is the error?

char* nextTerm(char* term) {
    int count = 1;
    char result[100];
    int index = 0;
    for (int i = 0; term[i] != '\0'; i++) {
        if (term[i] == term[i+1]) {
            count++;
        } else {
            result[index++] = count + '0';
            result[index++] = term[i];
            count = 1;
        }
    }
    result[index] = '\0';
    return result;
}

medium

A. Loop condition should be i < strlen(term)

B. Incorrect character comparison operator

C. Missing initialization of count variable

D. Returning pointer to local array causes undefined behavior

Solution

1. Step 1: Analyze return of local array

   The function returns pointer to local array 'result' which is destroyed after function ends.

2. Step 2: Identify undefined behavior

   Returning local array pointer leads to undefined behavior as memory is invalid outside function.

3. Final Answer:

   Returning pointer to local array causes undefined behavior -> Option D

4. Quick Check:

   Local array pointer return = undefined behavior [OK]

Hint: Never return pointer to local array in C [OK]

Common Mistakes:

• Ignoring local array lifetime
• Assuming loop condition is wrong
• Overlooking pointer return issues

5. How would you modify the Count and Say algorithm to handle very large terms efficiently in C?

hard

A. Store all terms in a global array for quick access

B. Use dynamic memory allocation and process terms iteratively

C. Sort each term before counting to reduce complexity

D. Use fixed-size arrays and recursion for each term

Solution

1. Step 1: Understand large term challenges

   Large terms require flexible memory to avoid overflow and efficient processing.

2. Step 2: Choose suitable approach

   Dynamic memory allocation allows resizing and iterative processing avoids stack overflow.

3. Final Answer:

   Use dynamic memory allocation and process terms iteratively -> Option B

4. Quick Check:

   Dynamic memory + iteration = efficient large term handling [OK]

Hint: Use malloc and loops for big terms, avoid recursion [OK]

Common Mistakes:

• Using fixed arrays causing overflow
• Recursion causing stack overflow
• Sorting terms breaks sequence logic