First Non Repeating Character Using Hash in DSA C - Build from Scratch

First Non Repeating Character Using Hash

📖 Scenario: Imagine you are building a simple text analyzer that finds the first character in a word that does not repeat. This can help in games like word puzzles or in text processing tools.

🎯 Goal: You will write a C program that finds the first non-repeating character in a given string using a hash (array) to count character occurrences.

📋 What You'll Learn

Create a character array called input with the exact value "swiss"

Create an integer array called count of size 256 initialized to 0

Use a for loop with variable i to count occurrences of each character in input

Use a for loop with variable i to find the first character with count 1

Print the first non-repeating character using printf

💡 Why This Matters

🌍 Real World

Finding the first non-repeating character is useful in text analysis, spell checking, and game development.

💼 Career

This technique helps in coding interviews and software development tasks involving string processing and hashing.

1

Create the input string

Create a character array called input and set it to the exact string "swiss"

DSA C

# Your code here

Hint

Use char input[] = "swiss"; to create the string.

2

Create and initialize the count array

Add an integer array called count of size 256 and initialize all elements to 0

DSA C

#include 

int main() {
    char input[] = "swiss";
    int count[256] = {0};
    // Your code here
    return 0;
}

Hint

Use int count[256] = {0}; to create and initialize the array.

3

Count character occurrences

Use a for loop with variable i to iterate over input until the null character '\0' and increment count[input[i]] for each character

DSA C

#include 

int main() {
    char input[] = "swiss";
    int count[256] = {0};

    // Count characters
    // Your code here

    return 0;
}

Hint

Use a for loop to go through each character until '\0' and increase the count for that character.

4

Find and print the first non-repeating character

Use a for loop with variable i to iterate over input until '\0'. Inside the loop, check if count[input[i]] is 1. If yes, print the character using printf("%c\n", input[i]) and break the loop.

DSA C

#include 

int main() {
    char input[] = "swiss";
    int count[256] = {0};

    for (int i = 0; input[i] != '\0'; i++) {
        count[(unsigned char)input[i]]++;
    }

    // Find and print first non-repeating character
    // Your code here

    return 0;
}

Hint

Loop again through input and print the first character whose count is 1.

Practice

1. What is the main purpose of using a hash (like a dictionary or map) to find the first non-repeating character in a string?

easy

A. To reverse the string quickly

B. To sort the characters alphabetically

C. To remove duplicate characters from the string

D. To count the frequency of each character efficiently

Solution

1. Step 1: Understand the role of hash in counting

   A hash (dictionary) stores characters as keys and their counts as values, allowing quick frequency counting.

2. Step 2: Identify why frequency counting helps

   Knowing how many times each character appears helps find the first character with count 1, which is the first non-repeating character.

3. Final Answer:

   To count the frequency of each character efficiently -> Option D

4. Quick Check:

   Hash counts characters = frequency count [OK]

Hint: Use hash to count chars, then find first with count 1 [OK]

Common Mistakes:

• Thinking hash sorts characters
• Assuming hash reverses string
• Using hash to remove duplicates only

2. Which of the following C code snippets correctly initializes a hash map (using an array) to count characters for ASCII strings?

easy

A. int count[256] = {0};

B. int count[] = NULL;

C. int count[256]; // uninitialized

D. int count = 256;

Solution

1. Step 1: Check correct array initialization

   In C, to count ASCII chars, an int array of size 256 initialized to zero is needed: int count[256] = {0};

2. Step 2: Identify incorrect options

   int count[] = NULL; sets pointer to NULL, no array. int count[256]; // uninitialized is uninitialized, may contain garbage. int count = 256; is a single int, not array.

3. Final Answer:

   int count[256] = {0}; -> Option A

4. Quick Check:

   Array size 256 with zero init = correct hash map [OK]

Hint: Use int count[256] = {0} for ASCII char frequency [OK]

Common Mistakes:

• Not initializing array to zero
• Using pointer instead of array
• Declaring single int instead of array

3. What will be the output of the following C code snippet?

char str[] = "swiss";
int count[256] = {0};
for (int i = 0; str[i] != '\0'; i++) {
    count[(int)str[i]]++;
}
for (int i = 0; str[i] != '\0'; i++) {
    if (count[(int)str[i]] == 1) {
        printf("%c", str[i]);
        break;
    }
}

medium

A. s

B. i

C. w

D. No output

Solution

1. Step 1: Count character frequencies in "swiss"

   Characters: s(3 times), w(1 time), i(1 time)

2. Step 2: Find first character with count 1 in order

   Order: s (count 3), w (count 1) -> first non-repeating is 'w'

3. Final Answer:

   w -> Option C

4. Quick Check:

   First unique char in "swiss" = w [OK]

Hint: Check counts, print first char with count 1 [OK]

Common Mistakes:

• Printing first char without checking count
• Confusing 'i' and 'w' counts
• Ignoring character order

4. Identify the bug in this C code snippet that tries to find the first non-repeating character:

char str[] = "hello";
int count[256];
for (int i = 0; str[i] != '\0'; i++) {
    count[(int)str[i]]++;
}
for (int i = 0; str[i] != '\0'; i++) {
    if (count[(int)str[i]] == 1) {
        printf("%c", str[i]);
        break;
    }
}

medium

A. count array is not initialized to zero

B. Loop condition is incorrect

C. Casting char to int is wrong

D. printf format specifier is incorrect

Solution

1. Step 1: Check count array initialization

   count array is declared but not initialized, so it contains garbage values causing wrong counts.

2. Step 2: Explain impact of uninitialized array

   Incrementing garbage values leads to incorrect frequency counts, so first non-repeating char cannot be found correctly.

3. Final Answer:

   count array is not initialized to zero -> Option A

4. Quick Check:

   Uninitialized count array causes wrong frequency [OK]

Hint: Always initialize count array to zero before counting [OK]

Common Mistakes:

• Forgetting to initialize count array
• Assuming default zero initialization
• Misusing loop conditions

5. Given the string "aabbccddeffg", which character will be identified as the first non-repeating character using a hash map counting method? Consider the code below:

char str[] = "aabbccddeffg";
int count[256] = {0};
for (int i = 0; str[i] != '\0'; i++) {
    count[(int)str[i]]++;
}
for (int i = 0; str[i] != '\0'; i++) {
    if (count[(int)str[i]] == 1) {
        printf("%c", str[i]);
        break;
    }
}

hard

A. g

B. e

C. d

D. f

Solution

1. Step 1: Count frequencies of each character

   Characters and counts: a(2), b(2), c(2), d(2), e(1), f(2), g(1)

2. Step 2: Find first character with count 1 in order

   Order: a,b,c,d,e,f,g; e and g have count 1, but e appears before g.

3. Step 3: Check carefully the string order

   String is "aabbccddeffg"; 'e' appears once at position 8, 'g' at position 11. So first non-repeating is 'e'.

4. Final Answer:

   e -> Option B

5. Quick Check:

   First unique char in string = e [OK]

Hint: Check counts and order carefully to find first unique char [OK]

Common Mistakes:

• Choosing last unique char instead of first
• Ignoring character order in string
• Miscounting character frequencies