Character Frequency Counting in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of character frequency count in a string

What is the output of the following C code that counts character frequencies in a string?

DSA C

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "hello world";
    int freq[256] = {0};
    for (int i = 0; i < strlen(str); i++) {
        freq[(unsigned char)str[i]]++;
    }
    for (int i = 0; i < 256; i++) {
        if (freq[i] > 0) {
            printf("%c:%d ", i, freq[i]);
        }
    }
    return 0;
}

A h:1 e:1 l:3 o:1 :1 w:1 r:1 d:1

B h:1 e:1 l:2 o:2 :1 w:1 r:1 d:1

C h:1 e:1 l:3 o:2 :1 w:1 r:1 d:1

D h:1 e:1 l:3 o:2 :2 w:1 r:1 d:1

💡 Hint

Count each character's occurrences carefully, including spaces.

The string "hello world" has 1 'h', 1 'e', 3 'l's, 2 'o's, 2 spaces, 1 'w', 1 'r', and 1 'd'.

🧠 Conceptual

intermediate

Understanding character frequency array size

Why is the frequency array usually declared with size 256 when counting characters in C?

A Because it is the size of the string being processed.

B Because 256 is the maximum number of characters in any string.

C Because ASCII characters fit in 256 values, covering all possible characters in extended ASCII.

D Because 256 is the number of letters in the English alphabet.

💡 Hint

Think about the range of unsigned char values.

The array size 256 covers all possible values of an unsigned char (0 to 255), which includes all ASCII and extended ASCII characters.

🔧 Debug

advanced

Identify the error in character frequency counting code

What error will the following code produce when counting character frequencies?

DSA C

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "abcabc";
    int freq[128];
    for (int i = 0; i < strlen(str); i++) {
        freq[(int)str[i]]++;
    }
    for (int i = 0; i < 128; i++) {
        if (freq[i] > 0) {
            printf("%c:%d ", i, freq[i]);
        }
    }
    return 0;
}

A The code will print frequencies but with wrong characters due to casting.

B The code will cause a runtime error due to uninitialized freq array values.

C The code will cause a compilation error because freq array size is too small.

D The code will print correct frequencies without error.

💡 Hint

Check if freq array elements are initialized before incrementing.

The freq array is not initialized, so it contains garbage values. Incrementing them causes undefined behavior and likely wrong output or runtime errors.

❓ Predict Output

advanced

Output of frequency count with non-alphabet characters

What is the output of this C code counting frequencies including digits and punctuation?

DSA C

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "a1!a2!";
    int freq[256] = {0};
    for (int i = 0; i < strlen(str); i++) {
        freq[(unsigned char)str[i]]++;
    }
    for (int i = 0; i < 256; i++) {
        if (freq[i] > 0) {
            printf("%c:%d ", i, freq[i]);
        }
    }
    return 0;
}

A !:2 1:1 2:1 a:2

B a:2 !:2 1:1 2:1

C a:2 1:1 2:1 !:1

D 1:1 2:1 a:2 !:2

💡 Hint

Check the order of characters printed by increasing ASCII code.

Characters are printed in ASCII order: '!' (33), '1' (49), '2' (50), 'a' (97). Frequencies: '!'=2, '1'=1, '2'=1, 'a'=2.

🧠 Conceptual

expert

Memory optimization for character frequency counting

Which approach best reduces memory usage when counting frequencies of only lowercase English letters in a large text?

A Use an int array of size 26, indexing by (character - 'a').

B Use an int array of size 256 to cover all ASCII characters.

C Use a hash map with characters as keys and counts as values.

D Use a linked list to store characters and their counts dynamically.

💡 Hint

Focus on the smallest fixed-size array that covers only needed characters.

Using an int array of size 26 indexed by (character - 'a') uses minimal memory and is efficient for lowercase letters only.