Frequency Counter Pattern Using Hash Map in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of frequency count for characters 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[] = "banana";
    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 b:1 a:2 n:3

B b:1 a:3 n:2

C a:2 b:1 n:3

D a:3 b:1 n:2

💡 Hint

Count how many times each character appears in the string "banana".

The string "banana" has 'b' once, 'a' three times, and 'n' twice. The code prints characters in ASCII order, so 'a' comes before 'b' and 'n'.

❓ Predict Output

intermediate

Frequency count of integers in an array

What is the output of this C code that counts frequency of integers in an array?

DSA C

#include <stdio.h>

int main() {
    int arr[] = {2, 3, 2, 4, 3, 2};
    int freq[5] = {0};
    int n = sizeof(arr) / sizeof(arr[0]);
    for (int i = 0; i < n; i++) {
        freq[arr[i]]++;
    }
    for (int i = 0; i < 5; i++) {
        if (freq[i] > 0) {
            printf("%d:%d ", i, freq[i]);
        }
    }
    return 0;
}

A 2:3 3:1 4:2

B 2:2 3:3 4:1

C 2:3 3:2 4:1

D 2:1 3:2 4:3

💡 Hint

Count how many times each number appears in the array.

The array has 2 three times, 3 two times, and 4 once. The code prints frequencies for numbers 0 to 4.

🔧 Debug

advanced

Identify the error in frequency counting code

What error does this C code produce when counting character frequencies?

DSA C

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

int main() {
    char str[] = "apple";
    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 Uses uninitialized values in freq array causing undefined behavior

B Syntax error due to missing semicolon

C Array index out of bounds error

D No error, prints correct frequencies

💡 Hint

Check if freq array is initialized before incrementing.

The freq array is declared but not initialized to zero, so incrementing freq[i] uses garbage values causing undefined behavior.

🧠 Conceptual

advanced

Frequency counter pattern with non-continuous keys

Which data structure is best to count frequencies of arbitrary integers (not limited range) in C?

A Use a fixed-size array indexed by integer values

B Use a hash map (hash table) to map integers to counts

C Use a stack to push integers and count later

D Use a linked list to store pairs of integer and count

💡 Hint

Think about how to efficiently store counts for any integer key.

A hash map allows mapping any integer key to its frequency efficiently without large memory waste.

❓ Predict Output

expert

Output of frequency count with mixed characters

What is the output of this C code that counts frequencies of letters and digits in a string?

DSA C

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

int main() {
    char str[] = "a1b2a1";
    int freq[256] = {0};
    for (int i = 0; i < strlen(str); i++) {
        freq[(unsigned char)str[i]]++;
    }
    for (int i = 48; i <= 57; i++) { // digits 0-9
        if (freq[i] > 0) {
            printf("%c:%d ", i, freq[i]);
        }
    }
    for (int i = 97; i <= 122; i++) { // lowercase letters a-z
        if (freq[i] > 0) {
            printf("%c:%d ", i, freq[i]);
        }
    }
    return 0;
}

A 1:2 2:1 a:2 b:1

B 1:1 2:1 a:2 b:1

C 1:2 2:1 a:1 b:1

D 1:2 2:2 a:2 b:1

💡 Hint

Count digits and letters separately and print in order.

The string "a1b2a1" has '1' twice, '2' once, 'a' twice, and 'b' once. The code prints digits first then letters.