C Program to find minimum occurrence of character in a string

In C programming, finding the minimum occurrence of a character in a string involves counting the frequency of each character and then determining which character appears the least number of times.

Syntax

char string[SIZE];
int frequency[CHARS];
// Count frequency of each character
// Find character with minimum frequency

String Declaration and Initialization

Before finding minimum occurrence, let's understand string basics −

Declaration:

char stringname[size];

For example − char string[50]; creates a string of length 50 characters.

Initialization:

• Using single character constants −

char string[10] = {'H', 'e', 'l', 'l', 'o', '\0'};

• Using string constants −

char string[10] = "Hello";

Algorithm for Finding Minimum Occurrence

The logic to find minimum occurrence of a character in a given string is as follows −

• Initialize frequency array to count occurrences of each character
• Traverse the string and count frequency of each character
• Find the character with minimum non-zero frequency

for(i=0; i<CHARS; i++){
    if(frequency[i]!=0){
        if(frequency[minimum] == 0 || frequency[i] < frequency[minimum])
            minimum = i;
    }
}

Example

Following is the C program to find minimum occurring character in a string −

Note: This program uses fgets() instead of deprecated gets() function for safe string input.

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

#define SIZE 100    // Maximum string size
#define CHARS 256   // ASCII characters

int main() {
    char string[SIZE];
    int frequency[CHARS];
    int i = 0, minimum;
    int value;
    
    printf("Enter the string: ");
    fgets(string, SIZE, stdin);
    
    // Remove newline character from fgets
    string[strcspn(string, "
")] = '\0';
    
    // Initialize frequency array
    for(i = 0; i < CHARS; i++) {
        frequency[i] = 0;
    }
    
    // Count frequency of each character
    i = 0;
    while(string[i] != '\0') {
        value = (int)string[i];
        frequency[value] += 1;
        i++;
    }
    
    // Find minimum frequency character
    minimum = -1;
    for(i = 0; i < CHARS; i++) {
        if(frequency[i] != 0) {
            if(minimum == -1 || frequency[i] < frequency[minimum]) {
                minimum = i;
            }
        }
    }
    
    if(minimum != -1) {
        printf("Minimum occurrence character is '%c' = %d times.
", minimum, frequency[minimum]);
    } else {
        printf("No characters found in string.
");
    }
    
    return 0;
}

Output

Enter the string: tutorialspoint
Minimum occurrence character is 'a' = 1 times.

How It Works

1. Create a frequency array to store count of each ASCII character
2. Initialize all frequencies to zero
3. Traverse the input string and increment frequency for each character
4. Find the character with minimum non-zero frequency
5. Display the result

Key Points

• The program uses ASCII values as array indices to count character frequencies
• Only characters that actually appear in the string are considered for minimum
• Time complexity is O(n) where n is the length of the string
• Space complexity is O(1) as we use fixed size array for ASCII characters

Conclusion

Finding the minimum occurrence character in a string requires frequency counting and comparison. This approach efficiently handles all printable ASCII characters and provides the result in linear time.