74. Remove Duplicate Characters from a String

#include <stdio.h>
#include <stdint.h>

void remove_duplicates(char *str) {
    uint8_t hash[256] = {0};  // 1-byte flags per ASCII char
    uint8_t read = 0, write = 0;

    while (str[read] != '\0') {
        uint8_t ch = (uint8_t)str[read];
        if (!hash[ch]) {
            hash[ch] = 1;
            str[write++] = str[read];
        }
        read++;
    }
    str[write] = '\0';  // Null terminate final string
}

int main() {
    char str[101];
    fgets(str, sizeof(str), stdin);

    // Remove newline
    uint8_t i = 0;
    while (str[i]) {
        if (str[i] == '\n') {
            str[i] = '\0';
            break;
        }
        i++;
    }

    remove_duplicates(str);
    printf("%s", str);
    return 0;
}

What is this about?

This simulates low-level filtering logic — useful for parsing protocol fields, or cleaning up user input in command-line firmware shells.

Why it matters in firmware?

• Embedded systems often need to sanitize input without dynamic memory
• Understanding in-place filtering improves memory-efficient design
• Useful in string tokenization, config processing

Solution Logic

• Use an integer array of size 256 to track characters seen (ASCII)
• Two indices: read to iterate, write to overwrite
• Skip already-seen characters and write only first occurrences