79. Check if the String Numeric or Alphabetic

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

void classify_string(const char *str) {
 
    uint8_t i = 0;
    uint8_t has_digit = 0;
    uint8_t has_alpha = 0;

    if (str[0] == '\0') {
        printf("MIXED");  // Empty string treated as mixed
        return;
    }

    while (str[i] != '\0') {
        char ch = str[i];
        if (ch >= '0' && ch <= '9') {
            has_digit = 1;
        } else if ((ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z')) {
            has_alpha = 1;
        } else {
            // Symbol or space = mixed immediately
            printf("MIXED");
            return;
        }
        i++;
    }

    if (has_alpha && !has_digit) {
        printf("ALPHABETIC");
    } else if (!has_alpha && has_digit) {
        printf("NUMERIC");
    } else {
        printf("MIXED");
    }
}

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++;
    }

    classify_string(str);
    return 0;
}

What’s the Goal?

Classify the entire string based on content:

• Only digits → "NUMERIC"
• Only letters → "ALPHABETIC"
• Anything else (mix of digits, letters, symbols) → "MIXED"

Firmware Relevance

• Used in serial command parsing: to determine if user passed config command or ID
• Helps filter valid arguments: e.g., "SET 123" vs "SET BAUD"
• Ensures robust input filtering in systems with no ctype.h

Logic Highlights

• Use two flags: has_digit, has_alpha
• On encountering a symbol or space → immediately return "MIXED"
• Check final state to decide type