78. Convert String to Integer

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

int custom_atoi(const char *str) {
    int result = 0;
    int sign = 1;
    uint8_t i = 0;

    // Skip leading non-numeric characters (optional)
    // We'll keep this strict — valid parsing starts with digit or sign

    // Handle optional sign
    if (str[i] == '-') {
        sign = -1;
        i++;
    } else if (str[i] == '+') {
        i++;
    }

    // Start reading digits
    while (str[i] >= '0' && str[i] <= '9') {
        result = result * 10 + (str[i] - '0');
        i++;
    }

    return result * sign;
}

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++;
    }

    printf("%d", custom_atoi(str));
    return 0;
}

What’s the goal?

• Convert valid number portion at the start of a string
• If first character is non-digit (except + or -), return 0
• Stop parsing at the first non-digit character after digits begin

Why it’s important in firmware?

• Serial input may be noisy or formatted loosely
• MCU command strings often need partial parsing
• Avoiding full string validation keeps code light & efficient

Logic Highlights

• Check for optional + or -
• Convert only digits until a non-digit is hit
• If string starts with non-digit (e.g., "abc"), skip parsing → result is 0