Convert a String to Float

Code

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

float custom_atof(const char *str) {
    uint8_t i = 0;
    float result = 0.0;
    float fraction = 0.0;
    float divisor = 10.0;
    int sign = 1;
    uint8_t after_dot = 0;

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

    while (str[i]) {
        if (str[i] >= '0' && str[i] <= '9') {
            if (!after_dot) {
                result = result * 10 + (str[i] - '0');
            } else {
                fraction += (str[i] - '0') / divisor;
                divisor *= 10;
            }
        } else if (str[i] == '.' && !after_dot) {
            after_dot = 1;
        } else {
            break;  // Stop at first non-numeric, non-dot character
        }
        i++;
    }

    return sign * (result + fraction);
}

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++;
    }

    float value = custom_atof(str);
    printf("%.2f", value);
    return 0;
}

Solving Approach

🪜 Step-by-Step Solving Algorithm

1. Initialize variables
   • result = 0.0 for the integer part
   • fraction = 0.0, divisor = 10.0 for the decimal part
   • sign = 1 to handle negative numbers
   • after_dot = 0 to track decimal point
2. Handle optional sign
   • If str[0] == '-', set sign = -1
   • If str[0] == '+', skip it
3. Iterate through characters
   • If character is a digit ('0' to '9'):
     • If before decimal: result = result * 10 + (ch - '0')
     • If after decimal: fraction += (ch - '0') / divisor, then divisor *= 10
   • If character is '.', set after_dot = 1
   • Stop at any other character
4. Return final value
   • return sign * (result + fraction)

🧪 Example

Input: " -12.34abc "
Output: -12.34