Question

How to extract the first digit from a floating-point number in C

Answer 1

#include <stdio.h>
#include <math.h>   // For fabs, floor

// Function to extract the first digit of a floating-point number
int getFirstDigit(double num) {
    // Handle zero explicitly
    if (num == 0.0) {
        return 0;
    }

    // Work with absolute value to ignore sign
    num = fabs(num);

    // Shift decimal point until number >= 1
    while (num < 1.0) {
        num *= 10.0;
    }

    // Shift until number < 10
    while (num >= 10.0) {
        num /= 10.0;
    }

    // Floor to get the integer digit
    return (int)floor(num);
}

int main(void) {
    double value = 48724.928;

    int firstDigit = getFirstDigit(value);
    printf("First digit: %d\n", firstDigit);

    firstDigit = getFirstDigit(0.9761);
    printf("First digit: %d\n", firstDigit);

    return 0;
}


/*
run:

First digit: 4
First digit: 9

*/

 

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Answer 2

#include <stdio.h>
#include <math.h>
#include <ctype.h> // isdigit

// Function to extract the first digit of a floating-point number
int getFirstDigit(double num) {
    char buffer[64];

    num = fabs(num); // handle negative numbers

    // Convert number to string
    snprintf(buffer, sizeof(buffer), "%.15g", num); // Use %.15g to avoid scientific notation

    // Find first digit character
    for (int i = 0; buffer[i] != '\0'; ++i) {
        if (isdigit((unsigned char)buffer[i])) {
            return buffer[i] - '0';
        }
    }

    return -1; // error case
}

int main() {
    double value = 48724.928;

    int firstDigit = getFirstDigit(value);
    printf("First digit: %d\n", firstDigit);

    firstDigit = getFirstDigit(0.9761);
    printf("First digit: %d\n", firstDigit);

    return 0;
}



/*
run:

First digit: 4
First digit: 0

*/

 

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