Convert float to cstring C

Write a C function ftoa() that converts a given floating-point number or a double to a string. Use of standard library functions for direct conversion is not allowed. The following is prototype of ftoa(). The article provides insight of conversion of C double to string.

Examples:

• ftoa(1.555, str, 2) should store “1.55” in res.
• ftoa(1.555, str, 0) should store “1” in res.

A simple way is to use sprintf(), but use of standard library functions for direct conversion is not allowed.

Approach: The idea is to separate integral and fractional parts and convert them to strings separately. Following are the detailed steps.

//code//:

// C program for implementation of ftoa()
#include <math.h>
#include <stdio.h>

// Reverses a string 'str' of length 'len'
void reverse(char* str, int len)
{
    int i = 0, j = len - 1, temp;
    while (i < j) {
        temp = str[i];
        str[i] = str[j];
        str[j] = temp;
        i++;
        j--;
    }
}

// Converts a given integer x to string str[].
// d is the number of digits required in the output.
// If d is more than the number of digits in x,
// then 0s are added at the beginning.
int intToStr(int x, char str[], int d)
{
    int i = 0;
    while (x) {
        str[i++] = (x % 10) + '0';
        x = x / 10;
    }

    // If number of digits required is more, then
    // add 0s at the beginning
    while (i < d)
        str[i++] = '0';

    reverse(str, i);
    str[i] = '\0';
    return i;
}

// Converts a floating-point/double number to a string.
void ftoa(float n, char* res, int afterpoint)
{
    // Extract integer part
    int ipart = (int)n;

    // Extract floating part
    float fpart = n - (float)ipart;

    // convert integer part to string
    int i = intToStr(ipart, res, 0);

    // check for display option after point
    if (afterpoint != 0) {
        res[i] = '.'; // add dot

        // Get the value of fraction part upto given no.
        // of points after dot. The third parameter
        // is needed to handle cases like 233.007
        fpart = fpart * pow(10, afterpoint);

        intToStr((int)fpart, res + i + 1, afterpoint);
    }
}

// Driver program to test above function
int main()
{
    char res[20];
    float n = 233.007;
    ftoa(n, res, 4);
    printf("\"%s\"\n", res);
    return 0;
}
// end code here//