Convert float to string without sprintf()

Question

I'm coding for a microcontroller-based application and I need to convert a float to a character string, but I do not need the heavy overhead associated with sprintf(). Is there any eloquent way to do this? I don't need too much. I only need 2 digits of precision.

Answer 1

Try this. It should be nice and small. I've output the string directly - doing a printf, rather than a sprintf. I'll leave it to you to allocate space for the return string, as well as copying the result into it.

EDIT: As noted in the comments, this answer does not work properly for negative numbers.

// prints a number with 2 digits following the decimal place
// creates the string backwards, before printing it character-by-character from
// the end to the start
//
// Usage: myPrintf(270.458)
//  Output: 270.45
void myPrintf(float fVal)
{
    char result[100];
    int dVal, dec, i;

    fVal += 0.005;   // added after a comment from Matt McNabb, see below.

    dVal = fVal;
    dec = (int)(fVal * 100) % 100;

    memset(result, 0, 100);
    result[0] = (dec % 10) + '0';
    result[1] = (dec / 10) + '0';
    result[2] = '.';

    i = 3;
    while (dVal > 0)
    {
        result[i] = (dVal % 10) + '0';
        dVal /= 10;
        i++;
    }

    for (i=strlen(result)-1; i>=0; i--)
        putc(result[i], stdout);
}

Answer 2

Here's a version optimized for embedded systems that doesn't require any stdio or memset, and has low memory footprint. You're responsible for passing a char buffer initialized with zeros (with pointer p) where you want to store your string, and defining CHAR_BUFF_SIZE when you make said buffer (so the returned string will be null terminated).

static char * _float_to_char(float x, char *p) {
    char *s = p + CHAR_BUFF_SIZE; // go to end of buffer
    uint16_t decimals;  // variable to store the decimals
    int units;  // variable to store the units (part to left of decimal place)
    if (x < 0) { // take care of negative numbers
        decimals = (int)(x * -100) % 100; // make 1000 for 3 decimals etc.
        units = (int)(-1 * x);
    } else { // positive numbers
        decimals = (int)(x * 100) % 100;
        units = (int)x;
    }

    *--s = (decimals % 10) + '0';
    decimals /= 10; // repeat for as many decimal places as you need
    *--s = (decimals % 10) + '0';
    *--s = '.';

    while (units > 0) {
        *--s = (units % 10) + '0';
        units /= 10;
    }
    if (x < 0) *--s = '-'; // unary minus sign for negative numbers
    return s;
}

Tested on ARM Cortex M0 & M4. Rounds correctly.

Answer 3

I can't comment on enhzflep's response, but to handle negative numbers correctly (which the current version does not), you only need to add

if (fVal < 0) {
     putc('-', stdout);
     fVal = -fVal;
  }

at the beginning of the function.

Answer 4

Its a Liitle large method, but It would work for both int and float, decimalPoint parameter is passed with zero value for Integer, Please let me know if you have smaller function than this.

void floatToStr(uint8_t *out, float x,int decimalPoint)
{
    uint16_t absval = fabs(x);
    uint16_t absvalcopy = absval;


    int decimalcount = 0;

    while(absvalcopy != 0)
    {

        absvalcopy /= 10;
        decimalcount ++;
    }

    uint8_t *absbuffer = malloc(sizeof(uint8_t) * (decimalcount + decimalPoint + 1));
    int absbufferindex = 0;
    absvalcopy = absval;
    uint8_t temp;

    int i = 0;
    for(i = decimalcount; i > 0; i--)
    {
        uint16_t frst1 = fabs((absvalcopy / pow(10.0, i-1)));
        temp = (frst1 % 10) + 0x30;
        *(absbuffer + absbufferindex) = temp;
        absbufferindex++;
    }

    if(decimalPoint > 0)
    {
        *(absbuffer + absbufferindex) = '.';
        absbufferindex ++;

        //------------------- Decimal Extractor ---------------------//
       for(i = 1; i < decimalPoint + 1; i++)
       {

           uint32_t valueFloat = (x - (float)absval)*pow(10,i);
           *(absbuffer + absbufferindex) = ((valueFloat) % 10) + 0x30;
           absbufferindex++;
       }
    }

   for(i=0; i< (decimalcount + decimalPoint + 1); i++)
   {
       *(out + i) = *(absbuffer + i);
   }

   i=0;
   if(decimalPoint > 0)
       i = 1;
   *(out + decimalcount + decimalPoint + i) = 0;

}

Answer 5

While you guys were answering I've come up with my own solution which that works better for my application and I figure I'd share. It doesn't convert the float to a string, but rather 8-bit integers. My range of numbers is very small (0-15) and always non-negative, so this will allow me to send the data over bluetooth to my android app.

//Assumes bytes* is at least 2-bytes long
void floatToBytes(byte_t* bytes, float flt)
{
  bytes[1] = (byte_t) flt;    //truncate whole numbers
  flt = (flt - bytes[1])*100; //remove whole part of flt and shift 2 places over
  bytes[0] = (byte_t) flt;    //truncate the fractional part from the new "whole" part
}
//Example: 144.2345 -> bytes[1] = 144; -> bytes[0] = 23

Answer 6

// convert float to string one decimal digit at a time
// assumes float is < 65536 and ARRAYSIZE is big enough
// problem: it truncates numbers at size without rounding
// str is a char array to hold the result, float is the number to convert
// size is the number of decimal digits you want


void FloatToStringNew(char *str, float f, char size)

{

char pos;  // position in string

    char len;  // length of decimal part of result

    char* curr;  // temp holder for next digit

    int value;  // decimal digit(s) to convert

    pos = 0;  // initialize pos, just to be sure

    value = (int)f;  // truncate the floating point number
    itoa(value,str);  // this is kinda dangerous depending on the length of str
    // now str array has the digits before the decimal

    if (f < 0 )  // handle negative numbers
    {
        f *= -1;
        value *= -1;
    }

     len = strlen(str);  // find out how big the integer part was
    pos = len;  // position the pointer to the end of the integer part
    str[pos++] = '.';  // add decimal point to string

    while(pos < (size + len + 1) )  // process remaining digits
    {
        f = f - (float)value;  // hack off the whole part of the number
        f *= 10;  // move next digit over
        value = (int)f;  // get next digit
        itoa(value, curr); // convert digit to string
        str[pos++] = *curr; // add digit to result string and increment pointer
    }
 }