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Topic: ftoa (Read 2185 times) previous topic - next topic

Martin Waller

Jun 11, 2011, 05:42 pm Last Edit: Jun 11, 2011, 06:58 pm by Martin Waller Reason: 1
I've been looking around for a ftoa function. The ones that I found seemed to fail for numbers such as 0.05, you'd get 0.5, it would loose any leading zeros in the fraction. Below is a version that should fix this.

Code: [Select]

char *ftoa(char *buffer, double d, int precision) {

long wholePart = (long) d;

// Deposit the whole part of the number.


// Now work on the faction if we need one.

if (precision > 0) {

// We do, so locate the end of the string and insert
// a decimal point.

char *endOfString = buffer;
while (*endOfString != '\0') endOfString++;
*endOfString++ = '.';

// Now work on the fraction, be sure to turn any negative
// values positive.

if (d < 0) {
d *= -1;
wholePart *= -1;

double fraction = d - wholePart;
while (precision > 0) {

// Multipleby ten and pull out the digit.

fraction *= 10;
wholePart = (long) fraction;
*endOfString++ = '0' + wholePart;

// Update the fraction and move on to the
// next digit.

fraction -= wholePart;

// Terminate the string.

*endOfString = '\0';

   return buffer;

Martin Waller

Sorry, I developed this in Microsoft C++ Express, you will need to remove the leading _ on the call to itoa! Sorry!


You still should be able to click modify and alter your post.

Also please use the [code ][/code ] tags


Hi Martin,
Can you give an example of how to use?

Yours Simon M.


Jun 15, 2011, 05:11 pm Last Edit: Jun 15, 2011, 05:46 pm by davekw7x Reason: 1

I've been looking around for a ftoa function.

The avr-libc library used with Arduino has two such conversion functions: dtostre() for scientific notation and dtstrf() for fixed point.

Quote from: Martin Waller
...Below is a version...

I would like to see rounded values in the output.

So, if I had a number, say, 0.014567 and I wanted three decimal places I think it should be 0.015.  With four decimal places I would expect to see 0.0146. Stuff like that.

Code: [Select]
void setup()
   float x = 0.014567;
   Serial.println("x was set to 0.014567");
   Serial.println("Here are results with ftoa, dtostrf and dtostre");

   char buffer[30]; // Could be smaller, but...
   for (int i = 0; i < 7; i++) {

       ftoa(buffer, x, i);
       Serial.print("  ");
       dtostrf(x, i+4, i, buffer); // avr-libc function for floats
       Serial.print("  ");
       dtostre(x, buffer, i, NULL); // avr-libc function for scientific notation


void loop() {}

// Your ftoa goes here


x was set to 0.014567
Here are results with ftoa, dtostrf and dtostre

0     0  1e-02
0.0    0.0  1.5e-02
0.01    0.01  1.46e-02
0.014    0.015  1.457e-02
0.0145    0.0146  1.4567e-02
0.01456    0.01457  1.45670e-02
0.014566    0.014567  1.456700e-02

Bottom line: It's interesting, but, as a general purpose conversion routine it has a few problems other than my preference for rounding.  What happens if you give it x = 1234567.0?  That one is pretty easy to fix, I'm thinking (don't forget to test -1234567.0 also).



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