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.

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;

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.

[quote author=Martin Waller link=topic=63721.msg463358#msg463358 date=1307806974]
I've been looking around for a ftoa function.[/quote]
The avr-libc library used with Arduino has two such conversion functions: dtostre() for scientific notation and dtstrf() for fixed point.

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.

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).