Writing a float value to a file

Projects Programming
1

I am trying to write a float value to a file on a SD card. I don't understand why serial.println(float) on works, but file.println(float) does not.

If someone can explain this I'd be most grateful. I've tried looking at dtostrf, but honestly I cannot find it in the reference.

Thanks.

2

I don't have an SD shield. Can you define "doesn't work"? Do you mean "doesn't compile", or "compiles, but doesn't write to the file"?

3

It compiles but does not write any output.

4

Are you using the SD library? According to the File.println() reference page (SD - Arduino Reference), it doesn't support floating point numbers:

data (optional): the data to print (char, byte, int, long, or string)

5

dxw00d:
Are you using the SD library? According to the File.println() reference page (SD - Arduino Reference), it doesn't support floating point numbers:

data (optional): the data to print (char, byte, int, long, or string)

Yes, so whats the solution?

6

After reviewing floattostring.h threads the suggested method is to use:

String tempC = dtostrf(celsius, 10, 2, 10);

However this generates errors...

 error: invalid conversion from 'int' to 'char*'
 error: initializing argument 4 of 'char* dtostrf(double, signed char, unsigned char, char*)'
7

fmpc001:
After reviewing floattostring.h threads the suggested method is to use:

String tempC = dtostrf(celsius, 10, 2, 10);

Why would the suggested method be something that is wrong?

The parameters for dtostrf says that the last argument is a character-buffer. You passed a 10. The compiler is telling you that it expected the last argument to be a character array.

dtostrf(floatVar, minStringWidthIncDecimalPoint, numVarsAfterDecimal, charBuf);

So you probably wanted to do:
char buffer[14]; // 10 min string with, decimal, 2 characters after decimal, plus a \0
dtostrf(celsius, 10, 2, buffer);

Then print buffer.