How to set float to variable with 7 digits after the decimal point in arduino?

Question

Example:

float x = 3154681.124 / 100000; //x = 31.54;

I want x to be:

x = 31.5468112;

• Edgar Bonet: When you write 3154681.124 / 100000, you are accumulating two rounding errors: first, in representing the numerator as a float, then in the division. Total error ≈ 1.09e-6. If you write instead 31.54681124 you have a single rounding error, and the total error is 25% lower.
• Dave X: An IEEE 32 bit float has only 23 bits for the fractional part, so you only get about log(2^23)/log(10)=6.9 decimal places total. Maybe you want a double?

Answer 1

The float type has slightly over 7 digits of precision. See https://en.wikipedia.org/wiki/Floating_point.

I want x to be:

 x = 31.5468112;

Bad luck. That's 9 digits of precision. You can get around 7 digits by converting it appropriately, eg. as Majenko said:

 float x = 3154681.124 / 100000; //x = 31.54;
 Serial.println(x, 7);

However that printed:

31.5468101

It got 7 digits right, as advertised.

---

You can use the BigNumber Library that I wrote. Available from GitHub.

Using that, you can get all the precision you want, within reason:

#include "BigNumber.h"
void setup ()
 {
 Serial.begin (115200);
 Serial.println ();
 BigNumber::begin (7); // 7 digits after the decimal place
 BigNumber x ("3154681.124");
 BigNumber y = x / BigNumber ("100000");
 
 Serial.println(y);
 } // end of setup
void loop ()
 {
 } // end of loop

Output:

31.5468112

Answer 2

float x = 3154681.124 / 100000; //x = 31.54;

x is now 31.5468112 (or close to).

Serial.println(x);

Output: 31.54

Why? Because Serial.println defaults to 2 decimal places for floats.

Syntax

Serial.println(val)

Serial.println(val, format)

Parameters

val: the value to print - any data type

format: specifies the number base (for integral data types) or number of decimal places (for floating point types)

So the solution: specift 7 decimal places when you print:

Serial.println(x, 7);

• Amir Hamdy: I don't want to print x , I want to send it as a parameter in URL to web service. when I send it as x its value became 31.54
• Majenko: Then whatever your code is that is doing the sending is truncating it. I can't see that code, so I can't advise on it. The simple fact, though, is a float doesn't have a fixed number of decimal places, that's not how floats work.
• Nick Gammon: when I send it as x its value became 31.54 - you need to post that code. We can't answer questions about code we can't see.