Arduino String to Float: Find out how to do it Easily. Is there a simple function that does this for you? How to convert strings to floats using c-style or C++ strings techniques.


Arduino String to Float: One common task when programming an Arduino is converting string data received from the serial port into numeric values that can be used in calculations or comparisons.

This tutorial will explain how to convert Arduino strings to floats - decimal numbers that allow for fractional values.
arduino float to string - how to do it easily with code examples

Converting a string floating point number is not for the faint hearted (floats are complicated) so the easiest way to do it is to use a pre-defined function.

Arduino floating point numbers are usually represented using 32bits when you use 8 bit processors; This is variable of type 'float' . For example Uno and Nano use 32bits.

When you use a 32bit processor you'll probably get 64 bits (this is type 'double'). In any case if you use the type double, for an 8 bit Arduino it is automatically changed to type float (32bit).

C++ Arduino String to float function

The simplest way to convert an Arduino string to a float when you use C++ Strings is to use the built-in stringToFloat() function. This takes a string as a parameter and returns the corresponding float value.

String input = "3.14";
float number;
number = stringToFloat(input);

In this C++ example, we start with a string "3.14" and assign it to the input variable, then declare a float called number to hold the output. The stringToFloat() function converts the string and stores the result (3.14) in variable 'number'.

C-style Arduino String to float function

For C coding in Arduino, you can use the standard C library function atof() to perform the same conversion. The function atof() stands for ascii to float meaning  "string to float" and works just like stringToFloat() except it takes a char pointer rather than a String object.

char input[] = "3.14";
float number;
number = atof(input);

This C example is nearly identical, but declares the input as a char array rather than a String. It then uses atof() to convert the char array to a float.

Handling Invalid Conversion

It's important to consider what happens if the string cannot be converted to a valid float value. Both stringToFloat() and atof() will return 0 in this case rather than producing an error. You may want to add validation to check for this outcome.

float number = stringToFloat(input);
if(number == 0){
  // conversion failed
}

By checking if the result is 0, this allows your code to handle invalid conversions gracefully rather than having unexpected behavior from bad input data.

Example Sketch - User Input with C++ strings

A common scenario is converting input from a the serial port interface to a float. The following demonstrates getting a string from Serial and validating the conversion.

String input;

float number;

if(Serial.available()>0){

  input = Serial.readString();
  number = stringToFloat(input);

  if(number != 0){
   // use converted number
  } else {
   // invalid input
  }
}

By reading the string, converting, and checking for errors, this allows safe conversion of dynamic user input to numeric values your program can understand.

Example Sketch - c-style string input

For instance you might have a PID project that requires three parameters kd, kp and ki. The following program parses the input string, and assigns the value to each floating point variable

// Input buffer to hold all serial data
char inputBuffer[50];

// PID variables
float kp, ki, kd;

void setup() {
  Serial.begin(115200);
}

void loop(){

  // Check for serial data
  if(Serial.available() > 0){
    // Get input string
    Serial.readBytesUntil('\n', inputBuffer, 50);

    char* token = strtok(inputBuffer, " ,");

    while(token != NULL){

      if(strcmp(token, "kp") == 0){
        token = strtok(NULL, " ,");
        kp = atof(token);
      }

      else if(strcmp(token, "kd") == 0){  
        token = strtok(NULL, " ,");
        kd = atof(token);
      }

      else if(strcmp(token, "ki") == 0){
        token = strtok(NULL, " ,");
        ki = atof(token);
      }

      token = strtok(NULL, " ,");
    }

    // Output values
    Serial.print("kp: "); Serial.println(kp,3);
    Serial.print("kd: "); Serial.println(kd,3);
    Serial.print("ki: "); Serial.println(ki,3);
  }
}

Entering the string "kp 1.0, kd 2.0, ki 3.0" results in the output:

kp: 1.000
kd: 2.000
ki: 3.000

You can also enter only on parameter. For instance entering the string "kd 0.24" results in output:

kp: 1.000
kd: 0.240
ki: 3.000

Conclusions

This tutorial covered the essential basics of converting Arduino strings to floats using either stringToFloat() or atof(). Being able to read text string inputs to and convert them to numeric values allows you to get floating point values into your project code without re-compiling it each time!


Written by John Main, who has a degree in Electronic Engineering.


Comments

Have your say about what you just read! Leave me a comment in the box below.

Don’t see the comments box? Log in to your Facebook account, give Facebook consent, then return to this page and refresh it.


Privacy Policy | Contact | About Me

Site Map | Terms of Use