How to sprintf a float with Arduino

Arduino has a small limitation in sprintf function that does not print correctly float numbers.
The following test sketch demonstrates the problem

void setup()
{
  Serial.begin(115200);
}
 
void loop()
{
  float f = 123.12F;
  Serial.print("Original value: ");
  Serial.println(f);

  char str[50];
  sprintf(str, "String value: %f", f);
  Serial.println(str);
  
  Serial.println();
  delay(2000);
}

You can see from the screenshot below that the float is always formatted a question mark '?'.

Using integer trick

There is a little trick to correctly print the float value using sprintf.

sprintf(str, "String value: %d.%02d", (int)f, (int)(f*100)%100);

This trick works well only with positive numbers and formats the float with two fixed decimals.

Using dtostrf function

A better solution is to use the dtostrf function. It it is more flexible and works well also with negative numbers.
These two lines of code will write a string into the buffer with strcpy function and the append the float value using the dtostrf function.

strcpy(str, "String value using dtostrf: ");
dtostrf(f, 2, 2, &str[strlen(str)]);