如何使用Arduino的Serial.println打印带有浮点数的字符串

Question

I've been printing a combination of strings and numbers with Arduino's Serial.println() like:

Serial.println(((String)"some value: " + some_value);

And this seems to work for most types, but fails for floats, which give me the compilation error:

ambiguous overload for 'operator+' (operand types are 'String' and 'float')

I've tried different calls like:

Serial.println(((String)"some float:" + ((String)some_float));
Serial.println(((String)"some float: " + String(some_float));
Serial.println(((String)"some float: " + String(some_float, 2));

but they give similar errors. Arduino's docs on String seem to imply that these should work. What am I doing wrong?

Update

After correcting my original errors, I now have this issue:

Serial.println(String("some value: ") + String(some_value));
>> call of overloaded ‘String(float&)’ is ambiguous

I also tried:

Serial.println(String("some float: ") + String(some_float, 2));
>> call of overloaded ‘String(float&, int)’ is ambiguous

Answer 1

First, you have an extra open bracket in all of your tests.

Next, as the error says, you are attempting to add a float value to a String type. The docs say the + operator only supports adding two String types together.

You'll need to convert your float into a String then add the two Strings together. There is an example of this on the doc page you linked:

String stringOne = String(5.698, 3); // using a float and the decimal places

The last example you gave that gives 'similar errors' looks like it should work when you remove the extra open bracket, according to the docs.

EDIT: in reference to the new error message ">> call of overloaded 'String(float&)' is ambiguous" can you paste some additional code to show at least: - the definitions of some_float and some_value - any namespaces you are including (eg std etc) Generally with these sorts of problems, you should reduce the code to the simplest example which shows the error and post that whole example in the thread. (Quite often when you reduce it to the simplest example, you learn what the problem is anyway!)

The error says that String(float&) is ambiguous , so I'm guessing you have another namespace with a String definition in it which is conflicting with the Arduino version.

Answer 2

It should be something else. I tried now with an Arduino UNO this code:

float some_float;

void setup() {
  Serial.begin(9600);
  some_float = 13.3558;
}

void loop() {
  Serial.println(String("some float: ") + String(some_float, 2));
  delay(1000);
}

and the output is

some_float: 13.35
some_float: 13.35
some_float: 13.35
...