SparkFun Forums

[sspence65]

What is the proper syntax for saving the scale output in a variable?

float value;

Serial.print(scale.getGram(), 1); <—– WORKS

value = (scale.getGram(), 1) ;

Serial.println(value); <—– NO GOOD! DISPLAYS: “1.00”

value = (scale.getGram(), 2) ;

Serial.println(value); <—– NO GOOD! DISPLAYS: “2.00”

[DanV]

I can't tell what you're programming platform is (or even what control you're using) but, is the syntax for the assignment
:=
and not just
=
??

[jremington]

Code: Select all

value = (scale.getGram(), 1) ;

The result is expected and correct.
Read up on the C "comma operator" here: https://en.wikipedia.org/wiki/Comma_operator

[sspence65]

The use of the comma token as an operator is distinct from its use in function calls and definitions, variable declarations, enum declarations, and similar constructs, where it acts as a separator.

This is being used as a separator in a function, to notify the function how many decimal points to return. In this case one, so it returns 10.5 grams as a float.

In arduino C/C++, = is used as an assignment, but it's assigning 10.0, not 10.5 to the variable.

[jremington]

In arduino C/C++, = is used as an assignment, but it's assigning 10.0, not 10.5 to the variable.

The "but" and the "10.0" in this statement suggests that you haven't yet figured it out, so here is a little program to try:

Code: Select all

void setup() {
  Serial.begin(9600);
}

// pretend to be a sensor
float sensor(void) {return 12.3;}

void loop() {

  float sensorValue = (sensor(), 1);
  Serial.println(sensorValue);

  sensorValue = sensor();
  Serial.println(sensorValue);

  delay(1000);
}

[sspence65]

In your example:

float sensorValue = (sensor(), 1);
Serial.print(sensorValue);
outputs 1.00

and

sensorValue = sensor();
Serial.println(sensorValue);
outputs 12.30

but your routine is not accepting ,1 as a designation how many decimal places sensor() is supposed to return. The library for the HX711 takes that input and passes back 12.3, or 12.35 as a result.

I know this because if I pass ,1 I get 12.3, and If I pass ,2, I get 12.35. The library documentation describes this.

[jremington]

You are still confused.
The following line calls scale.getGram with NO parameters, the value returned by scale.getGram() is discarded and the comma operator returns a 1, which is finally stored in value.

Code: Select all

value = (scale.getGram(), 1) ;

[sspence65]

In my original sketch,

Serial.print(scale.getGram(), 1);

returns 12.3

and

Serial.print(scale.getGram(), 2);

returns 12.35

[sspence65]

Ok, I'm getting it now. that ,1 or ,2 is a instruction to Serial.print how many decimals to print. I need to drop that off, and use

value = scale.getGram();
Serial.println(value);

[jremington]

In the following line the ",1" is a parameter to Serial.print(), which specifies how many decimal places to print.

Code: Select all

Serial.print(scale.getGram(), 1);

The ",1" does nothing to the value itself.

[sspence65]

I got that. Thanks.