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[on:]

I'm trying to concatenate a string and a floating point number to send to the serial monitor and the only way I can find to do it is using dtostrf(), but that seems a little clumsy.
For example (this works):

Code:

  int potValue = analogRead(pinSensor);
  float volts = 5*(float)potValue/1023;
  String strLab = "Pot Voltage = ";
  char tmp[10];
  dtostrf(volts,1,2,tmp);
  String strOut = strLab + tmp;

  Serial.println(strOut);

This doesn't work:

Code:

  int potValue = analogRead(pinSensor);
  float volts = 5*(float)potValue/1023;
  char tmp[10];
  dtostrf(volts,1,2,tmp);

  Serial.println("Pot Voltage = " + tmp);
 
  BlinkLED(pinLED,potValue,potValue);

nor does:

Code:

  int potValue = analogRead(pinSensor);
  float volts = 5*(float)potValue/1023;
  char tmp[10];
  dtostrf(volts,1,2,tmp);
  String strOut = "Pot Voltage = " + tmp;

  Serial.println(strOut);

both give the error:  invalid operands of types 'const char [15]' and 'char [10]' to binary 'operator+'

What I'd really like to do is somthing like:

Code:

  int potValue = analogRead(pinSensor);
  float volts = 5*(float)potValue/1023;

  Serial.println("Pot Voltage = " + String(Volts));

but String doesn't work with floating point numbers. 

I've looked at the String constructor tutorial, the String Addition Examples and googled this as well to no avail.  Any suggestions?
Thanks,
Jason

[Reply #1 on:]

Any suggestions?

Use null terminated character arrays instead of Strings and use sprintf to "concatenate"

[Reply #2 on:]

Code:

char voltageMsg[25];
sprintf(voltageMsg, "Pot Voltage = %f", volts);
Serial.println(voltageMsg);

[Reply #3 on:]

try this,

Code:

int potValue = analogRead(pinSensor);
float volts = 5*(float)potValue/1023;

char tmp[25] = "Pot Voltage = ";

dtostrf(volts,1,2, &tmp[12]);

Serial.println(tmp);

The char array is declared big enough to hold the fixed string and the float.  (25)
The whole trick is to let the dtostrf add to the tmp array at the right position. (you might need to change 12 in 13 or so)

give it a try.

[Reply #4 on:]

use sprintf to "concatenate"

Great idea, except that sprintf() on the Arduino does not deal with floating point values.

OP: Why do you feel it is necessary to concatenate the data before sending it?

Code:

Serial.print("Pot Voltage = ");
Serial.print(volts);

The device on the other end of the serial port will not know that two Serial calls were made to send the data.

[Reply #5 on:]

The device on the other end of the serial port will not know that two Serial calls were made to send the data.

For Serial you are 100% right. however if you use the print on a TCP socket, multiple calls might cause fragmentation of IP packets.

[Reply #6 on:]

use sprintf to "concatenate"

Great idea, except that sprintf() on the Arduino does not deal with floating point values.

Didn't realize that, thanks.

[Reply #7 on:]

try this,

Code:

int potValue = analogRead(pinSensor);
float volts = 5*(float)potValue/1023;

char tmp[25] = "Pot Voltage = ";

dtostrf(volts,1,2, &tmp[12]);

Serial.println(tmp);

The char array is declared big enough to hold the fixed string and the float.  (25)
The whole trick is to let the dtostrf add to the tmp array at the right position. (you might need to change 12 in 13 or so)

give it a try.

I'll give this a shot--I wonder if it'd be possible to write a wrapper function for dtostrf so that you could just use it in-line. 
Is there a reason that adding the string literal and variable (see my first post for code) doesn't work?  seems like it should from the string constructor tutorial.

[Reply #8 on:]

String strOut = "Pot Voltage = " + tmp;

does not work as the addition of two char[] is not defined..

String strOut = String("Pot Voltage = ") + String(tmp); 

might work. String("not tried"); 

[Reply #9 on:]

I'm trying to concatenate a string and a floating point number to send to the serial monitor  ...

... however if you use the print on a TCP socket ...

Which, fortunately, we don't have to worry about.