String concatenation with arduino - arduino

Question

I have a very short example program running on an Arduino Fio device. This program is sending serial data. A connected Xbee router device is receiving this data and sending it to a Xbee coordinator device connected to my notebook. The program also reading data from the serial port. I can send a 1 or a 0 to switch the LED of the Fio device on or off.
Switching the LED on or off by sending a 1 or a 0 from a terminal on my notebook is working well.
But when I try to read the data which the Fio device is sending, I get this:
his direction works
is direction works
s direction works
direction works
direction works
irection works
rection works
ection works
ction works
... and so on.
But I'm expecting a string ("This direction works " + counter++;) as you can see in the following code example.
Here is the short Arduino sketch:
int incomingByte = 0; // for incoming serial data
int counter = 0;
void setup()
{
Serial.begin(57600);
pinMode(13,OUTPUT);
// blink twice at startup
digitalWrite(13, LOW);
delay(1000);
digitalWrite(13, HIGH); // first blink
delay(50);
digitalWrite(13, LOW);
delay(200);
digitalWrite(13, HIGH); // second blink
delay(50);
digitalWrite(13, LOW);
}
void loop()
{
// send data only when you receive data:
if (Serial.available() > 0)
{
// read the incoming byte:
incomingByte = Serial.read();
if(incomingByte == '0')
{
digitalWrite(13, LOW);
}
else if(incomingByte == '1')
{
digitalWrite(13, HIGH);
}
// say what you got:
Serial.print("Fio received: ");
Serial.write(incomingByte); // Arduino 1.0 compatibility
Serial.write(10); // send a line feed/new line, ascii 10
}
else
{
String sendData = "This direction works " + counter++;
Serial.println(sendData);
delay(1500);
}
}
What I'm doing wrong? Why do I don't get:
This direction works 0
This direction works 1
This direction works 2
This direction works 3
... and so on?

Answer 1

As #Elric and #Olaf mentioned in the comments, it is not possible to do that like in my posted code.
After reading the documentation again I found an "explanation":
Caution: You should be careful about concatenating multiple variable
types on the same line, as you may get unexpected results. For
example:
int sensorValue = analogRead(A0); String stringOne = "Sensor
value: "; String stringThree = stringOne + sensorValue;
Serial.println(stringThree);
results in "Sensor Value: 402" or whatever the analogRead() result is,
but
int sensorValue = analogRead(A0); String stringThree = "Sensor
value: " + sensorValue; Serial.println(stringThree);
gives unpredictable results because stringThree never got an initial
value before you started concatenating different data types.
Firstly you have to initialize the String variable to assign two concatenated strings to it.