Arduino Anomalous Second Run on Serial - arduino

Question

When running the following program, it works as expected but adds an additional output for -38 as an input. It doesn't matter what input I use, it will print out for -38 as well.
int number;
void setup()
{
Serial.begin(9600);
}
void loop()
{
number=0;
Serial.flush();
while(Serial.available() == 0)
{
//just waiting while nothing entered
}
while (Serial.available()>0)
{
number = Serial.read() - '0';
// read the number in buffer and
//remove ASCII text for "0"
}
Serial.print("You entered: ");
Serial.println(number);
Serial.print(number);
Serial.print(" multiplied by two is ");
number = number * 2;
Serial.println(number);
}
Output looks like:
>You entered: 4
>4 multiplied by two is 8
>You entered: -38
>-38 multiplied by two is -76

Answer 1

Your problem might be the Serial.flush. As the documentation states, it
Waits for the transmission of outgoing serial data to complete. (Prior to Arduino 1.0, this instead removed any buffered incoming serial data.)
You seem to try to achieve the pre 1.0 behavior of flushing the incoming buffer. So basically Serial.flush does nothing in your sketch, which leads to the second run reading and calculating with the line feed character (which is 10 in ASCII).
You could clear the buffer like so:
while ( Serial.available() > 0 ) {
Serial.read();
}
But please note that this part
while (Serial.available()>0)
{
number = Serial.read() - '0';
// read the number in buffer and
//remove ASCII text for "0"
}
may not do exactly what you intend it to to. If for example you would send 12 via serial, it may only print the result for the last character (the 2). It would be equally possible for your program to only output the -38 result, because the last character always is a line feed. You basically just got lucky and the serial data wasn't sent fast enough for this to happen, but as soon as your loop gets longer it likely will happen.
I would rather suggest you moving the calculation part inside the loop too and checking the input data instead of flushing the buffer after the first character. Consider this:
while ( Serial.available() > 0 )
{
// read the number in buffer and
number = Serial.read();
// make sure the character is 0 - 9 in ASCII
if( number < '0' || number > '9' ){
// invalid character, skip
continue;
}
//remove ASCII text for "0"
number = number - '0';
Serial.print("You entered: ");
Serial.println(number);
Serial.print(number);
Serial.print(" multiplied by two is ");
number = number * 2;
Serial.println(number);
}
This would read your buffer byte by byte and would apply the multiplication to each (valid) character.