Hello,
Right now, I'm trying to use this program to translate pen movement to computer strings of text. I'm very experienced with Processing but pretty new to the Arduino, so I apologize if this is a noob question that's really dumb 
My problem is the arduino file the program uses for input. Whenever a physical button is pressed, the program sends data that a python script uses that tracks the movement of an accelerometer. I'd like to instead use just a key on a keyboard instead of a physical button, and thus, I'm trying to change the code to allow for this.
My idea was to use Processing that sends a '1' as a char to the Arduino which translates to LOW in the program, and '0' which signifies HIGH. This was pretty easy to make in processing and the logic is just
 if (keyPressed && isPressed != "1") {
 isPressed = "LOW";
 port.write(isPressed);
 println("LOW");
 }
 else if (!keyPressed && isPressed != "0") {
 isPressed = "HIGH";
 port.write(isPressed);
 println("HIGH");
 }
The problem is, since I'm using both a Python script for the data, but I'm also trying to send data through the Processing program, I'm getting the error that the serial port is busy. I tried to use a SoftwareSerial port for processing that mimics wires at port 9, but I'm not sure if I understood it properly as Processing couldn't find any COM ports besides the normal Arduno one. I'm guessing I'm probably fundamentally not understanding it. I'm kind of confused on what I should do to replace a physical button in the code with keyboard strokes instead, so any help is appreciated.
Here's the Arduino code :
// AUTHOR: Federico Terzi
// Arduino sketch for the gesture controlled keyboard.
// This sketch is the BASIC version, and does not include Bluetooth capabilities,
// If you want Bluetooth, use the normal version instead.
// Part of this sketch is taken from an example made by JohnChi
#include <Wire.h>
// Pins used for I/O
int btnPin1 = 9;
// I2C address of the MPU-6050
const int MPU_addr=0x68;
// Variables that will store sensor data
int16_t AcX,AcY,AcZ,Tmp,GyX,GyY,GyZ;
// Status variables, used with buttons
int precBtn1 = HIGH;
void setup(){
 // Set the pin mode of the buttons using the internal pullup resistor
 pinMode(btnPin1, INPUT_PULLUP);
 // Start the comunication with the MPU-6050 sensor
 Wire.begin();
 Wire.beginTransmission(MPU_addr);
 Wire.write(0x6B); // PWR_MGMT_1 register
 Wire.write(0);  // set to zero (wakes up the MPU-6050)
 Wire.endTransmission(true);
 // Start the serial communication
 Serial.begin(38400);
}
void loop(){
 // Read the values of the buttons
 int resultBtn1 = digitalRead(btnPin1);
 // ON btn1 pressed, start the batch and light up the yellow LED
 if (precBtn1 == HIGH && resultBtn1 == LOW)
 {
  startBatch();
 }
 // If the btn1 is pressed, reads the data from the sensor and sends it through the communication channel
 if (resultBtn1==LOW)
 {
  // Start the transmission with the MPU-6050 sensor
  Wire.beginTransmission(MPU_addr);
  Wire.write(0x3B); // starting with register 0x3B (ACCEL_XOUT_H)
  Wire.endTransmission(false);
  Wire.requestFrom(MPU_addr,14,true); // request a total of 14 registers
 Â
  // Reads the data from the sensor
  AcX=Wire.read()<<8|Wire.read(); // 0x3B (ACCEL_XOUT_H) & 0x3C (ACCEL_XOUT_L) Â
  AcY=Wire.read()<<8|Wire.read(); // 0x3D (ACCEL_YOUT_H) & 0x3E (ACCEL_YOUT_L)
  AcZ=Wire.read()<<8|Wire.read(); // 0x3F (ACCEL_ZOUT_H) & 0x40 (ACCEL_ZOUT_L)
  Tmp=Wire.read()<<8|Wire.read(); // 0x41 (TEMP_OUT_H) & 0x42 (TEMP_OUT_L)
  GyX=Wire.read()<<8|Wire.read(); // 0x43 (GYRO_XOUT_H) & 0x44 (GYRO_XOUT_L)
  GyY=Wire.read()<<8|Wire.read(); // 0x45 (GYRO_YOUT_H) & 0x46 (GYRO_YOUT_L)
  GyZ=Wire.read()<<8|Wire.read(); // 0x47 (GYRO_ZOUT_H) & 0x48 (GYRO_ZOUT_L)
  Serial.print("START");
  Serial.print(" "); Serial.print(AcX);
  Serial.print(" "); Serial.print(AcY);
  Serial.print(" "); Serial.print(AcZ);
  Serial.print(" "); Serial.print(GyX);
  Serial.print(" "); Serial.print(GyY);
  Serial.print(" "); Serial.print(GyZ);
  Serial.println(" END");
 }
 // Closes the batch when the button is released
 if (precBtn1 == LOW && resultBtn1 == HIGH)
 {
  closeBatch();
 }
 // Saves the button states
 precBtn1 = resultBtn1;
}
// Sends the started batch signal
void startBatch()
{
 Serial.println("STARTING BATCH");
}
// Sends the closed batch signal
void closeBatch()
{
 Serial.println("CLOSING BATCH");
}