need helpwith a pid control at a home care robot

hi at all, actually i´m working with the HCR Mobile Robot Platform from DFRobot with an Arduino Romeo.
https://www.dfrobot.com/product-361.htm ... gKOcPD_BwE

To control the robot in one way i found the following example:

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//The sample code for driving one way motor encoder
#include <PID_v1.h>
const byte encoder0pinA = 2;//A pin -> the interrupt pin 0
const byte encoder0pinB = 3;//B pin -> the digital pin 3
int E_left =5; //The enabling of L298PDC motor driver board connection to the digital interface port 5
int M_left =4; //The enabling of L298PDC motor driver board connection to the digital interface port 4
byte encoder0PinALast;
double duration,abs_duration;//the number of the pulses
boolean Direction;//the rotation direction
boolean result;

double val_output;//Power supplied to the motor PWM value.
double Setpoint;
double Kp=0.6, Ki=5, Kd=0;
PID myPID(&abs_duration, &val_output, &Setpoint, Kp, Ki, Kd, DIRECT);

void setup()
{
Serial.begin(9600);//Initialize the serial port
pinMode(M_left, OUTPUT); //L298P Control port settings DC motor driver board for the output mode
pinMode(E_left, OUTPUT);
Setpoint =80; //Set the output value of the PID
myPID.SetMode(AUTOMATIC);//PID is set to automatic mode
myPID.SetSampleTime(100);//Set PID sampling frequency is 100ms
EncoderInit();//Initialize the module
}

void loop()
{
advance();//Motor Forward
abs_duration=abs(duration);
result=myPID.Compute();//PID conversion is complete and returns 1
if(result)
{
Serial.print("Pluse: ");
Serial.println(duration);
duration = 0; //Count clear, wait for the next count
}


}

void EncoderInit()
{
Direction = true;//default -> Forward
pinMode(encoder0pinB,INPUT);
attachInterrupt(0, wheelSpeed, CHANGE);
}

void wheelSpeed()
{
int Lstate = digitalRead(encoder0pinA);
if((encoder0PinALast == LOW) && Lstate==HIGH)
{
int val = digitalRead(encoder0pinB);
if(val == LOW && Direction)
{
Direction = false; //Reverse
}
else if(val == HIGH && !Direction)
{
Direction = true; //Forward
}
}
encoder0PinALast = Lstate;

if(!Direction) duration++;
else duration--;

}
void advance()//Motor Forward
{
digitalWrite(M_left,LOW);
analogWrite(E_left,val_output);
}
void back()//Motor reverse
{
digitalWrite(M_left,HIGH);
analogWrite(E_left,val_output);
}

void Stop()//Motor stops
{
digitalWrite(E_left, LOW);
}
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my problem is, that i want to control the robot not only in one way, rather in two. so i want to switch between function "advance()", "reverse()" and "stop()", maybe with a switch-case instruction.

I tried, too, but the regulation does not work as soon as I try it this way:

void loop()
{

if (Serial.available()){

if (x =='1'){
advance();//Motor Forward
abs_duration=abs(duration);
result=myPID.Compute();//PID conversion is complete and returns 1
if(result)
{
Serial.print("Pluse: ");
Serial.println(duration);
duration = 0; //Count clear, wait for the next count
}
}
if (x =='2'){
reverse();//Motor Backyard
abs_duration=abs(duration);
result=myPID.Compute();//PID conversion is complete and returns 1
if(result)
{
Serial.print("Pluse: ");
Serial.println(duration);
duration = 0; //Count clear, wait for the next count
}
}

}
}

Please help!