how to use arduino PID library?

hi
i new arduino programming , want use pid library using motor position encoder, example provided on arduino pid website confusing
my current code :

#include <pid_v1.h>

//define variables we'll connecting to
double setpoint, input, output;
int counta=0;
int last_a;
float error_a, error_int_a, error_calc_a, d_a;
int forward = 9;
int backward = 7;
int pwm = 10;
int directionforward = 5;
int directionbackward = 2;
//specify links , initial tuning parameters
pid mypid(&input, &output, &setpoint,5,0,30, direct);

void setup()
{ //motota
  pinmode(pwm, output);
  pinmode(forward, output);
  pinmode(backward, output);
  pinmode(directionbackward , input);
  pinmode(directionforward , input);
  attachinterrupt(digitalpintointerrupt(directionbackward ), pulsecounta, rising);
  serial.begin(19200);

  //initialize variables we're linked to
  input = digitalread(5);
  setpoint = 100;
//  setoutputlimits(-255, 255);
  //turn pid on
  mypid.setmode(automatic);
}

void loop()
{

  input = digitalread(5);


  mypid.compute();
  analogwrite(10,output);
      digitalwrite(backward, high);
    digitalwrite(forward, low);
   serial.print("counta: "); serial.println(counta);

}

but code not performing needed, instead motor runs contieously position 6000 , not stop @ 100.
can please? or share code?
thanks

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