@Anno I didn't modify the code. My teacher did (I think he had no idea what he was doing). So anyways, I asked around, and people told me to use the newping library. So I followed the instructions and I came up with the following, but it won't work. What did I do wrong?

#include <NewPing.h>
#define SensorN 2
#define max_dis 5
#define PING_INTERVAL 33
const int motor=9;
volatile int average_dis=0;

unsigned long pingTimer[SensorN];
unsigned int cm[SensorN];
uint8_t currentSensor = 0;

NewPing sonar[SensorN] = {

I get the following:
ultrasonic:6: error: expected `)' before ';' token
ultrasonic:6: error: expected unqualified-id before ')' token
ultrasonic:6: error: expected unqualified-id before ')' token
ultrasonic.ino: In function 'void loop()':
ultrasonic:13: error: 'sen2' was not declared in this scope
ultrasonic:21: error: 'distance' was not declared in this scope
ultrasonic:27: error: expected `)' before ';' token
ultrasonic:27: error: expected primary-expression before ',' token
ultrasonic:27: error: expected `;' before ')' token

#define maxdistance 20 //TODO: replace this with the maximun CM your sensor can read; look at the specs
#define minspeed 60 //TODO: replace this with the minimum speed of your motor
#define tolerance 8 //TODO: replace this with the toleramce divided by 2; higher == small movements will be ignored
#include <NewPing.h>
int motorPin = 9, trigPin1 = 13, echoPin1 = 12, trigPin2 = 5, echoPin2 = 4, postdist1 = 112, postdist2 = 112;
NewPing sen1(trigPin1, echoPin1, maxdistance), sen2(trigPin2, echoPin2, maxdistance);

so right now, this is what I got: #include <NewPing.h>
#define maxdistance 10 //TODO: replace this with the maximun CM your sensor can read; look at the specs
#define minspeed 120 //TODO: replace this with the minimum speed of your motor
#define tolerance 8 //TODO: replace this with the toleramce divided by 2; higher == small movements will be ignored

int motorPin = 9, trigPin1 = 13, echoPin1 = 12, trigPin2 = 5, echoPin2 = 4, postdist1 = 112, postdist2 = 112;
NewPing sen1(trigPin1, echoPin1, maxdistance), sen2(trigPin2, echoPin2, maxdistance);

#define maxdistance 20 //TODO: replace this with the maximun CM your sensor can read; look at the specs
#define minspeed 60 //TODO: replace this with the minimum speed of your motor
#define maxswipe 50 //TODO: replace this with the maximum speed change per swipe out of 256
#include <NewPing.h>
int motorPin = 9, trigPin1 = 13, echoPin1 = 12, trigPin2 = 5, echoPin2 = 4, postspeed = 0;
NewPing sen1(trigPin1, echoPin1, maxdistance), sen2(trigPin2, echoPin2, maxdistance);

void setup() {}

#define maxdistance 20 //TODO: replace this with the maximun CM your sensor can read; look at the specs
#define minspeed 60 //TODO: replace this with the minimum speed of your motor
#define maxswipe 50 //TODO: replace this with the maximum speed change per swipe out of 256
#include <NewPing.h>
int motorPin = 9, trigPin1 = 13, echoPin1 = 12, trigPin2 = 5, echoPin2 = 4, postspeed = 0;
NewPing sen1(trigPin1, echoPin1, maxdistance), sen2(trigPin2, echoPin2, maxdistance);

void setup() {}