Arduino turtle avoidance program

turtle1.ino

#include <Servo.h>
#include <Ultrasonic.h>
Servo servo;
Ultrasonic ultrasonic(13,12);
int l=0;
int r=0;
void forward() {
digitalWrite(5, 1);
digitalWrite(7, 1);
digitalWrite(6, 0);
digitalWrite(8, 0);
}
void back() {
digitalWrite(5, 0);
digitalWrite(7, 0);
digitalWrite(6, 1);
digitalWrite(8, 1);
}
void right() {
digitalWrite(5, 0);
digitalWrite(7, 1);
digitalWrite(6, 1);
digitalWrite(8, 0);
}
void left() {
digitalWrite(5, 1);
digitalWrite(7, 0);
digitalWrite(6, 0);
digitalWrite(8, 1);
}
void halt() {
digitalWrite(5, 0);
digitalWrite(7, 0);
digitalWrite(6, 0);
digitalWrite(8, 0);
}
void head_right() {
servo.write(0);
}
void head_center() {
servo.write(90);
}
void head_left() {
servo.write(180);
}
void setup() {
  // put your setup code here, to run once:
servo.attach(11);
pinMode(3, OUTPUT);
pinMode(5, OUTPUT);
pinMode(6, OUTPUT);
pinMode(7, OUTPUT);
pinMode(8, OUTPUT);
digitalWrite(3, 1);
servo.write(90);
}

void loop() {
  // put your main code here, to run repeatedly:
head_center();
forward();
while (ultrasonic.Ranging(CM) > 20){delay(10);}
halt();
head_left();
delay(500);
l = ultrasonic.Ranging(CM);
delay(500);
head_right();
delay(500);
r = ultrasonic.Ranging(CM);
delay(250);
if (l>r){head_left();left();}
else {head_right();right();}
delay(500);
halt();
delay(500);
}