Mathf.PingPong

Description

PingPongs the value t, so that it is never larger than length and never smaller than 0.

The returned value will move back and forth between 0 and length.

function Update () {
    // Set the x position to loop between 0 and 3
    transform.position = Vector3(
            Mathf.PingPong(Time.time, 3), transform.position.y, transform.position.z);
}

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    void Update() {
        transform.position = new Vector3(Mathf.PingPong(Time.time, 3), transform.position.y, transform.position.z);
    }
}