Only assignment, call, increment, decrement, and new object expressions can be used as a statement

I am trying to flatten a 2D Array into a 1D Array and get this error when converting the values over via index.

 oneDimTable[x*tableA.GetLength(0)+y];

Assets/Timetable.cs(75,77): error CS0201: Only assignment, call, increment, decrement, and new object expressions can be used as a statement.

Here is the section with the error:

 for (int x = 0; x < tableA.GetLength(1); x++){
                  for (int y = 0; y < tableA.GetLength(0); y++){
                      String = tableA[x,y];
                      oneDimTable[x*tableA.GetLength(0)+y];
                  }
              }

Here is the whole script "Timetable":

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class Timetable : MonoBehaviour {
 
     public bool setupMode = true;
     int dayPosition;
     int lessonPosition;
     int realDay, realLesson;
     public Transform Lesson, Room, Teacher, Info, ConfirmPage;
     public GameObject LessonInfo, RoomInfo, TeacherInfo;
     string[,] tableA;
     string[,] tableB;
 
     // Use this for initialization
     void Start () {
         if(PlayerPrefs.GetInt("SetupMode") == 0){
             setupMode = false;
             tableA = PlayerPrefsX.GetStringArray("TableA");
         }
         tableA = new string[5,5];
         tableB = new string[5,5];
         dayPosition = 1;
         lessonPosition = 1;
         realDay = 1;
         realLesson = 1;
     }
     
     // Update is called once per frame
     void Update () {
         if(!setupMode){
             string info = tableA[realDay-1,realLesson-1];
             print (info);
             string[] infoSplit = info.Split(","[0]);
             print (info.Split(","[0]));
             LessonInfo.GetComponent<Text>().text = infoSplit[0];
             RoomInfo.GetComponent<Text>().text = infoSplit[1];
             TeacherInfo.GetComponent<Text>().text = infoSplit[2];
         }
     }
 
     void Submit (){
         string lesson = Lesson.FindChild("Text").GetComponent<Text>().text;
         Lesson.FindChild("Text").GetComponent<Text>().text = "";
 
         string room = Room.FindChild("Text").GetComponent<Text>().text;
         Room.FindChild("Text").GetComponent<Text>().text = "";
 
         string teacher = Teacher.FindChild("Text").GetComponent<Text>().text;
         Teacher.FindChild("Text").GetComponent<Text>().text = "";
 
         print ("Lesson: "+lesson+" Room: "+room+" Teacher: "+teacher);
 
         tableA[dayPosition-1,lessonPosition-1] = lesson+","+room+","+teacher;
         lessonPosition++;
         if(lessonPosition==6){
             lessonPosition=1;
             dayPosition++;
         }
         Info.GetComponent<Text>().text = "Day: "+dayPosition+"\nPeriod: "+lessonPosition;
         if(dayPosition==6 && lessonPosition==1){
             for (int x = 0; x < tableA.GetLength(1); x++){
                 for (int y = 0; y < tableA.GetLength(0); y++){
                     print(tableA[x,y]);
                 }
             }
             setupMode = false;
             PlayerPrefs.SetInt("SetupMode", 0);
             string[] oneDimTable = new string[tableA.GetLength(1)*tableA.GetLength(0)];
             string String;
             for (int x = 0; x < tableA.GetLength(1); x++){
                 for (int y = 0; y < tableA.GetLength(0); y++){
                     String = tableA[x,y];
                     oneDimTable[x*tableA.GetLength(0)+y];
                 }
             }
             PlayerPrefsX.SetStringArray("TableA", tableA);
         }
     }
 
     void Reset() {
         ConfirmPage.gameObject.SetActive(true);
     }
 
     void Cancel() {
         ConfirmPage.gameObject.SetActive(false);
     }
 
     void RealReset() {
         setupMode = true;
         PlayerPrefs.SetInt("SetupMode", 1);
         ConfirmPage.gameObject.SetActive(false);
     }
 }