Undefined reference to class::function

[Univcplus]

Hello Everyone, I am getting an error in my code and I don't know why.
The error is

sh-4.3$ g++ -std=c++11 -o main *.cpp                                                                                                                                                                                       
/tmp/ccH7pMMe.o: In function `main':                                                                                                                                                                                       
main.cpp:(.text+0x4c2): undefined reference to `NumberList::mergeArray(double*, int)'                                                                                                                                      
collect2: error: ld returned 1 exit status  

My code is below:

#include <iostream>  // For cout

using namespace std;
class NumberList
{
private:
   // Declare a structure for the list
   struct ListNode
   {
      double value;           // The value in this node
      struct ListNode *next;  // To point to the next node
   };

   ListNode *head;            // List head pointer

public:
   // Constructor
   NumberList()
      { head = nullptr; }
void mergeArray(double[],int);
   // Destructor
   ~NumberList();

   // Linked list operations
   void appendNode(double);
   void insertNode(double);
   void deleteNode(double);
   void displayList() const;

   //Added for assignment



};



//**************************************************
// appendNode appends a node containing the        *
// value pased into num, to the end of the list.   *
//**************************************************



void NumberList::appendNode(double num)
{
   ListNode *newNode;  // To point to a new node
   ListNode *nodePtr;  // To move through the list

   // Allocate a new node and store num there.
   newNode = new ListNode;
   newNode->value = num;
   newNode->next = nullptr;

   // If there are no nodes in the list
   // make newNode the first node.
   if (!head)
      head = newNode;
   else  // Otherwise, insert newNode at end.
   {
      // Initialize nodePtr to head of list.
      nodePtr = head;

      // Find the last node in the list.
      while (nodePtr->next)
         nodePtr = nodePtr->next;

      // Insert newNode as the last node.
      nodePtr->next = newNode;
   }
}

//**************************************************
// displayList shows the value                     *
// stored in each node of the linked list          *
// pointed to by head.                             *
//**************************************************

void NumberList::displayList() const
{
   ListNode *nodePtr;  // To move through the list

   // Position nodePtr at the head of the list.
   nodePtr = head;

   // While nodePtr points to a node, traverse
   // the list.
   while (nodePtr)
   {
      // Display the value in this node.
      cout << nodePtr->value << endl;

      // Move to the next node.
      nodePtr = nodePtr->next;
   }
}

//**************************************************
// The insertNode function inserts a node with     *
// num copied to its value member.                 *
//**************************************************

void NumberList::insertNode(double num)
{
   ListNode *newNode;					// A new node
   ListNode *nodePtr;					// To traverse the list
   ListNode *previousNode = nullptr;	// The previous node

   // Allocate a new node and store num there.
   newNode = new ListNode;
   newNode->value = num;

   // If there are no nodes in the list
   // make newNode the first node
   if (!head)
   {
      head = newNode;
      newNode->next = nullptr;
   }
   else  // Otherwise, insert newNode
   {
      // Position nodePtr at the head of list.
      nodePtr = head;

      // Initialize previousNode to nullptr.
      previousNode = nullptr;

      // Skip all nodes whose value is less than num.
      while (nodePtr != nullptr && nodePtr->value < num)
      {
         previousNode = nodePtr;
         nodePtr = nodePtr->next;
      }

      // If the new node is to be the 1st in the list,
      // insert it before all other nodes.
      if (previousNode == nullptr)
      {
         head = newNode;
         newNode->next = nodePtr;
      }
      else  // Otherwise insert after the previous node.
      {
         previousNode->next = newNode;
         newNode->next = nodePtr;
      }
   }
}

//**************************************************
// The deleteNode function searches for a node     *
// with num as its value. The node, if found, is   *
// deleted from the list and from memory.          *
//**************************************************

void NumberList::deleteNode(double num)
{
   ListNode *nodePtr;       // To traverse the list
   ListNode *previousNode;  // To point to the previous node

   // If the list is empty, do nothing.
   if (!head)
      return;

   // Determine if the first node is the one.
   if (head->value == num)
   {
      nodePtr = head->next;
      delete head;
      head = nodePtr;
   }
   else
   {
      // Initialize nodePtr to head of list
      nodePtr = head;

      // Skip all nodes whose value member is
      // not equal to num.
      while (nodePtr != nullptr && nodePtr->value != num)
      {
         previousNode = nodePtr;
         nodePtr = nodePtr->next;
      }

      // If nodePtr is not at the end of the list,
      // link the previous node to the node after
      // nodePtr, then delete nodePtr.
      if (nodePtr)
      {
         previousNode->next = nodePtr->next;
         delete nodePtr;
      }
   }
}

//**************************************************
// Destructor                                      *
// This function deletes every node in the list.   *
//**************************************************

NumberList::~NumberList()
{
   ListNode *nodePtr;   // To traverse the list
   ListNode *nextNode;  // To point to the next node

   // Position nodePtr at the head of the list.
   nodePtr = head;

   // While nodePtr is not at the end of the list...
   while (nodePtr != nullptr)
   {
      // Save a pointer to the next node.
      nextNode = nodePtr->next;

      // Delete the current node.
      delete nodePtr;

      // Position nodePtr at the next node.
      nodePtr = nextNode;
   }
}

int main () //Here is the main...It will demonstrate the assignment
{
    NumberList num; //Object of Nmberlist...(num)
    //Now we must make a list of numbers
int sizeofarray = 7;
    num.insertNode(1.3);
    num.insertNode(8);
    num.insertNode(4.5);
    num.insertNode(6);
    num.insertNode(8.8);
    cout<<"The current list of values are:  "<<endl;
    num.displayList();
double myArray[7] ={1.0,2.3,5.8,9.7,8.5,4.2,19.0}; //myArray of doubles
    cout<<"These are in the double array"<<endl;

    //passing

    for(int i =0; i <7; i++)
    {
        if(i<6)
            cout<<myArray[i]<<" -- "<<endl;
        else
            cout <<myArray[i];
    }
    cout << "\n"<<endl;
    num.mergeArray(myArray,7); //HERE is the error 
    cout<<"The passing results in : "<<endl;
    num.displayList();


    return 0;
}

[cire]

I don't see any implementation for NumberList::mergeArray.