I see a runtime error wz message "Unhandled exception at 0x009f5900 in trial4.exe: 0xC0000005: Access violation reading location 0xcdcdcdd5." each time I run the following code.
// trial4.cpp : Defines the entry point for the console application.
//

#include <iostream>
#include <fstream>
#include <string>
using namespace std;
#define NULL    0
struct node{
char letter;
int frequency;
node* next;
};

struct treeq{
char letter;
int frequency;
treeq* leftch;
treeq* rightch;
treeq* parent;
char code;
treeq* next;
};
 
struct lut{
string code;
char letter;
lut* next;};
int freCount(string c, node* start)
{
node* ptr=start;
ptr->letter=NULL;
ptr->frequency=0;
ptr->next=NULL;
node* temp=ptr;
node* ptr1;
int i=0,k=0,len=0;
bool found=false;
const int leng=c.length();
while(i<leng)
{
	k=0;
	ptr1=temp;
	while(k<=i&&ptr1!=ptr)
{
	if(ptr1->letter==c[i])
	{
		ptr1->frequency++;
		found=true;
		break;
	}
	ptr1=ptr1->next;
	k++;
  }
if(!found)
{	ptr->letter=c[i];
	ptr->frequency=1;
	ptr->next=new node;
	ptr=ptr->next;
	len++;
		}
i++;
found=false;
}
return len;
}
void enque(treeq* item, treeq* start){
		treeq* ptrt=start;
		if(ptrt!=NULL){
		while((ptrt->next!=NULL))
		{if(ptrt->next->frequency>item->frequency)
		break;
		ptrt=ptrt->next;}}
	treeq* temp=new treeq;
	temp->letter=item->letter;
	//temp->next=NULL;
	temp->frequency=item->frequency;
	if(ptrt==NULL)
	ptrt=temp;
	else
	{temp->next=ptrt->next;
	ptrt->next=temp;}
	
	}


treeq* deque(treeq* head){
	treeq* item;
	item=head;
	head=head->next;
return item;} 
treeq* add(treeq* item1, treeq* item2,treeq* parent){
		parent->frequency=item1->frequency+item2->frequency;
		item1->code='0';
		item2->code='1';
		item1->parent=parent;
		item2->parent=parent;
		parent->leftch=item1;
		parent->rightch=item2;
		return parent;}
treeq* huffman(treeq* start){
		treeq* parent;
	while(start!=NULL){
		parent=new treeq;
		treeq* item1=new treeq;
		treeq* item2=new treeq;
		item1->frequency=start->frequency;
		item1->letter=start->letter;
		start=start->next;
		item2->frequency=start->frequency;
		item2->letter=start->letter;
		start=start->next;
	//cout<<item2->letter<<"->    "<<item2->frequency<<endl;
		parent=add(item1, item2,parent);
		enque(parent, start);
		cout<<parent->letter<<"->"<<parent->frequency<<endl;
		}
				return parent;
} 
lut* inorder(treeq* root,string code){
lut* start=new lut;
lut* ptr=start;
if(root->leftch!=NULL){
	inorder(root->leftch,code);
	code+=root->leftch->code;
	cout<<endl<<code;
	}
else 
	{lut* k=new lut;
		k->code=code;
		k->letter=root->letter;
		ptr->next=k;
		ptr=ptr->next;
		if(root->parent->rightch!=NULL){
			code="";
			inorder(root->parent->rightch,code);
		}
}
return start;}


int main () {
	int x;
    ifstream myfile ("C:\\Users\\Sara\\Desktop\\trial.txt");
	string copy;
  char c; 
	while(myfile.get(c))
	  copy+=c;
    myfile.close();
    cout<<copy<<endl;
	node* start=new node;
	int len=freCount(copy, start);
	node* ptr=start;
	int i=0;
	while(i<len)
		{cout<<ptr->letter<<"   "<<ptr->frequency<<endl;
		ptr=ptr->next;
		i++;}

	treeq* star=new treeq;
	star->frequency=0;
	star->letter=NULL;
	star->next=NULL;
	treeq* item=NULL;
	treeq* ptrt;
	ptr=start;
	ptrt=star;
	treeq* ptrt1=star;
	/*ptrt1->leftch=NULL;
	ptrt1->rightch=NULL;*/
	i=0;

	while(i<len)
		{ptrt1->frequency=ptr->frequency;
		ptrt1->letter=ptr->letter;
		enque(ptrt1,star);
		ptr=ptr->next;
		i++;}

	ptrt=star->next;
	i=0;

	while(ptrt!=NULL)
		{cout<<ptrt->letter<<"              "<<ptrt->frequency<<endl;
		ptrt=ptrt->next;
		i++;}
	star=star->next;
	treeq* root=huffman(star);
	string code="";
	lut* begin=inorder(root,code);
	cin>>x;
  return 0;
}
Comments
Did you think of that title yourself?

I may be missing something but in the inorder method, you have an if statement. Either path that you take calls the function recursively and there doesn't appear to be a stopping condition.

One thing I noticed too on lines 29 and 159 you are trying to assign the value NULL to a char. The compiler complained about that one. You had redefined NULL but you didn't have to it's in <iostream> and a bunch of others.

I tried compiling and debugging your code.

On line 115 you return the variable parent which is not initialised, meaning you haven't set a value for it. When declaring it on line 99, you might wan't to NULL initialise it, saying treeq *parent = NULL;

I did that and next problem occurs on line 120 where you check

if root->leftch != NULL)

But what you don't consider, is that root is NULL, so it when you ask for root's member.

After changing it to this on line 120:

if(root != NULL) if(root->leftch != NULL){

There occured no more run time errors, and a blank console showed up.

Hope this helps you on with your code :)

Happy new year :P

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