One loop in the link list is having some logical problem

[Rehan FASTian]

At line 148:

for (p = head; (strcmp(p->color,color)) != 0 && (p!=NULL); p = p->link);

My program crashes

First time pen is inserted. Second Time this loop starts having problem.

Question statement: https://www.box.com/s/kx16fae6276wnjb5rhx9

#include <iostream>
#include <conio.h>
#include <windows.h>
using namespace std;
////////////////////////////////////////////////////////////////////////////////////////////////////////
struct pen{
	
	char color[20];
	
	int qty;
	
	pen* link;
	
};
////////////////////////////////////////////////////////////////////////////////////////////////////////
class ll{
	
public:
	
	ll();
	
	void insert(char color[]);
	
	void sell(char color[]);
	
	void print();	
	
private:
	
	pen *head, *tail;
	
};

////////////////////////////////////////////////////////////////////////////////////////////////////////

int main(){
	ll o;
	while(1){
		cout << 
			
			"Select your choice"
			
			<< "\n1. Add Pen"
			
			<< "\n2. Sell Pen"
			
			<< "\n3. Display Stock"
			<< "\n4. Exit\n";
		
		char opt = getch();
		switch(opt){
		case '1':
			cout
				
				<< "\nSelect Choice"
				
				<< "\n1. Add Red"
				
				<< "\n2. Add Green"
				
				<< "\n3. Add Blue"
				
				<< "\n4. Add Yellow"
				
				<< "\n5. Add Aqua\n";
			opt = getch();
			switch(opt){
			case '1':
				o.insert("Red");
				break;
			case '2':
				o.insert("Green");
				break;
			case '3':
				o.insert("Blue");
				break;
			case '4':
				o.insert("Yellow");
				break;
			case '5':
				o.insert("Aqua");
				break;
			}
			break;
			
			case '2':
				cout
					
					<< "\nSelect Choice"
					
					<< "\n1. Sell Red"
					
					<< "\n2. Sell Green"
					
					<< "\n3. Sell Blue"
					
					<< "\n4. Sell Yellow"
					
					<< "\n5. Sell Aqua\n";
				opt = getch();
				switch(opt){
				case '1':
					o.sell("Red");
					break;
				case '2':
					o.sell("Green");
					break;
				case '3':
					o.sell("Blue");
					break;
				case '4':
					o.sell("Yellow");
					break;
				case '5':
					o.sell("Aqua");
					break;
				}
				case '3':
					o.print();
					break;
				case '4':
					return 0;
		}		
		system("cls");	
	}
	return 0;
	
}

////////////////////////////////////////////////////////////////////////////////////////////////////////

ll::ll(){
	
	head = tail = 0;
	
}

////////////////////////////////////////////////////////////////////////////////////////////////////////

void ll::insert(char color[]){
	
	cout << "Quantity: ";	int quantity;	cin >> quantity;
	
	//find if pen already exists?
	pen* p = 0;
	bool pen_exists = 0;
	if (head != NULL && tail != NULL){
		for (p = head; (strcmp(p->color,color)) != 0 && (p!=NULL); p = p->link);
	}
	if (p!=NULL){
		
		pen_exists = 1;
		
	}
	
	if (pen_exists){
		
		p->qty+=quantity; //Pen added.
		
	}
	
	else{ //create new node for this color pen and insert it.
		
		
		
		pen *t = new pen;
		
		t->link = NULL;
		
		strcpy(t->color , color);
		
		t->qty = quantity;
		
		
		
		if (head == NULL && tail == NULL){
			
			head = t;
			
			tail = t;
			
		}
		
		else if (color <= (head->color)){//insert at head
			
			t->link = head;
			
			head = t;
			
		}
		
		else if ( color >= (tail->color) ){ //insert at tail
			
			tail->link = t;
			
			tail = t;
			
		}
		
		else{ 				//insert somewhere in the middle.	
			
			for (p = head; color < p->color && p!=NULL; p = p->link);
			
			//p points at node before which we've to insert.
			//so
			for (pen *b = head; b->link != p; b = b->link);// b points before p and at node after which we'll insert.
			//isert
			t->link = p;
			b->link = t;
		}
	}
}



////////////////////////////////////////////////////////////////////////////////////////////////////////
void ll::sell(char color[]){
	//whether color exists?
	for (pen *p = head; p!=NULL && color != p->color; p = p->link);
	if (p!=NULL){
		if (p->qty-1 != 0){
			p->qty--;
		}
		else{//delete that node.
			for (pen *b = head; b->link != p; b = b->link);//b points just before p.
			//we've to delete p.
			b->link = p->link;
			delete p;
			p = NULL;
		}
	}
	else{
		cout << color << " does not exists" << endl;
	}
}
////////////////////////////////////////////////////////////////////////////////////////////////////////
void ll::print(){
	if (head == NULL && tail == NULL){
		cout << "No Record." << endl;
	}
	else{ 
		cout << "Pen Color \t\t Quantity" << endl;
		for (pen *p = head; p!=NULL; p = p->link){
			cout <<"   " << p->color <<"                        " << p->qty << endl;
		}
	}
	getch();
}