Member Avatar for xeption12

Ok i'm trying to build a header file which then i can include it for programs that use singly linked lists but somthing is fishy here...:

here's the code

#include <iostream.h>

struct nod
{
	int info;
	nod* next_adr;};

void add(nod* &v, nod* &sf, int val)
{
	nod *c;
	if(v==0) //if list empty
	{
			v=new nod;
			v->info=val;
			v->next_adr=0;
			sf=v;
		}
	else
	{
		c=new nod;
		sf->next_adr=c;
		c->info=val;
		c->next_adr=0;
		sf=c;
	}
}

void insert_after(nod* v, nod*& sf, int val, int val1)
{
	nod* c=v,*d;
	while(c->info!=val) c=c->next_adr;
	d=new nod;
	d->info=val1; //the member _info_ of the node represented by _d_ takes the value of _val1_
	d->next_adr=c->next_adr;
	if(d->next_adr==0) sf=d;
}

void insert_before(nod* &v, int val, int val1)
{
	nod* c,* d;
	if(v->info==val)
	{
		d=new nod;
		d->info=val1;
		d->next_adr=v;
		v=d;
	}
	else
	{
		c=v;
		while(c->next_adr->info!=val)
			c=c->next_adr;
		d=new nod;
		d->info=val1;
		d->next_adr=c->next_adr;
		c->next_adr=d;
	}
}

void _delete(nod* &v, nod* &sf, int val)
{
	nod *c,*man;
	if(v->info==val)
	{
		man=v;
		v=v->next_adr;
	}
	else
	{
		c=v;
		while(c->next_adr->info!=val) c=c->next_adr;
		man=c->next_adr;
		c->next_adr=man->next_adr;
		if(man==sf) sf=c;
	}
	delete man;
}

void list_it(nod* v)
{
	nod* c=v;
	while(c)
	{
		cout<<c->info<<endl;
		c=c->next_adr;
	}
	cout<<endl;
}

and here's the program which tests these functions:

#include <liste.h>
#include <conio.h>

nod* v,* sf;

void main()
{
	clrscr();
	
	int i;
	for(i=1;i<=10;i++) add(v,sf,i);
	list_it(v);
	
	insert_after(v,sf,7,11);
	insert_after(v,sf,10,12);
	insert_after(v,sf,1,13);
	list_it(v);

	/// PROBLEM: There are no changes made

	insert_before(v,13,14);
	insert_before(v,1,15);
	list_it(v);
	
            /// PROBLEM: Program enters in an infinite loop and when breaked points me to "while(c->next_adr->info!=val)" @ insert_before() function in the header file
	_delete(v,sf,15);
	_delete(v,sf,13);
	_delete(v,sf,12);
	list_it(v);
	
            /// PROBLEM: same problem as before except this time points me to the while in the _delete() function
	getch();
}
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  • Latest Post Latest Post by xeption12

Dani AI

Generated

Good catch, — the symptom and your fix line up. Creating a node without connecting it leaves the list unchanged (the new node is allocated but not reachable), and later searches that assume well-formed links will either crash or loop because they dereference null pointers or keep walking a broken chain.

Practical hardening to avoid similar problems:

  • Always initialize head/tail pointers explicitly (for example set them to nullptr) before first use. Global scope may zero them by default, but explicit init avoids surprises.
  • Never dereference without checking. Replace patterns like "walk until value" with guarded loops: advance while the current pointer is non-null, and handle the not-found case instead of assuming the value exists.
  • When inserting, update both the new node’s next pointer and the previous node’s next pointer; if inserting at the end, update the tail pointer as well. When deleting, if the list becomes empty, set head and tail to null.
  • Decide and document behavior for duplicates (operate on first match only, or provide variants). Make functions return status (bool or enum) so callers can detect failures instead of silently continuing on broken state.

Design suggestions: use a dummy/sentinel head to simplify insert-before logic, or, when using modern C++, prefer std::forward_list or smart pointers to reduce manual pointer errors (std::forward_list on cppreference). For debugging, step through the faulty operation in a debugger and print node addresses and next pointers, or use tools like Valgrind to find leaks and invalid accesses.

The immediate fix was correct; the remaining stability concerns are the null checks, tail updates, and clear error reporting so future mistakes fail fast instead of producing infinite loops.

Member Avatar for xeption12

solved :) the bug was in insert_after() ... i forgot to link the node that has the value val to the new node...

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