Why is the pointer's value changing in while loop? - c++

Question

I have the following code:
#include <iostream>
#include <cstring>
using namespace std;
class lexicon{
private:
class node{
public:
string s = "";
int freq;
node *left=nullptr;
node *right=nullptr;
friend class lexicon;
};
node *root;
public:
void display(node *pd)const{
if(pd==nullptr)return;
display(pd->left);
cout << root->s << " " << root->freq << endl;
display(pd->right);
}
lexicon(){
root=nullptr;
}
~lexicon(){
delete root;
}
void insert(const string &s1){
if(root==nullptr){
root= new node;
root->s=s1;
root->freq=1;
root->left=root->right=nullptr;
}else{
node *point=root;
string s6 = point->s;
if(point->s!=s1)cout << "1";
node *pppp=point;
while(s1!=(point->s) && point!=nullptr){ //this is where the problem occurs
if(s1>(point->s))point=point->right;
else if(s1<(point->s))point=point->left;
}
if(point==nullptr){
point = new node;
point->s=s1;
point->freq = 1;
point->left=point->right=nullptr;
}else{
++(point->freq);
}
}
}
int lookup(const string &s)const{
node *point=root;
if(point==nullptr)return 0;
while(point->s!=s && point!=nullptr){
if(s>point->s)point=point->right;
else if(s<point->s)point=point->left;
}
if(point==nullptr)return 0;
else return point->freq;
}
int depth(const string &s)const{
int count = 1;
node *point=root;
if(point==nullptr)return 0;
while(point->s!=s && point!=nullptr){
if(s>point->s)point=point->right;
else if(s<point->s)point=point->left;
++count;
}
if(point==nullptr)return 0;
else return count;
}
void replace(const string &s1, const string &s2){
int side;
node *point1=nullptr;
node *point=root;
if(point==nullptr)return;
while(point->s!=s1 && point!=nullptr){
point1=point;
if(s1>point->s){point=point->right; side=1;}
else if(s1<point->s){side=-1; point=point->left;}
}
if(point==nullptr)return;
int k=point->freq;
if(point->right==nullptr && point->left!=nullptr){
if(side==1)point1->right=point->left;
if(side==-1)point1->left=point->left;
point->left=nullptr;
delete point;
}else if(point->left==nullptr && point->right!=nullptr){
if(side==1)point1->right=point->right;
if(side==-1)point1->left=point->right;
point->right=nullptr;
delete point;
}else if(point->left==nullptr && point->right==nullptr){
if(side==1)point1->right=nullptr;
if(side==-1)point1->left=nullptr;
delete point;
}else{
node *small = point->left;
if(small->right==nullptr){
small->right=point->right;
if(side==1)point1->right=small;
if(side==-1)point1->left=small;
point->right=point->left=nullptr;
delete point;
}else{
node *smallb, *small=point->left;
while(point->right!=nullptr){
smallb=small;
small=small->right;
}
smallb->right=small->left;
if(side==1)point1->right=small;
if(side==-1)point1->left=small;
small->right=point->right;
small->left=point->left;
point->left=point->right=nullptr;
delete point;
}
}
node *start=root, *startb;
int ns=0;
while(start->s!=s2 && start!=nullptr){
startb=start;
if(s2>start->s){ns=1; start=start->right;}
if(s2<start->s){start=start->left; ns=-1;}
}
if(start==nullptr){
if(ns==1){
startb->right=new node;
(startb->right)->s=s2;
(startb->right)->freq=k;
}if(ns==-1){
startb->left=new node;
(startb->left)->s=s2;
(startb->left)->freq=k;
}
}else{
start->freq+=k;
}
}
friend ostream & operator<<(ostream &out, const lexicon &l){
l.display(l.root);
return out;
}
};
int main(){
cout <<"1";
lexicon l;
l.insert("the");
l.insert("boy");
l.insert("and");
l.insert("the");
l.insert("wolf");
cout << "the word 'the' is found " << l.lookup("the") << " tine(s)" << endl;
cout << "the word 'and' is found at depth " << l.depth("and") << endl;
cout << l;
l.replace("boy", "wolf");
cout << "Aftr replacement:" << endl;
cout << l;
cout << "Now the word 'and' is found at depth: "<<l.depth("and")<<endl;
return 0;
}
When i run it through a debugger it stops at the line i have the comment at, with a segmentation fault error. As you can see i set the point to be equal to root and then i set pppp to be equal to point. The debugger shows that root has a value of 0x615c50, but the point has a value of 0x0. And the weirdest thing is that pppp has the correct value 0x615c50. I don't know why point's value is set to 0x0 when i call the while loop. The debugger I used is onlinegdb. The pictures i attached show the value of the variables at the right side, just after is shows the segmentation fault at this line. Thanks for any help in advance.

Answer 1

Why is the pointer's value changing in while loop?
while(s1!=(point->s) && point!=nullptr){ //this is where the problem occurs
if(s1>(point->s))point=point->right;
else if(s1<(point->s))point=point->left;
}
You are changing the value of point in both branches of the if block. Of course point is changing in the loop.
The problem in your code is that you are dereferencing the pointer in point->s even when point has been set to nullptr.
You need to change the conditional of the while statement to:
while ( point != nullptr && s1 != (point->s) { ... }

Answer 2

Your condition is the wrong way around:
while(s1!=(point->s) && point!=nullptr)
You should first check if point is valid before you dereference it. Ie change that to
while(point!=nullptr && s1!=(point->s))
There might be more problems in your code though..