Merging 3 lists in one big list - c++

Question

So I have a few lists(HeadFirstCl, HeadNoSm, HeadSm) that I want to connect and make a big list (HeadByPlace) while the pointers for the others still remain. My question is why does my ConnectLists function not work?
#include <iostream>
#include <string>
using namespace std;
struct Item {
string naprav;
string chasizl;
string chaskac;
int termizl;
int termkac;
char fime[5];
int mqsto;
Item *NextByPlace;
};
typedef Item *Point;
Point HeadByPlace, HeadFirstCl, HeadNoSm, HeadSm;
void ConnectLists(Point &P, Point A) {
while (A) {
if (P->NextByPlace == NULL)
P->NextByPlace = A;
P = P->NextByPlace;
}
}
void PrintOut(Point P) {
while (P) {
cout << P->fime<<endl;
cout << P->chasizl << endl;
cout << P->chaskac << endl;
cout << P->mqsto << endl;
cout << P->naprav << endl;
cout << P->termizl << endl;
cout << P->termkac << endl;
P = P->NextByPlace;
}
}
void Create(Point &Head, int i) {
Point Last, P;
Last = NULL;
P = new Item;
P->mqsto = i;
cout << "Enter destination" << endl;
cin >> P->naprav;
cout << "Enter departure HOUR" << endl;
cin >> P->chasizl;
cout << "Enter arrival HOUR" << endl;
cin >> P->chaskac;
cout << "Enter # of leaving terminal" << endl;
cin >> P->termizl;
cout << "Enter # of entering terminal" << endl;
cin >> P->termkac;
cout << "Last name of traveler" << endl;
cin >> P->fime;
P->NextByPlace = NULL;
if (Head == NULL) {
Head = P;
} else {
Last->NextByPlace = P;
}
Last = P;
}
void Delete(char name[], Point &Head) {
Point Pprev, P;
P = new Item;
Pprev = new Item;
cin >> name;
while (Head) {
if (strcmp(Head->fime, name) == 1) {
Pprev = P->NextByPlace;
*P = *Pprev;
delete Pprev;
}
}
}
void main() {
char ch;
HeadByPlace = NULL;
HeadFirstCl = NULL;
HeadNoSm = NULL;
HeadSm = NULL;
int i;
cout << "New element? (Y/N)? : ";
cin >> ch;
while (ch == 'Y' || ch == 'y') {
cout << "Enter seat #: ";
cin >> i;
if (i < 7) Create(HeadFirstCl,i);
else if (i > 7 && i < 25) Create(HeadNoSm,i);
else if (i > 25) Create(HeadSm,i);
cout << " New element? (Y/N) ?: ";
cin >> ch;
}
ConnectLists(HeadByPlace, HeadFirstCl);
ConnectLists(HeadByPlace, HeadNoSm);
ConnectLists(HeadByPlace, HeadSm);
PrintOut(HeadByPlace);
system("pause");
}

Answer 1

I think I found some of your problems. Your connect list function has one main issue, your while loop never terminates. In C++, the implicit conversion from a pointer to a bool returns true if the pointer is not null, and false if the pointer is null. Your loop checks infinitely to see if A is null or not. Obviously, if you give the function a null pointer, it won't do anything, but if you give it a valid pointer for A, it will loop forever because A isn't being changed in the function. Here's a simple function that will actually combine your lists without this issue:
void ConnectLists(Point P, Point A) {
while (P->NextByPlace != nullptr) {
P = P->NextByPlace;
}
P->NextByPlace = A;
}
This function loops until the end of the list, then adds A to the end. Note that it doesn't do any error handling for situations where A or P are null. If you replace your function with this one, it will do what you asked.
There are also very fundamental problems with the code you provided. Your create function segfaults if you make more than one entry of the same type. Also, your test case is flawed. You are passing the connectLists function the pointer HeadByPlace for P each time it is called. HeadByPlace is null so attempting to access any of it's values, such as nextByPlace, results in a segmentation fault. If you want to make sure the code for the connectLists function I provided works, initialize one of each of your list types (HeadFirstCl, HeadNoSM, and HeadSM) and try to connect those, or simply allocate memory for HeadByPlace.
Also, I have one piece of unsolicited advice. You may want to take a look at the C++ STL containers: http://www.cplusplus.com/reference/stl/. I think some of these data structures, such as the vector or list, may be useful to you for this project.