General Linked List C++ - c++

Question

New to C++ and I have a general linked list question. Say I have:
struct Node
{
string name;
Node *next
};
in a header file and I have a function
Node ReadIntoList(const string INPUT_FILE)
{
ifstream inFile;
Node *head;
head = NULL;
Node *perPtr;
perPtr = new Node;
inFile.open(INPUT_FILE);
while(inFile && perPtr!= NULL)
{
getline(inFile, perPtr->name);
perPtr -> next = head;
head = perPtr;
perPtr = new Node;
}
delete perPtr;
perPtr = NULL;
return *head;
}
My question is what do I need to return from the ReadIntoList function to main
so that I can access the list and then be able to setup a function to output it
to a file. Here is my main so far...
int main()
{
ofstream oFile;
string inputFile;
Node *head;
cout << left;
cout << "Please enter the name of the input file you would like to "
"use: ";
getline(cin, inputFile);
head = ReadIntoList(inputFile);
oFile.open("OFile.txt");
return 0;
}
This Node in main is not setup properly obviously but I'm not sure how to access the info from ReadIntoList.

Answer 1

In your code, ReadIntoList() is returning the content of the last node in the list (which will crash if the input file fails to open). ReadIntoList() needs to instead return a pointer to the first node in the list. That will allow main() to loop through the list, eg:
Node* ReadIntoList(const string &inputFile)
{
ifstream inFile;
Node *head = NULL;
Node *last = NULL;
Node *perPtr = NULL;
inFile.open(inputFile);
if (inFile)
{
do
{
perPtr = new Node;
if (!getline(inFile, perPtr->name))
{
delete perPtr;
break;
}
perPtr->next = NULL;
if (!head) head = perPtr;
if (last) last->next = perPtr;
last = perPtr;
}
while (true);
}
return head;
}
int main()
{
ofstream oFile;
string inputFile;
Node *head;
Node *perPtr;
Node *tmp;
cout << left;
cout << "Please enter the name of the input file you would like to use: ";
getline(cin, inputFile);
head = ReadIntoList(inputFile);
if (head)
{
oFile.open("OFile.txt");
if (oFile)
{
perPtr = head;
do
{
oFile << perPtr->name << endl;
perPtr = perPtr->next;
}
while (perPtr != NULL);
}
perPtr = head;
do
{
tmp = perPtr->next;
delete perPtr;
perPtr = tmp;
}
while (perPtr != NULL);
}
return 0;
}
That being said, since you are using C++, you should be using std::list (or std::forward_list in C++11 and later) instead, eg:
#include <list>
bool ReadIntoList(const string &inputFile, list<string> &outList)
{
outList.clear();
ifstream inFile(inputFile);
if (!inFile)
return false;
string name;
while (getline(inFile, name))
outList.push_back(name);
return true;
}
int main()
{
ofstream oFile;
string inputFile;
list<string> items;
cout << left;
cout << "Please enter the name of the input file you would like to use: ";
getline(cin, inputFile);
if (ReadIntoList(inputFile, items))
{
oFile.open("OFile.txt");
if (oFile)
{
for (list<string>::const_iterator iter = items.begin(), end = items.end(); iter != end; ++iter)
{
oFile << *iter << endl;
}
}
}
return 0;
}