Linked List

[jabeh]

100 Mary
110 John
111 David
112 Sofia
121 Maria
144 Mia

Lets say given above is the user.txt file and i want make a function to read all these data using a linked list with struct .

Any idea how to create this??

[salem c]

https://www.cplusplus.com/forum/beginner/280683/
Well you already know about reading files and structures.
Enough to at least make a start on the problem.

Can you use https://en.cppreference.com/w/cpp/container/list or is the exercise to roll your own list?

[jabeh]

well yeah i know that but my concern is to roll my own list using linked list

[salem c]

https://en.wikipedia.org/wiki/Linked_list

You could make a start by just using std::list while you get the rest of the code working.

So when you replace std::list with mylist, you'll know where the problem is going to be when it doesn't work, and you'll know that you've done it right when it does work.

[lastchance]

#include <iostream>
#include <fstream>
using namespace std;

struct node
{
   string id, name;
   node *next;
};

void print( node *n )
{
   for ( ; n; n = n->next ) cout << n->id << ' ' << n->name << '\n';
}

int main()
{
   ifstream in( "user.txt" );
   node *root = nullptr, *tail = nullptr;
   for ( string id, name; in >> id && getline( in, name ); tail = ( root ? tail->next : root ) = new node{ id, name, nullptr } );
   print( root );
}

100  Mary
110  John
111  David
112  Sofia
121  Maria
144  Mia

[jabeh]

when i run the this program it has an error on nullptr how do i correct this?

[salem c]

Use a C++11 or newer compiler.
https://en.cppreference.com/w/cpp/language/nullptr

[lastchance]

jabeh wrote:

when i run the this program it has an error on nullptr how do i correct this?

Well, you could use NULL instead of nullptr.

However, if your compiler won't cope with at least C++11 (as @salem c noted) then you are missing out on a lot.

[seeplus]

This version doesn't use explicit nullptr:

#include <iostream>
#include <fstream>
#include <string>
#include <iomanip>
using namespace std;

struct node {
	string id, name;
	node *next {};
};

void print(const node* n) {
	for (; n; n = n->next)
		cout << n->id << ' ' << n->name << '\n';
}

int main() {
	ifstream in("user.txt");

	if (!in)
		return (cout << "Cannot open file\n"), 1;

	node *root {}, *tail {};

	for (string id, name; in >> id && getline(in >> ws, name); tail = (root ? tail->next : root) = new node {.id = id, .name = name});

	print(root);
}

[closed account G21UpDi1]

tail = ( root ? tail->next : root ) = new node{ id, name, nullptr }

I decided so that I can get myself up to par I'm going to spend the next week or so reading beginners questions.

I found the above code really clever, and think I understand it.

Let me see if I can put it in words: if root is false then root equals a new node, else the take the tail's next and make it a new node, regardless the tail becomes next.

Please let me know if you find this sentence correct?

[lastchance]

It's to do really with placement of the first node, depending on whether we have an empty list or not.

The right-hand assignment is done first.

If root is non-null then there is an existing list and the (current) tail->next pointer is set to point to a new node. In the one-off case that root is null then the list is currently empty and we have to set root to the new node.

After all these shenanigans the left-hand assignment is done and the tail pointer is set to point to the newly-assigned node.


It has probably been abbreviated beyond utility, I admit. I usually do linked-list assignment with several more easy-to-read if blocks. The assembled code probably looks the same.


To be fair, I think I pinched the idea from @dutch at some point - just can't remember where. It was one of his single-line statements that left me in awe.




If you don't mind the reversed order in a forward list then you can construct more easily by adding at the start instead and ditch the tail pointer.

#include <iostream>
#include <fstream>
using namespace std;

struct node
{
   string id, name;
   node *next;
};

void print( node *n )
{
   for ( ; n; n = n->next ) cout << n->id << ' ' << n->name << '\n';
}

int main()
{
   ifstream in( "user.txt" );
   node *root = nullptr;
   for ( string id, name; in >> id && getline( in, name ); root = new node{ id, name, root } );
   print( root );
}

Entries come out in reverse order:

144  Mia
121  Maria
112  Sofia
111  David
110  John
100  Mary

though you can fix that by changing the print() function:

void print( node *n )
{
   if ( n )
   {
      print( n->next );
      cout << n->id << ' ' << n->name << '\n';
   }
}

to give

100  Mary
110  John
111  David
112  Sofia
121  Maria
144  Mia