inserting into ordered LLL - c++

Question

So far I've been able to correctly add items into my linear linked list but I cannot compare data members from two separate places in my list so that I can find the correct place to insert my new node. I try to create a dummy pointer so that no data maybe lost when I get the next object in the linear linked list so that I may compare the current and previous items. However the previous item just get overridden by the new item. How do I create a dummy pointer to store data without writing over the last object?
(code below)
int political_party::insert_republican_party(candidate_info & a_candidate) {
republican_party * current = new republican_party(); //intiates the new node
current->a_candidate = a_candidate; //gives the new node a perticular value:
current->next = NULL; // declares next the next as null
if (rep_head == NULL) //no nodes exist so far
{
current->next = rep_head; //the node after current contains the value rep_head
rep_head = current; //rep head equals the value of current
}
else if (rep_head->next == NULL) {
republican_party * current2 = rep_head;
current2 = current2->next;
republican_party * previous = rep_head;
// previous -> a_candidate = current -> a_candidate;
// current -> a_candidate = a_candidate;
if (current->a_candidate.get_rank() > previous->a_candidate.get_rank()) {
current->next = rep_head;
rep_head = current;
}
}
else {
republican_party * current2 = rep_head;
while (current->next != NULL) {
current = current->next;
}
current->next = current2->next;
current2->next = current;
return 2;
}
}

Answer 1

Welcome to C++ and welcome to StackOverflow.
You have some hurdles to clear, so I'm going to do the following:
1) make some observations about your code.
2) Then I'm going to show you some sample code that runs with eough "printing" so you can tell what is going on.
3) Then I'll end with a hint or two on how you can proceed.
The big thing I want you to get is learning how to put enough print-outs in your code so you can follow what is going on.
They don't have to be pretty, they're just there to help your troubleshooting, then you can comment them out.
1) observations about your code.
This will be kind of long, I'm including your version of the code to talk about it.
Let's sketch out how the code you posted will run:
0: (step 0, the beginning)
rep_head = NULL
pretty much an empty list.
1: insert_republican_party( c1 );
current points to c1
execute /*A*/ if() block since rep_head is null (see below for /*A*/,
I might edit the original question to have these values for discussion).
after:
rep_head = current, so...
rep_head points to c1.
rep_head next = NULL
at the end of /*A*/ if-block, the list looks something like this:
rep_head-----+
|
v
current----> R[c1] ---next--> NULL
I'm kind of smashing R (republican_party pointer) together with c1 (a_candidate reference).
2: insert_republican_party( c2 );
rep_head points to c1, rep_head.next = NULL
current points to c2, current.next = NULL
We execute the "/*B*/ else if() block" since rep_head->next == NULL.
local var current2 gets pointed to same place rep_head points to.
current2 then pointed to rep_head->next, so current2 = NULL
At this point we don't do anything else with current2.
we'll skip the commented-out lines.
And you're looking to do an in-order insertion for your linked list based on candidate rank...
so we have 3 possible conditions:
maybe c1.rank < c2.rank
maybe c1.rank == c2.rank
maybe c1.rank > c3.rank
The /*C*/ if statement is checking to see if c2.rank > c1.rank
Suppose c2.rank > c1.rank then fine, c2 becomes the new head of the list.
after that:
current->next points to c1.
rep_head points to c2.
So the list might look like this, more or less (assuming c2.rank > c1.rank).
rep_head---+
|
v
current--> R[c2]--next--> R[c1]--next--> NULL
At this stage your previous pointers seem out synch.
Suppose c2.rank <= c1.rank, then... nothing happens and c2 never makes it onto your linked list.
This seems like a hole.
However I would recommend patching that hole in your else-block, /*E* below.
3: insert_republican_party( c3 );
Lets say we somehow get down to /*D*/ else.
and this list looks like step 2, above.
current is pointing to c3 (current.next points to NULL)
rep_head is pointing to c2.
rep_head.next is pointing to c1.
current2 is pointed to same as rep_head, which means c2.
the while() loop walks current down the linked list,
which seems like a logic errror? Should it be walking current2 (instead of current)?
int political_party::insert_republican_party(candidate_info & a_candidate)
{
republican_party * current = new republican_party(); //intiates the new node
current -> a_candidate = a_candidate; //gives the new node a perticular value:
current -> next = NULL; // declares next the next as null
/*A*/ if(rep_head == NULL) //no nodes exist so far
{
current -> next = rep_head; //the node after current contains the value rep_head
rep_head = current; //rep head equals the value of current
}
/*B*/ else if (rep_head -> next == NULL)
{
republican_party * current2 = rep_head;
current2 = current2 -> next;
republican_party * previous = rep_head;
// previous -> a_candidate = current -> a_candidate;
// current -> a_candidate = a_candidate;
/*C*/ if(current -> a_candidate.get_rank() > previous -> a_candidate.get_rank())
{
current -> next = rep_head;
rep_head = current;
}
}
/*D*/ else
{
republican_party * current2 = rep_head;
/*E*/ while(current -> next != NULL)
{
current = current -> next;
}
current -> next = current2 -> next;
current2 -> next = current;
return 2;
}
}
2) sample code that runs
I've hacked up a pretty rough "linked_list.cpp" version that has just enough code to run.
I'm going to suggest you adopt the printout part (Overrides for the output stream << since we're in c++ land).
Then add liberal printouts to your code. Search for "ostream&" in the code to find those overrides.
fwiw, this would have been easier to comment on if you had posted enough C++ code to run; I encourage you to do that in the future.
=== begin sample output ===
$ c++ linked_list.cpp
$ ./a.out
Hello from main()
c1=C[c1, rank=1]
c2=C[c2, rank=1]
c3=C[c3, rank=1]
before, r=R[C[?, rank=-1]]--next--> NULL
after, r=R[C[c1, rank=1]]--next--> NULL
party=Party.rep_head=0
1. calling party.insert_republican_party( C[c1, rank=1] )
insert: adding current=0x1498c20, *current=R[C[c1, rank=1]]--next--> NULL
insert.A: now rep_head=0x1498c20, *rep_head=R[C[c1, rank=1]]--next--> NULL
1. party=Party.rep_head=0x1498c20---> R[C[c1, rank=1]]--next--> NULL
2. calling party.insert_republican_party( C[c2, rank=1] )
insert: adding current=0x1498c50, *current=R[C[c2, rank=1]]--next--> NULL
insert.B: now rep_head=0x1498c20
2. party=Party.rep_head=0x1498c20---> R[C[c1, rank=1]]--next--> NULL
$
=== end sample output ===
=== begin linked_list.cpp ===
#include<iostream>
// disclaimer: I'm not trying to solve anybody's homework problem.
// Just adding enough code (not even good style code at that) to make
// it run well enough to offer a hint or two.
using namespace std;
class candidate_info {
public:
int rank;
const char *name;
int get_rank() { return rank; }
candidate_info( );
candidate_info( int rank, const char *name );
friend ostream& operator<<(ostream& os, const candidate_info& c);
};
class republican_party {
public:
republican_party * next;
republican_party * prev;
candidate_info a_candidate;
republican_party();
friend ostream& operator<<(ostream& os, const republican_party& r);
};
class political_party {
public:
republican_party * rep_head;
political_party();
int insert_republican_party(candidate_info & a_candidate);
friend ostream& operator<<(ostream& os, const political_party & p);
};
int main( int argc, char **argv ) {
cout << "Hello from main()\n";
candidate_info c1( 1, "c1" );
candidate_info c2( 1, "c2" );
candidate_info c3( 1, "c3" );
cout << "c1=" << c1 << "\n";
cout << "c2=" << c2 << "\n";
cout << "c3=" << c3 << "\n";
republican_party r;
cout << "before, r=" << r << "\n";
r.a_candidate = c1;
cout << "after, r=" << r << "\n";
political_party party;
cout << "party=" << party << "\n";
cout << "1. calling party.insert_republican_party( " << c1 << " )\n";
party.insert_republican_party( c1 );
cout << "1. party=" << party << "\n";
cout << "2. calling party.insert_republican_party( " << c2 << " )\n";
party.insert_republican_party( c2 );
cout << "2. party=" << party << "\n";
}
// === CANDIATE_INFO things ===
candidate_info::candidate_info( ) {
this->rank = -1;
this->name = "?";
}
candidate_info::candidate_info( int rank, const char *name ) {
this->rank = rank;
this->name = name;
}
ostream& operator<<(ostream& os, const candidate_info& c)
{
os << "C[" << c.name << ", rank=" << c.rank << "]";
return os;
}
// === REPUBLICAN_PARTY things ===
republican_party::republican_party()
{
next = prev = NULL;
}
ostream& operator<<(ostream& os, const republican_party& r)
{
// note about flush: sometimes when I feed a bad pointer to this
// it can blow up and hit "segmentation fault" so I'm adding
// flushes here and there to give us an idea of how far along we
// actually got before then.
// I strongly encourage you to do something like << for all of your
// classes to make it easier to see what is going on.
// Maybe you did this already, dont know (didn't see the full
// definition for republican_party and other classes).
os << "R[" << flush << r.a_candidate << "]" << flush;
republican_party *p = r.next;
do {
os << "--next--> ";
if( p == NULL ) {
os << "NULL";
} else {
os << " R[" << p->a_candidate << "]";
p = p->next;
}
} while( p != NULL );
return os;
}
// === POLITICAL_PARTY things ===
political_party::political_party() {
rep_head = NULL;
}
ostream& operator<<(ostream& os, const political_party & p) {
os << "Party.rep_head=" << p.rep_head << flush;
if( p.rep_head != NULL ) {
os << "---> " << *p.rep_head;
}
return os;
}
int political_party::insert_republican_party(candidate_info & a_candidate)
{
republican_party * current = new republican_party(); //intiates the new node
current -> a_candidate = a_candidate; //gives the new node a perticular value:
current -> next = NULL; // declares next the next as null
cout << "insert: adding current=" << current << ", *current=" << *current << "\n";
/*A*/ if(rep_head == NULL) //no nodes exist so far
{
current -> next = rep_head; //the node after current contains the value rep_head
rep_head = current; //rep head equals the value of current
cout << "insert.A: now rep_head=" << rep_head << ", *rep_head=" << *rep_head << "\n";
}
/*B*/ else if (rep_head -> next == NULL)
{
republican_party * current2 = rep_head;
current2 = current2 -> next;
republican_party * previous = rep_head;
// previous -> a_candidate = current -> a_candidate;
// current -> a_candidate = a_candidate;
/*C*/ if(current -> a_candidate.get_rank() > previous -> a_candidate.get_rank())
{
current -> next = rep_head;
rep_head = current;
cout << "insert.C: now rep_head=" << rep_head << "\n";
}
cout << "insert.B: now rep_head=" << rep_head << "\n";
}
/*D*/ else
{
republican_party * current2 = rep_head;
/*E*/ while(current -> next != NULL)
{
current = current -> next;
cout << "insert.E: current=" << current << "\n";
if( current != NULL ) {
cout << " *current=" << current << "\n";
}
}
current -> next = current2 -> next;
current2 -> next = current;
cout << "insert.D: now rep_head=" << rep_head << "\n";
return 2;
}
}
=== end linked_list.cpp ===
3) some hints about how you could proceed
I think you are actually fairly close with the /E/ while loop.
I would encourage you to focus your efforts on the /E/ while loop, and comment out everything in the previous /A/ and /B/ if blocks. Comment it out for now because some if it you'll want to salvage and re-use after you teach your while() loop how to figure out where the new candidate_info should go.
This is a bit complicated because the "prev" link part isn't working yet, so I would suggest fixing your code so that --next--> works (singly linked list).
Then come back and modify it so that <--prev-- also works (making it a doubly linked list).
Some questions to consider about making just the "next" linking part work all the way through...
Let's say you were inserting c25 (some candidate name with a rank of 25)..
existing list: rep_head--> R[c10]--next--> R[c20]--next--> R[c30]--next--> NULL
goal list: rep_head--> R[c10]--next--> R[c20]--next--> R[c25]--next--> R[c30]--next--> NULL
Question 1) What do you need to know to say where to splice in c25 ?
Question 2) How can you modify your /E/ while loop to find that splice point?
hint: in additon to checking for hitting end-of-list, what else to you need to consider? (e.g. why do we want c25 to go after c20, but before c30 ?
Question 3) Are you looking for a single splice point? Or would it be easier to solve with both before and after splice points?
You already have effective code that creates a new R[] and puts the candidate_info in them.
The next-handling is a bit rough, so let's close by focusing on that.
Question 4) to splice in R[c25] as show, which of the R's have next values that need to change?
How can your while-loop find those values?
Lastly, some potential null values to watch out for.
If you were inserting at the beginning of the list, what value will your "before" splice point have?
If you were adding to the end of the list, what value will your "after" splice point have?
Is there any difference between inserting into an empty list vs. inserting at both the beginning and end ?
Good luck, I think with a little bit better feedback from cout << printing you'll be able to get where you want to go.