Dereferencing and Equality?

Sooo I'm working on a problem that requires me to use iterators. What im trying to do is test for equality of items in two lists that is being traversed with 2 separate iterators. However I am getting an error when I run my code, saying list iterator not dereferencable. I am pretty sure my two things are list iterators. Trying to debug it, it goes to these few lines first...

Code:

if(*current != *sorted_index){
	while(*current != *sorted_index){
		current++;
	}
}

So I was wondering if what I wrote is actually proper to do. Thank you!

As a basic principle, there is nothing wrong with that code. The if-statement is unnecessary, as the while-loop will not be entered if the current is matching sorted_index.

If the object has a not-equal operator (operator!=), you may want to check that this operator does what it should do.

--
Mats

Perhaps you run out over the end of a container (if the items are never equal)? In this case you also must check both iterators for end().

If none of the items are equal to *sorted_index then current will eventually reach end() and attempt to compare that to *sorted_index.
You would need something like this:

Code:

while (current != myContainer.end() && *current != *sorted_index) {
	++current;
}

However you're basically doing the job of std::find. Assuming you are either using a built-in type in the list, or you have a compatible less-than operator defined as well, just do this:

Code:

current = std::find(myContainer.begin(), myContainer.end(), *sorted_index);
if (current != myContainer.end())
{
	// we found it! - do our thing with it now...
}

Part of the point of std::find is that since you don't have to write the loop yourself, you wont get it wrong (as you have here).

hmm.. I think it may be better if I posted my whole code...At this point I believe there are a lot of errors, and I haven't cleaned it up yet to make it shorter. However I can't do that if it doesn't work at all....

Code:

#include <iostream>
#include <list>
#include <string>

using std::cout;
using std::endl;
using std::string;
using std::cin;
using std::list;

int main(){
	
	//asks for user input
	cout << "Please type in your input: ";
	
	//the container
	list<string> sentence;
	string x;
	
	//adds user input to container
	while(cin >> x){
		sentence.push_back(x);
		}
	
	list<string> sorted = sentence;
	sorted.sort();

    //starts container from beginning, and continues until the end
	for(list<string>::iterator iter = sentence.begin(); iter != sentence.end(); iter++){
		//set the current index to the iterator
		list<string>::iterator current = sentence.begin();
		//set the sorted index
		list<string>::iterator sorted_index = sorted.begin();
        

		//while the count is not equal to the end
		while(sorted_index != sorted.end()){
			//if the current is not equal to the end of the container
			//output object 
			if(*current != *sorted_index){
					current++;
				}
			//else set the current to the beginning and output
			else{
				cout << *current << ' ';
				for(list<string>::iterator count = sentence.begin(); count != sentence.end(); count++){
					if(*current != *(sentence.end())){
							current++;
							cout << *current << ' ';
						}
					else{
						//set current back to the beginning
						current = sentence.begin();
						//output the current
						cout << *current << ' ';
						//increment current
						current++;
					}
		//for the sake of a new line
		cout << endl;
				}
			}
		//increment the sorted index
		sorted_index++;
		}
	}

	return 0;
}

The code here should take in a phrase, for example "The quick brown fox", and output

Code:

brown fox The quick       
fox The quick brown       
quick brown fox The      
The quick brown fox

I am trying to do a permuted index problem and this is only the beginning step. It would be great if I could get further assistance though.

This is the problem:

Code:

			if(*current != *(sentence.end())){

You can never dereference an 'end' iterator, and again *current will also fail if the collection is empty.
What you want in this case is just

Code:

 if (current != sentence.end())

so I fixed that and added

Code:

if(current != sentence.end() && *current != *sorted_index){
	current++;
				}

from

Code:

if(*current != *sorted_index){
	current++;
				}

Here is my entire code...

Code:

#include <iostream>
#include <list>
#include <string>

using std::cout;
using std::endl;
using std::string;
using std::cin;
using std::list;

int main(){
	
	//asks for user input
	cout << "Please type in your input: ";
	
	//the container
	list<string> sentence;
	string x;
	
	//adds user input to container
	while(cin >> x){
		sentence.push_back(x);
		}
	
	list<string> sorted = sentence;
	sorted.sort();

    //starts container from beginning, and continues until the end
	for(list<string>::iterator iter = sentence.begin(); iter != sentence.end(); iter++){
		
		//set the current index to the iterator
		list<string>::iterator current = sentence.begin();
		//set the sorted index
		list<string>::iterator sorted_index = sorted.begin();
        

		//while the count is not equal to the end
		while(sorted_index != sorted.end()){
			
			//if the current is not equal to the end of the container
			//output object 
			if(current != sentence.end() && *current != *sorted_index){
					current++;
				}
			
			else{
				
				//output the current since it must equal to the sorted_index object
				cout << *current << ' ';
				
				//start the for loop on the "second" count because of the sorted_index output
				for(list<string>::iterator count = sentence.begin()++; count != sentence.end(); count++){
					
					//if the current is not the end of the sentence 
					//increment current and output the next word
					if(current != sentence.end()){
							current++;
							cout << *current << ' ';
						}

                    //since current is at the end
					else{
						//set current back to the beginning
						current = sentence.begin();
						//output the current
						cout << *current << ' ';
						//increment current
						current++;
						}
				}
				
			//for the sake of a new line
			cout << endl;
			}

		//increment the sorted index
		sorted_index++;
		}
	}

	return 0;
}

Now it simply does nothing...I type in a sentence and the program just simply ends, telling me to press enter to continue. I went through my code, and to me at the moment nothing stands out that could be wrong. Could someone just nudge me into the right direction please? Thanks!

It sounds like you have no debugger? Or is it that you just don't know how to use it?
Either way, I think that to get any furthur in programming you're going to need to get used to using a debugger.

Honestly I really do not know how to use the debugger. I use Visual Studio to do my programming. I've tried using it, but end up not understanding what its telling me. Right now Im not at home, so I can not pull up the debugging messages, but is there a guide out there on how to understand it? I've searched but a lot of it just tells me how to start the debugging but not understand it.

soo...I found out if I have this

Code:

if(current != sentence.end() && *current != *sorted_index){
	current++;
				}

My program would crash giving me the "list iterator not dereferencable" error.

However if I changed it back to

Code:

if(current != sentence.end()){
	current++;
				}

It simply does nothing! "Press Enter to continue..."

I've tried using the VS debugging tool and all I get is...

Code:

'permuted.exe': Loaded 'C:\Documents and Settings\Dung\My Documents\Visual Studio 2008\Projects\permuted\Debug\permuted.exe', Symbols loaded.
'permuted.exe': Loaded 'C:\WINDOWS\system32\ntdll.dll'
'permuted.exe': Loaded 'C:\WINDOWS\system32\kernel32.dll'
'permuted.exe': Loaded 'C:\WINDOWS\WinSxS\x86_Microsoft.VC90.DebugCRT_1fc8b3b9a1e18e3b_9.0.21022.8_x-ww_597c3456\msvcp90d.dll', Symbols loaded.
'permuted.exe': Loaded 'C:\WINDOWS\WinSxS\x86_Microsoft.VC90.DebugCRT_1fc8b3b9a1e18e3b_9.0.21022.8_x-ww_597c3456\msvcr90d.dll', Symbols loaded.
The program '[4080] permuted.exe: Native' has exited with code 0 (0x0).

This looks more like something is wrong with my system files...

Originally Posted by dnguyen1022

soo...I found out if I have this

Code:

if(current != sentence.end() && *current != *sorted_index){
	current++;
				}

My program would crash giving me the "list iterator not dereferencable" error.

However if I changed it back to

Code:

if(current != sentence.end()){
	current++;
				}

It simply does nothing! "Press Enter to continue..."

I've tried using the VS debugging tool and all I get is...

Code:

'permuted.exe': Loaded 'C:\Documents and Settings\Dung\My Documents\Visual Studio 2008\Projects\permuted\Debug\permuted.exe', Symbols loaded.
'permuted.exe': Loaded 'C:\WINDOWS\system32\ntdll.dll'
'permuted.exe': Loaded 'C:\WINDOWS\system32\kernel32.dll'
'permuted.exe': Loaded 'C:\WINDOWS\WinSxS\x86_Microsoft.VC90.DebugCRT_1fc8b3b9a1e18e3b_9.0.21022.8_x-ww_597c3456\msvcp90d.dll', Symbols loaded.
'permuted.exe': Loaded 'C:\WINDOWS\WinSxS\x86_Microsoft.VC90.DebugCRT_1fc8b3b9a1e18e3b_9.0.21022.8_x-ww_597c3456\msvcr90d.dll', Symbols loaded.
The program '[4080] permuted.exe: Native' has exited with code 0 (0x0).

This looks more like something is wrong with my system files...

So the only time you print something is when that if statement you quoted is false. The only time that if statement you quoted is false is when current == sentence.end(). Trying to print *current, when current is an end of a list, will launch various nuclear missiles. If you don't have any nuclear missiles installed, then your program will just stop.

Originally Posted by dnguyen1022

I've tried using the VS debugging tool and all I get is...

Code:

'permuted.exe': Loaded 'C:\Documents and Settings\Dung\My Documents\Visual Studio 2008\Projects\permuted\Debug\permuted.exe', Symbols loaded.
'permuted.exe': Loaded 'C:\WINDOWS\system32\ntdll.dll'
'permuted.exe': Loaded 'C:\WINDOWS\system32\kernel32.dll'
'permuted.exe': Loaded 'C:\WINDOWS\WinSxS\x86_Microsoft.VC90.DebugCRT_1fc8b3b9a1e18e3b_9.0.21022.8_x-ww_597c3456\msvcp90d.dll', Symbols loaded.
'permuted.exe': Loaded 'C:\WINDOWS\WinSxS\x86_Microsoft.VC90.DebugCRT_1fc8b3b9a1e18e3b_9.0.21022.8_x-ww_597c3456\msvcr90d.dll', Symbols loaded.
The program '[4080] permuted.exe: Native' has exited with code 0 (0x0).

This looks more like something is wrong with my system files...

You can ignore those lines.
What you need to familiarize yourself with is basic debugging functionality first.
Those are: stepping (step in, step out, step over), breakpoints and watching variables.
If you know those, then you can accomplish a lot.

(...) you're going to need to get used to using a debugger.

Real Programmers can read core dumps :-)

http://www.pbm.com/~lindahl/real.programmers.html

But dnguyen1022 isn't a real programmer now, eh?
Besides, while real programmers can read core dumps, real programmers shouldn't be afraid to use debuggers, simply because it's faster!

But admit, the site is funny...