C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Delete node from singly linked list when header is not known          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Raj Guest Posted: Mon Jan 30, 2006 10:00 am    Post subject: Delete node from singly linked list when header is not known Is there any way to delete a particular node from a singly linked list where the header of the list is unknown.Only pointer available is the one which points to the node to be deleted Back to top Raj Guest Posted: Mon Jan 30, 2006 10:00 am    Post subject: Re: Delete node from singly linked list when header is not k But I want to retain the list. I would be losing the link with the previous node if I do wats mentioned Back to top Tosha Guest Posted: Mon Jan 30, 2006 10:00 am    Post subject: Re: Delete node from singly linked list when header is not k "Raj" <raj.reshmi (AT) gmail (DOT) com> wrote in message news:1138613016.129170.56910 (AT) f14g2000cwb (DOT) googlegroups.com... Quote: Is there any way to delete a particular node from a singly linked list where the header of the list is unknown.Only pointer available is the one which points to the node to be deleted type *temp = ptr; ptr = ptr->next; free(temp); Back to top Tosha Guest Posted: Mon Jan 30, 2006 11:00 am    Post subject: Re: Delete node from singly linked list when header is not k "Raj" <raj.reshmi (AT) gmail (DOT) com> wrote in message news:1138614678.060715.40600 (AT) g14g2000cwa (DOT) googlegroups.com... Quote: But I want to retain the list. I would be losing the link with the previous node if I do wats mentioned No you would not, because the the pointer you have has the adress of the node you want to delete and value of previous_node->next at the same time (that's is the same thing), and when you say "ptr = ptr->next" you change the value of previous_node->next to point to previous_node->next->next or ptr->next. I don't know if you have double indirection connection with list or single, because if you have single, than it's only possible to delete node in front of pointed node, like "ptr->next = ptr->next->next". Back to top Jaspreet Guest Posted: Mon Jan 30, 2006 11:00 am    Post subject: Re: Delete node from singly linked list when header is not k Raj wrote: Quote: Is there any way to delete a particular node from a singly linked list where the header of the list is unknown.Only pointer available is the one which points to the node to be deleted This one's a popular interview question. Just copy the value of the next node to the current node and delete the next node. So, it would be something like : node *tmp = NULL; tmp = ptr->next; ptr->value = tmp->value; ptr->next=tmp->next; delete tmp; Back to top Howard Guest Posted: Mon Jan 30, 2006 4:00 pm    Post subject: Re: Delete node from singly linked list when header is not k "Tosha" <tosha.dev (AT) gmail (DOT) com> wrote in message news:drkp4k$iub$1 (AT) ss405 (DOT) t-com.hr... Quote: "Raj" <raj.reshmi (AT) gmail (DOT) com> wrote in message news:1138614678.060715.40600 (AT) g14g2000cwa (DOT) googlegroups.com... But I want to retain the list. I would be losing the link with the previous node if I do wats mentioned No you would not, because the the pointer you have has the adress of the node you want to delete and value of previous_node->next at the same time (that's is the same thing), and when you say "ptr = ptr->next" you change the value of previous_node->next to point to previous_node->next->next or ptr->next. What? When you change the pointer variable ptr, which currently points to the node to be deleted, in what way does that change the value of the previous node's "next" member? Just because the current values of those pointers are the same, does not mean they're the same pointers! After assigning a new value to ptr, ptr now points to the next node, but the previous node's next pointer is unchanged! After deleting the current node, it becomes an invalid pointer. The correct method is given by Jaspreet (although the first two lines of that example might as well be combined into one.) -Howard Quote: I don't know if you have double indirection connection with list or single, because if you have single, than it's only possible to delete node in front of pointed node, like "ptr->next = ptr->next->next". I don't know what "double indirection" means in this context, but the requirement was for a "singly linked list". To me, "double indirection" means a pointer-to-a-pointer, as in node**. -Howard Back to top Tosha Guest Posted: Mon Jan 30, 2006 6:00 pm    Post subject: Re: Delete node from singly linked list when header is not k "Howard" <alicebt (AT) hotmail (DOT) com> wrote in message news:UcqDf.571910$zb5.456275@bgtnsc04-news.ops.worldnet.att.net... Quote: "Tosha" <tosha.dev (AT) gmail (DOT) com> wrote in message news:drkp4k$iub$1 (AT) ss405 (DOT) t-com.hr... "Raj" <raj.reshmi (AT) gmail (DOT) com> wrote in message news:1138614678.060715.40600 (AT) g14g2000cwa (DOT) googlegroups.com... But I want to retain the list. I would be losing the link with the previous node if I do wats mentioned No you would not, because the the pointer you have has the adress of the node you want to delete and value of previous_node->next at the same time (that's is the same thing), and when you say "ptr = ptr->next" you change the value of previous_node->next to point to previous_node->next->next or ptr->next. What? When you change the pointer variable ptr, which currently points to the node to be deleted, in what way does that change the value of the previous node's "next" member? Just because the current values of those pointers are the same, does not mean they're the same pointers! After assigning a new value to ptr, ptr now points to the next node, but the previous node's next pointer is unchanged! After deleting the current node, it becomes an invalid pointer. I meant if you have adress of previous->next or to say adress of original ptr value holder. Back to top Howard Guest Posted: Mon Jan 30, 2006 7:00 pm    Post subject: Re: Delete node from singly linked list when header is not k "Tosha" <tosha.dev (AT) gmail (DOT) com> wrote in message news:drli67$o6k$1 (AT) ss405 (DOT) t-com.hr... Quote: "Howard" <alicebt (AT) hotmail (DOT) com> wrote in message news:UcqDf.571910$zb5.456275@bgtnsc04-news.ops.worldnet.att.net... "Tosha" <tosha.dev (AT) gmail (DOT) com> wrote in message news:drkp4k$iub$1 (AT) ss405 (DOT) t-com.hr... "Raj" <raj.reshmi (AT) gmail (DOT) com> wrote in message news:1138614678.060715.40600 (AT) g14g2000cwa (DOT) googlegroups.com... But I want to retain the list. I would be losing the link with the previous node if I do wats mentioned No you would not, because the the pointer you have has the adress of the node you want to delete and value of previous_node->next at the same time (that's is the same thing), and when you say "ptr = ptr->next" you change the value of previous_node->next to point to previous_node->next->next or ptr->next. What? When you change the pointer variable ptr, which currently points to the node to be deleted, in what way does that change the value of the previous node's "next" member? Just because the current values of those pointers are the same, does not mean they're the same pointers! After assigning a new value to ptr, ptr now points to the next node, but the previous node's next pointer is unchanged! After deleting the current node, it becomes an invalid pointer. I meant if you have adress of previous->next or to say adress of original ptr value holder. The original post said that the ONLY thing we had was a pointer to the node to be deleted. Nothing in the original post suggested that the address of the previous node's next pointer was known,or even if there IS a previous node. (The node to be deleted might be the head of the list, after all.) The only way your code would affect the previous node's next pointer would be if you were given the pointer to the current node as a [non-const] reference to the previous node's next pointer, or if you were operating directly on that pointer, neither of which is suggested by the original post. -Howard Back to top Howard Guest Posted: Mon Jan 30, 2006 7:00 pm    Post subject: Re: Delete node from singly linked list when header is not k "Jaspreet" <jsingh.oberoi (AT) gmail (DOT) com> wrote in message news:1138617835.633515.223180 (AT) g14g2000cwa (DOT) googlegroups.com... Quote: Raj wrote: Is there any way to delete a particular node from a singly linked list where the header of the list is unknown.Only pointer available is the one which points to the node to be deleted This one's a popular interview question. Just copy the value of the next node to the current node and delete the next node. So, it would be something like : node *tmp = NULL; tmp = ptr->next; ptr->value = tmp->value; ptr->next=tmp->next; delete tmp; [re-posting, because previous try doesn't seem to have gone through...] Two things to note in your example: 1) (minor point): Might as well combine the frist two lines. No point in assigning NULL to the pointer and then assigning a value to it. Just declare and initialize in one statement. 2) (major point): Before using tmp to assign the next and value members, you need to test that tmp is not NULL. Dereferencing a NULL pointer constitutes "undefined behavior". And if the current node is at the end of the list (assuming it's not circular), then its "next" pointer would be NULL (and so tmp would also be NULL). -Howard Back to top Guest Posted: Tue Jan 31, 2006 12:00 pm    Post subject: Re: Delete node from singly linked list when header is not k Jaspreet wrote: Quote: Raj wrote: Is there any way to delete a particular node from a singly linked list where the header of the list is unknown.Only pointer available is the one which points to the node to be deleted This one's a popular interview question. Just copy the value of the next node to the current node and delete the next node. So, it would be something like : And if there's no next node? How do you delete the last node? Quote: node *tmp = NULL; tmp = ptr->next; ptr->value = tmp->value; ptr->next=tmp->next; delete tmp; You're writing to tmp twice. First you set it to 0, then to ptr->next. The first statement is obviously useless. Worse, you don't check for ptr->next==0. HTH, Michiel Salters Back to top Raj Guest Posted: Wed Feb 01, 2006 6:00 am    Post subject: Re: Delete node from singly linked list when header is not k Thanks all of u for the replies.... The suggestion is working out....But as Howard said the logic does not work if the node to be deleted is the last one. Any solutions?? Back to top Ben Pope Guest Posted: Wed Feb 01, 2006 1:00 pm    Post subject: Re: Delete node from singly linked list when header is not k Raj wrote: Quote: Thanks all of u for the replies.... The suggestion is working out....But as Howard said the logic does not work if the node to be deleted is the last one. Any solutions?? Perhaps the tail must be a special case. Some kind of "null" node which is copyable and always points to null. Ben Pope -- I'm not just a number. To many, I'm known as a string... Back to top Howard Guest Posted: Wed Feb 01, 2006 4:01 pm    Post subject: Re: Delete node from singly linked list when header is not k "Raj" <raj.reshmi (AT) gmail (DOT) com> wrote in message news:1138772801.320705.142540 (AT) g47g2000cwa (DOT) googlegroups.com... Quote: Thanks all of u for the replies.... The suggestion is working out....But as Howard said the logic does not work if the node to be deleted is the last one. Any solutions?? Nope. If the node to be deleted is the last one, then you're stuck. You should not delete it, because then you may at some point access it illegally through the next pointer of the previous node. In this case, I'd report an error. Another problem would be if the node were the first node in the list. Then, deleting it would invalidate the actual pointer to the head of the list. (Which you say you don't have... but I'm assuming this is just for the sake of this "puzzle", since any "real" program would certainly have to retain the pointer to the head of the list!) There is _no_way_, using a singly-linked list, and not having a pointer to the head of that list, to "properly" delete the given node from that list. You _can_ do like the example, making sure to check for NULL, of course, but you'd need to throw an error in that case. And, you'd need to tell whoever was supposedly using this method (your teacher? job interviewer?) that if this _were_ by chance the head node, then any existing pointers to the head node would no longer be valid. By the way, when given questions in interviews (if that's the kind of question this is), it's not always required that you come up with a perfect solution. Sometimes, what the interviewer wants is to hear your analysis of the problem. These kind of problems with the proposed solution are exactly the kind of things you need to be able to consider for yourself when designing real-world solutions. The more you're able to demonstrate your analytical abilities, the more likely you'll get the job. Of course, if this is just a puzzle posited by a friend or something, then the answer is that it's simply not possible to do correctly in all cases. Some, but not all. -Howard Back to top Raj Guest Posted: Thu Feb 02, 2006 7:00 am    Post subject: Re: Delete node from singly linked list when header is not k Ok..fine Deleting header wont be a probelm bcos if we knew it was header the qusetion itself would have been irrelevant Thanks a lot Howard... 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