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问题:
#include <iostream> using namespace std; struct list { char name[20]; int age; double height; list *next; }; list *first = NULL, *current; int optn = 0; void currentfor() { if (current->next == NULL) cout << "List has ended!" << endl; else current = current->next; } void currentbac() { if (current == first) cout << "This is the beginning of the list." << endl; else { list *previous; previous = first; while (previous->next != current) { previous = previous->next; } current = previous; } } void addbeginning() { list *newlist; newlist = new list; cout << "Enter your name:" ; cin >> newlist->name; cout << "Enter your age:" ; cin >> newlist->age; cout << "Enter your height:" ; cin >> newlist->height; newlist->next=first; first=newlist; } void addending() { list *newlist, *newlist2; newlist = new list; cout << "Enter your name: "; cin >> newlist->name; cout << "Enter your age : "; cin >> newlist->age; cout << "Enter your height : "; cin >> newlist->height; newlist->next = NULL; if (first == NULL) { first = newlist; current=first; } else { newlist2 = first; while (newlist2->next != NULL) { newlist2 = newlist2->next; } newlist2->next = newlist; } } void addmiddle() { if ( current->next=NULL) addending(); else { list *newlist; newlist=new list; cout << "Enter your name:" ; cin >> newlist->name; cout << "Enter your age:" ; cin >> newlist->age; cout << "Enter your height:" ; cin >> newlist->height; newlist->next=current->next; current->next=newlist; } } void deletebegin() { list *newlist; newlist = first; first = first->next; delete newlist; } void deletemiddle() { if ( current->next=NULL) cout<<"There is no one after this."; else { list *newlist; newlist=current; current=current->next; delete newlist; } } void deleteend() { list *newlist, *newlist2; if (first == NULL) cout << "End of list" << endl; else { newlist = first; if (newlist->next == NULL) { delete newlist; first = NULL; } else { while (newlist->next != NULL) { newlist2 = newlist; newlist = newlist->next; } delete newlist; newlist2->next = NULL; } } } void display() { list *newlist; newlist = first; cout << endl; do { if (newlist == NULL) cout << "End of List" << endl; else { cout << "Name is: " << newlist->name << " "; cout << "Age is: " << newlist->age << " "; cout << "Height is: " << newlist->height; cout<<" <-- Current position "; cout<< endl; newlist = newlist->next; } } while (newlist!=NULL); cout << "End of list" << endl; } int main() { first = NULL; do { display(); cout << endl; cout << "Choose an option: " << endl; cout << "1. Move the current position forward once." << endl; cout << "2. Move the current position backwards once." << endl; cout << "3. Add a member at the beginning of the list." << endl; cout << "4. Add a member at the current position of the list." << endl; cout << "5. Add a member at the ending of the list." << endl; cout << "6. Delete the first member from the list." << endl; cout << "7. Delete the member at current position from the list." << endl; cout << "8. Delete the last member from the list." << endl; cout << "9. End program." << endl; cout << endl << " >> " ; cin >> optn; switch (optn) { case 1 : currentfor(); break; case 2 : currentbac(); break; case 3 : addbeginning(); break; case 4 : addmiddle(); break; case 5 : addending(); break; case 6 : deletebegin(); break; case 7 : deletemiddle(); break; case 8 : deleteend(); break; } } while (optn!= 9); }
I execute this program and number 3 works.But the others seem to crash my program :( can someone please show me the correct coding to this? Im making a linked list with nodes filled with name height and age and it should work for any user. And I m supposed to display the entire list right?Or dont linked lists appear that way ? can some good soul please help?
回答1:
I corrected your program and indicated all errors you should pay attention to in the comments. Please take a good look at them and try to see what you did wrong, so you can learn from this.
#include <iostream> using namespace std; struct list { char name[20]; int age; double height; list *next; }; list *first = NULL, *current = NULL; // Init current to NULL so you can test whether current is set at all int optn = 0; void currentfor() { if(current == NULL) { cout << "You don't have any members yet!" << endl; } else { if (current->next == NULL) cout << "This is the end of the list." << endl; else current = current->next; } } void currentbac() { if (current == first) cout << "This is the beginning of the list." << endl; else { list *previous; previous = first; while (previous->next != current) { previous = previous->next; } current = previous; } } void addbeginning() { list *newlist; newlist = new list; cout << "Enter your name:" ; cin >> newlist->name; cout << "Enter your age:" ; cin >> newlist->age; cout << "Enter your height:" ; cin >> newlist->height; newlist->next = first; first = newlist; if(current == NULL) // Set the current pointer to first, because this is the first element you add current = first; } void addending() { list *newlist, *newlist2; newlist = new list; cout << "Enter your name: "; cin >> newlist->name; cout << "Enter your age : "; cin >> newlist->age; cout << "Enter your height : "; cin >> newlist->height; newlist->next = NULL; if (first == NULL) { first = newlist; current=first; } else { newlist2 = first; while (newlist2->next != NULL) { newlist2 = newlist2->next; } newlist2->next = newlist; } } void addmiddle() { if (current->next == NULL) // You were assigning here. Use == instead of = or you will assign NULL to // current->next! Which is incorrect. addending(); else { list *newlist; newlist = new list; cout << "Enter your name:" ; cin >> newlist->name; cout << "Enter your age:" ; cin >> newlist->age; cout << "Enter your height:" ; cin >> newlist->height; newlist->next = current->next; current->next = newlist; } } void deletebegin() { list *newlist; newlist = first; first = first->next; // You need to update the current pointer first if(newlist == current) { current = current->next; } delete newlist; } void deletemiddle() { list *newlist; newlist = first; // If we delete the first element if(current == first) { list *deleteMe = first; first = first->next; delete deleteMe; current = current->next; } else { // Otherwise // Search until newlist->next == current // Als test for newlist != NULL or you will try to get a next value from NULL -> crash! while(newlist != NULL && newlist->next != current) newlist = newlist->next; if(newlist != NULL) { delete newlist->next; newlist->next = current->next; // Also update the next from the previous node in the list! Or it will not disappear when displaying if (current->next == NULL) // You did it again here. Use == for comparing values instead of = current = first; // It doesn't mean that you don't have to delete if // you don't have a current->next. If you don't have a current->next, // just set it to first. The element does need to be deleted. else current = current->next; } } } void deleteend() { list *newlist, *newlist2; if (first == NULL) cout << "End of list" << endl; else { newlist = first; if (newlist->next == NULL) { delete newlist; first = NULL; current = NULL; // Current should also be null } else { while (newlist->next != NULL) { newlist2 = newlist; newlist = newlist->next; } delete newlist; newlist2->next = NULL; current = newlist2; // You forgot to update the current pointer. } } } void display() { list *newlist; newlist = first; cout << endl; do { if (newlist == NULL) cout << "End of List" << endl; else { cout << "Name is: " << newlist->name << " "; cout << "Age is: " << newlist->age << " "; cout << "Height is: " << newlist->height; if(current == newlist) // You need to check whether you really are at the current position cout<<" <-- Current position "; cout<< endl; newlist = newlist->next; } } while(newlist!=NULL); if(newlist != NULL) // What if the newList was initially NULL? You will print twice. cout << "End of list" << endl; } int main(void) { first=NULL; do { display(); cout << endl; cout << "Choose an option: " << endl; cout << "1. Move the current position forward once." << endl; cout << "2. Move the current position backwards once." << endl; cout << "3. Add a member at the beginning of the list." << endl; cout << "4. Add a member at the current position of the list." << endl; cout << "5. Add a member at the ending of the list." << endl; cout << "6. Delete the first member from the list." << endl; cout << "7. Delete the member at current position from the list." << endl; cout << "8. Delete the last member from the list." << endl; cout << "9. End program." << endl; cout << endl << " >> " ; cin >> optn; switch (optn) { case 1 : currentfor(); break; case 2 : currentbac(); break; case 3 : addbeginning(); break; case 4 : addmiddle(); break; case 5 : addending(); break; case 6 : deletebegin(); break; case 7 : deletemiddle(); break; case 8 : deleteend(); break; } } while (optn!= 9); }
回答2:
One problem I can see is that *current is not initialised to anything.
So option 1 will call currentfor(), and because *current is pointing to garbage, it will attempt to execute current->next which will cause an exception.
So I recommend initialising current to NULL:
list *first = NULL; list *current = NULL;
and checking it in currentfor (and other such places):
void currentfor() { if (current == NULL) { cout << "Current invalid" << endl; return; } if (current->next == NULL) cout << "List has ended!" << endl; else current = current->next; }
回答3:
current is not initialized. Initialize it to 0 and test in the functions that access *current.
