I was trying to do an example about linked list. First, I added the values to the variables and there was no problem. But when I tried to get values from user, the program crashed when entering midterm 2 grade. I tried other input functions but the result is same. Where is the problem?
#include <stdio.h>
struct student
{
char *name;
int m1,m2,final;
struct student* next;
};
main()
{
addStudent();
system("PAUSE");
}
addStudent()
{
struct student *node = NULL;
struct student *firstnode;
firstnode = (struct student *)malloc(sizeof(struct student));
node = firstnode;
printf("press 0 to exit \n");
while(1)
{
printf("Student name: ");
scanf("%s", node->name)
if(node->name == "0") break;
printf("Midterm 1: ");
scanf("%d", node->m1);
printf("Midterm 2: ");
scanf("%d", node->m2);
printf("Final: ");
scanf("%d", node->final);
node->next = (struct student *)malloc(sizeof(struct student));
node = node->next;
}
node->next = NULL;
node = firstnode;
while(node->next);
while(node->next != NULL)
{
printf("%s - ",node->name);
printf("%d ", node->m1);
printf("%d ", node->m2);
printf("%d ", node->final);
node = node->next;
}
system("PAUSE");
return 0;
}
Fix 1
Remove the line
while(node->next);
Reason: It will put you on an infinite loop in most cases and it is unnecessary.
Fix 2
Replace the loop
while(node->next != NULL) {
}
with
if (node->next != NULL) {
while (node->next->next != NULL) {
}
}
Reason: You are allocating one additional struct each time and keeping it empty for reading next time. So the Linked List will end before the next becomes NULL.
Fix 3
Replace following in struct
char *name;
with
char name[80];
Reason: Memory not being allocated.
Fix 4
Replace at all occurrences of scanf (except for name)
scanf("%d", node->m1);
with
scanf("%d", &node->m1);
Reason: scanf needs memory location of data to be read.
Good luck
Your code has multiple errors.
To start with, the first scanf("%s", node->name) is missing its terminating semicolon.
Next, your function signatures are sloppy. main() should be int main(void). addStudent() should be int addStudent(void). (Or, get rid of its return 0 and let it return void.) Since you don't pre-declare addStudent(), you should define it before main() so that main() can know about it.
The crash, though, is because you haven't allocated memory for node->name. You've allocated memory for a node, but that doesn't give you space to put the name.
Related
I'm trying to create a linked list using C. This is the code that I have now.
//fig 12_4.c
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
struct gradeNode{
char lastName[20];
double grade;
struct gradeNode *nextPtr;
};
void insert(GradeNodePtr);
int main(void)
{
//part a
struct gradeNode *startPtr = NULL;
//part b
struct gradeNode *newPtr = malloc(sizeof(GradeNode));
startPtr = newPtr;
// checking to see if memory was allocated properly
if(newPtr != NULL)
{
newPtr->grade = 91.5;
strcpy(newPtr->lastName,"Jones");
newPtr->nextPtr = NULL;
}
//part c
for(int i = 0; i<4; i++)
{
struct gradeNode *newPtr = malloc(sizeof(GradeNode));
if(newPtr != NULL)
{
printf("Please Type A grade: ");
scanf("%lf",newPtr->grade);
printf("Please Enter a LastName: ");
scanf("%s",newPtr->lastName);
puts("");
}
else
{
puts("No memory available. Critical Error!");
}
}
}
If you look at the for loop under the comment labeled part c you may be able to find my error. After the scanf("%lf",newPtr->grade); line of code is ran the execution process stop. Therefore, the scanf function to enter lastname is never reached and the for loop doesn't loop 3 times. Does anyone know where I'm going wrong?
scanf expects to receive pointers. newPtr->grade is not a pointer.
scanf("%lf", &newPtr->grade);
As always, you should check the return from scanf to ensure the data was successfully read.
This is my algorithm for adding nodes to a linked list which is in a sorted way for surnames of persons.
Here is the code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
struct Node {
char Name[1024];
char Surname[1024];
char Telno[1024];
struct Node* next;
};
void Add(struct Node** firstnode, struct Node* NewNode)
{
struct Node* tempo;
//printf("%d",strcmp((*firstnode)->Surname,NewNode->Surname));
if (*firstnode == NULL || (strcmp((*firstnode)->Surname,NewNode->Surname) > 0)) {
NewNode->next = *firstnode;
*firstnode = NewNode;
}
else {
tempo = *firstnode;
while (tempo->next != NULL && strcmp(tempo->Surname,NewNode->Surname) < 0) {
tempo = tempo->next;
}
NewNode->next = tempo->next;
tempo->next = NewNode;
}
}
struct Node* CreateNode(char name[1024], char surname[1024], char telno[1024])
{
struct Node* NewNode = (struct Node*)malloc(sizeof(struct Node));
strcpy(NewNode->Name,name);
strcpy(NewNode->Surname,surname);
strcpy(NewNode->Telno,telno);
NewNode->next = NULL;
return NewNode;
}
void Printlinkedlist(struct Node* head)
{
struct Node* temp = head;
while (temp != NULL) {
printf("%s %s %s\n", temp->Name,temp->Surname,temp->Telno);
temp = temp->next;
}
}
struct Node* head = NULL;
struct Node* temp;
int main()
{
int personcount;
char name[1024],surname[1024],telno[1024];
printf("Please give the count of person:");
scanf("%d", &personcount);
for (int i = 0; i < personcount; i++) {
printf("Please give the name of %d. person:", i + 1);
scanf(" %s", &name);
printf("Please give the surname of %d. person:", i + 1);
scanf(" %s", &surname);
printf("Please give the phone number of %d. person:", i + 1);
scanf(" %s", &telno);
temp = CreateNode(name,surname,telno);
Add(&head, temp);
}
printf("\n -------------- Linkedlist --------------\n");
Printlinkedlist(head);
return 0;
}
The first problem is this: For example, if I enter people's surnames as G, A, L, E, K (So first person's last name will be "G", second person's last name will be "A" etc..), it gives an incorrectly ordered output.
And the second one is: If I delete the comment line characters behind the printf inside the add function, I get a segmentation fault that I don't understand why
Thanks for the answer.
It should first be said that you could, and should, have figured it out yourself by either:
Debugging the program:
On Linux: How Can I debug a C program on Linux?
On Windows: How do you debug a C program on Windows?
Enabling core dumps and analyzing the core file you get when your program crashes; see this explanation.
But, more to the point, let's have a look at (some of) your code:
if (*firstnode == NULL || /* another condition */) {
// do stuff
}
else {
// so *firstnode may be NULL here
tempo = *firstnode;
while (tempo->next != /* some value */ && /* another condition*/ ) {
// do stuff
}
// do stuff
}
See the problem? tempo could get assigned a NULL point, and then de-referenced to get to the next field. That would likely cause a segmentation fault.
I want to programm a linked list in C, trying to apply everything I've learned so far.
I wrote this program to create a linked list:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct node *head = NULL;
//Define struct node
struct node{
char * name;
int num;
struct node * next;
};
//Print List
void printList(){
struct node * temp = head;
while (temp != NULL){
printf("%s", temp->name);
printf("%d", temp->num);
temp = temp->next;
}
}
//Create New Node
struct node * newNode (char * name, int num){
struct node * newNode = (struct node*)malloc(sizeof(struct node));
newNode->name = name;
newNode->num = num;
newNode->next = NULL;
return newNode;
}
//Function to insert and sort Elements
void insertSorted (char * name, int num){
//Empty Liste -> Head is NULL
if (head == NULL){
head = newNode(name, num);
}
else if (strcmp(name, head->name) <=0){
// strcmp-output = -1 ->string 1 > string 2; 0 -> both strings are the same
struct node * temp = newNode(name, num);
temp->next = head;
head = temp;}
struct node * current = head; struct node *prev = NULL;
if (strcmp(name, current->name)>0){ //-> 1, string1 < string 2 ->insert after
while(current != NULL && strcmp(name, current->name)<=0){
prev = current;
current = current->next;
}
struct node * temp = newNode(name, num);
prev->next = temp;
temp->next = current;
}
}
//Test of the linked list
int main()
{
char name; int num;
//Testprogram
printf("Enter a name\n");
scanf("%s", &name);
printf("Enter a number\n");
scanf("%d", &num);
insertSorted(&name, num);
char name2; int num2;
printf("Enter a name\n");
scanf("%s", &name);
printf("Enter a number\n");
scanf("%d", &num2);
insertSorted(&name2, num2);*/
char name3; int num3;
printf("Enter a name\n");
scanf("%s", &name);
printf("Enter a number\n");
scanf("%d", &num3);
insertSorted(&name3, num3);
printList();
return 0;
}
Output example:
Input: Anna 1, Claudio 2, Berta 3
Output: 32Berta1
It somehow...makes the Names vanish and the numbers are in the wrong order too. I'm pretty new to programming, so I have troubles fixing this by myself.
Any help would be hugely appreciated :) Not only to fix the error, but also tips on how to program more...elegantly, so to say.
Thanks :)
//Edit:
Thank you for all the input so far! I messed up the input of the string while testing the program.
As suggested I tried to skip the part with the input, testing the linked list like this in main() (thanks Julien!):
insertSorted("Anna", 1);
insertSorted("Claudio", 2);
insertSorted("Berta", 3);
printList();
it results in the programm not executing and exiting with a negative number error code. Does this point to an infinite loop?
I haven't looked at the linked list details, but one issue I see is that you are using single char variable to store the names (which should be an array or characters). This lack of enough space to store the input make you program have an undefined behaviour after the call to scanf.
As #franji1 stated, try working step by step. If you want to check the code of your list, try testing:
insertSorted("Anna", 1);
insertSorted("Claudio", 2);
insertSorted("Berta", 3);
And check the result is what you expect. Once this is working, add the code asking for input from the user using scanf.
Im not an expert at C but since you mentioned code elegancy I can tell you this. You didn't need to use all these different if statements, you could use the while loop from the begining and insert after you find a name that is bigger than current. This will work even if it has to be inserted before the head node as long as you check that prev is not null. I hope I helped you and that you will find a solution to your problem!
I've written a linked list program and want to take input with spaces but it's not working.It works fine when I simply use "scanf" with %s but since I want to take input with multiple spaces I tried using "gets" and "puts" I've also tried using scanf("%[^\n]*c"); but on the console it gives me random garbage value for scanf("%[^\n]*c"); and for "gets" it reads blank space,
now let me tell you guys some info about the code and how it works
createNode(); function basically just creates a new node to store in the list and returns the address of this newly created node to the insertend(); function where it aligns the new node at the end of the list and in start=t=newnode start is the head pointer which points to the very first node and t is used to traverse the list until t's value becomes NULL,As you could see in the else part of the insertend(); function we're using another pointer t and storing the value of start in it so that we can traverse the list without losing the the address of the first node which is originally kept in the start pointer.
here's the code ->
#include<stdio.h>
#include<stdlib.h>
#include<conio.h>
struct Node
{
char first[20];
struct Node* next;
};
struct Node* start=NULL;
struct Node* t,*u;
int i=1;
struct Node* createNode() //this function creates a newnode everytime it's called
{
struct Node* create=(struct Node*)malloc(sizeof(struct Node));
return create;
}
int length() //to measure the length of the list.
{
int count = 0;
struct Node* temp;
temp=start;
while(temp!=NULL)
{
count++;
temp = temp->next;
}
return count;
}
void insertend() //to insert a node at the end of the list.
{
int l;
struct Node* newnode = createNode();
printf("Enter Name : ");
fgets(newnode->first,sizeof(newnode->first),stdin);
if(start==NULL)
{
start=t=newnode;
start->next=NULL;
}
else
{
t=start;
while(t->next!=NULL)
t=t->next;
t->next=newnode;
t=newnode;
t->next=NULL;
printf("%s successfully added to the list!",newnode->first);
}
l=length();
printf("The length of the list is %d",l);
}
void display() //to display the list
{
struct Node* dis;
dis=start;
if(start==NULL)
{
system("cls");
printf("No elements to display in the list");
}
else
{
system("cls");
for(int j=1;dis!=NULL;j++)
{
printf("%d.) %s\n",j,dis->first);
dis=dis->next;
}
}
}
int menu() //this is just a menu it returns the user input to the main function
{
int men;
printf("Please select a choice from the options below :-\n\n");
printf("1.) Add at the end of the list\n");
printf("2.) Display list\n");
printf("3.) exit\n");
printf(" Enter your choice : ");
scanf("%d",&men);
return men;
}
int main()
{
while(1)
{
system("cls");
switch(menu())
{
case 1 : insertend();
break;
case 2 : display();
break;
case 3: exit(0);
default : system("cls"); printf("Ivalid choice!Please select an appropriate option!");
fflush(stdin);
break;
}
getch();
}
return 0;
}
gets is not to be used, it has been removed from C standard due to it's lack of security.
If you want to know more read Why is the gets function so dangerous that it should not be used?
If you use [^\n] it should work, though it's also problematic since this specifier does not limit the lenght of the stream to be read only that it must stop when finding a newline character.
I suspect the problem might be in the container rather than in the reading, maybe uninitialized memory, If you provide the struct code it'll be easier to diagnose.
You can try:
fgets(newnode->first, sizeof(newnode->first), stdin)
There is a caveat:
If the inputed stream is larger than the container, the extra characters will remain in the input buffer, you might need to discard them.
EDIT:
So the main problem was the fact that through your code you have lingering characters in the buffer, in the particular case of your fgets input it would catch a '\n' left in the buffer, so it would read it before the inputed stream, leaving it, again, in the buffer.
I added a function to clean up buffer, note that fflush(stdin) leads to undefined behaviour so it's a bad option.
I also added a few small tweaks.
- Note that conio.h is windows specific as is system("cls") and getch()(ncurses.h in Linux systems) so I commented it for this sample.
Live sample here
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <conio.h>
struct Node
{
char first[20];
struct Node *next;
};
struct Node *start = NULL;
struct Node *t, *u;
void clear_buf(){ //clear stdin buffer
int c;
while((c = fgetc(stdin)) != '\n' && c != EOF){}
}
struct Node *createNode() //this function creates a newnode everytime it's called
{
struct Node *create = malloc(sizeof(struct Node));
return create;
}
int length() //to measure the length of the list.
{
int count = 0;
struct Node *temp;
temp = start;
while (temp != NULL)
{
count++;
temp = temp->next;
}
return count;
}
void insertend() //to insert a node at the end of the list.
{
int l;
struct Node *newnode = createNode();
printf("Enter Name : ");
clear_buf(); //clear buffer before input
fgets(newnode->first, sizeof(newnode->first), stdin);
newnode->first[strcspn(newnode->first, "\n")] = '\0'; //remove '\n' from char array
if (start == NULL)
{
start = t = newnode;
start->next = NULL;
printf("%s successfully added to the list!", newnode->first);
}
else
{
t = start;
while (t->next != NULL)
t = t->next;
t->next = newnode;
t = newnode;
t->next = NULL;
printf("%s successfully added to the list!", newnode->first);
}
l = length();
printf("The length of the list is %d", l);
}
void display() //to display the list
{
const struct Node *dis;
dis = start;
if (start == NULL)
{
system("cls");
printf("No elements to display in the list");
}
else
{
system("cls");
for (int j = 1; dis != NULL; j++)
{
printf("%d.) %s\n", j, dis->first);
dis = dis->next;
}
}
}
int menu() //this is just a menu it returns the user input to the main function
{
int men;
printf("\nPlease select a choice from the options below :-\n\n");
printf("1.) Add at the end of the list\n");
printf("2.) Display list\n");
printf("3.) exit\n");
printf(" Enter your choice : ");
scanf("%d", &men);
return men;
}
int main()
{
while (1)
{
system("cls");
switch (menu())
{
case 1:
insertend();
break;
case 2:
display();
break;
case 3:
exit(0);
default:
system("cls");
printf("Ivalid choice!Please select an appropriate option!");
clear_buf();
break;
}
getch();
}
return 0;
}
I already posted something similar before but this is another part of a problem that I'm having and I would appreciate your ideas and if you can help me fix my mistake.
As stated on my title i'm building a Doubly Linked List and I would like to know if my function insert_after works or not. When I run the full code I can't insert after the value that I chose which is why I'm posting this. Please let me know if I'm not clear or if this question is wrongly asked. I'm new in C and just need your help understanding.
This is my struct:
struct node
{
char data[100];
struct node *previous; // Points to the previous node
struct node *next; // Points out to the next node
}*head, *last;
This is my function to insert after a the chosen word:
char insert_after(char words[99], char loc[99])
{
struct node *temp, *var, *temp1;
var=(struct node *)malloc(sizeof(struct node));
strncpy(var->data, words,100);
if (head==NULL)
{
head=var;
head->previous=NULL;
head->next=NULL;
}
else
{
temp=head;
while ((temp!=NULL) && (temp->data!=loc))
{
temp=temp->next;
}
if (temp==NULL)
{
printf("\n %s not presented at list\n", loc);
}
else
{
temp1=temp->next;
temp->next=var;
var->previous=temp;
var->next=temp1;
temp1->previous=var;
}
}
last=head;
while (last->next!=NULL)
{
last=last->next;
}
return 0;// take it out after
}
And this is my main: --> Case 3 is my problem
int main()
{
char loc[99];
char words[99];
int i, dat;
head=NULL;
printf("Select the choice of operation on link list");
printf("\n1.) Insert At Begning\n2.) Insert At End\n3.) Insert At Middle");
printf("\n4.) Delete From End\n5.) Reverse The Link List\n6.) Display List\n7.)Exit");
while(1)
{
printf("\n\n Enter the choice of operation you want to do ");
scanf("%d",&i);
switch(i)
{
case 1:
{
printf("Enter a word you want to insert in the 1st node ");
scanf(" %s",words);
insert_beginning(words);
display();
break;
}
case 2:
{
printf("Enter a word you want to insert in the last node ");
scanf(" %s",words);
insert_end(words);
display();
break;
}
case 3:
{
printf("After which data you want to insert your new data ");
scanf(" %s",words);
printf("Enter the data you want to insert in list ");
scanf(" %s",loc);
insert_after(words, loc);
display();
break;
}
I didn't put the full code because it's pretty long which is why i posted the important stuff.
Please let me know if my question is not clear.
Thanks
This is a problem:
while ((temp!=NULL) && (temp->data!=loc))
temp->data != loc is not comparing strings, it just compares pointer addresses.
You could use strcmp or a similar string comparision function. strcmp returns non-zero if
the two parameters differ.
while ((temp!=NULL) && (strcmp(temp->data, words))
Note that it is words that is to compared to the values in the list as words is the value that loc is to be inserted after.
If you use char arrays as the parameters to insert_after function care will be needed to ensure that the entire length of the array is initialised correctly.
Note that a check on the following node is necessary in case there is only one node in the list (the following node will be NULL in this case).
char insert_after(char *words, char *loc)
{
struct node *temp, *var;
var=(struct node *)malloc(sizeof(struct node));
strcpy(var->data, loc);
if (head==NULL)
{
head=var;
head->previous=NULL;
head->next=NULL;
}
else
{
temp=head;
while ((temp!=NULL) && (strcmp(temp->data, words)))
{
temp=temp->next;
}
if (temp==NULL)
{
printf("\n %s not presented at list\n", words);
}
else
{
struct node * followingNode = temp->next;
temp->next=var;
var->previous=temp;
var->next= followingNode;
if (followingNode)
followingNode->previous = var;
}
}
}
Issues
char words[99] has 99 elements but you're allowing up to 100 here, strncpy(var->data, words,100);. That may cause a problem if words is not '\0' terminated. You could unintentially write word[99] to var->data which shouldn't be allowed. Change the statement to this:
strncpy(var->data, words, 99);
var->data[99] = '\0'; // In case words wasn't a string
You cannot compare strings like this, temp->data!=loc. You're actually comparing addresses. Change this line to the following:
while((temp != NULL) && (strcmp(temp->data, loc) != 0));
var->next=temp1 is incorrect. We're at the end of the list so this line should be rewritten as follows:
var->next = NULL;
You must use memcmp or, better yet, strcmp for string comparison instead of !=.