Structures & Linked List

[Dylan Forsyth]

A buddy and I trying to make a program that can take in records and, by using linked list and a structure, process them in a variety of task.
I've put several checks in the code to see where it goes wrong.

#include <stdlib.h>#include <string.h>
#define size 21


int check = 0;


struct student{
    struct student* back;
    char first[size];
    char last[size];
    double score;
    int zip;
    struct student* next;
};
struct student *head;
struct student *tail;
struct student *current;
struct student *temp;


void insert(){
    struct student *lead = (struct student*)malloc(sizeof(struct student));
    lead->back = current;
    lead->next = tail;
    temp = tail;
    printf("First name: ");
    scanf("%s",&lead->first);
    printf("Last name: ");
    scanf("%s",&lead->last);
    printf("Score: ");
    scanf("%f",&lead->score);
    printf("Zip: ");
    scanf("%d",&lead->zip);
    tail->back = temp;
    printf("Check 3\n");
    while(&temp->back != &head){
        temp = temp->back;
    }
    head->next = temp;
    printf("Check 4\n");
    current->next = lead;
    printf("Check 5\n");
    current = lead;
    printf("\nRecord inserted successfully!\n\n");
}
int main(){
    printf("Check 1\n");
    head = (struct student*)malloc(sizeof(struct student));
    tail = (struct student*)malloc(sizeof(struct student));
    tail->back = head;
    head->next = tail;
    current = head;
    printf("Check 2\n");
    char fTemp[size],lTemp[size];
    int records,i,j,k;
    printf("How many records would you like to enter? (min. 5)");
    scanf("%d",&records);
    for(i = 0;i<records;i++){
        insert();
    }

For some reason it will stick after Check 3 and go into a infinite while loop. Help please!

[WoodSTokk]

Line 26:

temp = tail;

Line 35:

tail->back = temp;

tail->back point to tail, also temp->back points to temp!

Line 37 is wrong and build the infinity loop:

while(&temp->back != &head){

The address of temp->back can never be the address of head.

should be:

while(temp->back != head){

Line 38 build also a infinity loop because temp and temp->back point to the same address.

temp = temp->back;

[Matticus]

You should be sure you post the complete code. In the posted code, there is a missing closing semi-colon at the end of "main". Also, be sure to include "stdio.h" when using functions like "printf" and "scanf".

Are you getting any compiler warnings? If not, you need to increase the warning level of your compiler. Here are some important ones I got:

main.c|29|warning: format '%s' expects type 'char *', but argument 2 has type 'char (*)[21]'|
main.c|31|warning: format '%s' expects type 'char *', but argument 2 has type 'char (*)[21]'|
main.c|33|warning: format '%f' expects type 'float *', but argument 2 has type 'double *'|

A couple of other observations:

• Be sure to check of "malloc" succeeds before attempting to use the memory.
• Do not cast the return value of "malloc": FAQ > Casting malloc - Cprogramming.com
• Avoid using global variables

---

I think you need to spend a little more time planning out your program before attempting to write code.

head = (struct student*)malloc(sizeof(struct student));
tail = (struct student*)malloc(sizeof(struct student));
tail->back = head;
head->next = tail;

What are the values of "head->back" and "tail->next"?

temp = tail;

So now "temp" is set equal to "tail"...

tail->back = temp;

... and "tail->back" is set equal to "temp", which is just "tail". It appears you're tying your list into a knot.

while(&temp->back != &head){
    temp = temp->back;
}

You probably want to be comparing the variables here, not their addresses.

Also, the multiple pointer variables you're using can lead to confusion, making your code difficult to analyze.