C linked list pointer issue

Question

I am practicing the c language and am trying to create a linked list with structures that tells you if the day of the week entered is in the list.

#include <stdio.h>
#include <stdbool.h>

bool isTrue=1, *ptrisTrue=&isTrue;
struct weekday {
    char *ptrday;
    struct weekday *next;
} sunday, monday, tuesday, wednesday, thursday, friday, saturday;
struct weekday *head=&sunday;
struct weekday *cursor;
struct weekday *ecursor;

void matchtest(char *eday, struct weekday *head, struct weekday *cursor) {
    cursor=head;
    while (cursor!=(struct weekday *)0){
        while (*eday!='\0') {
            if (*eday!=*cursor->ptrday)
            *ptrisTrue=0;
            ++eday; ++cursor->ptrday;
        }
        if (*ptrisTrue==1)
            printf("Yes, %s is in the list\n", cursor->ptrday);
        cursor=cursor->next;
    }
}

int main (void) {
    char enteredday[]="Monday", *ptreday=enteredday;
    sunday.ptrday="Sunday"; monday.ptrday="Monday"; tuesday.ptrday="Tuesday";
        wednesday.ptrday="Wednesday"; thursday.ptrday="Thursday";
        friday.ptrday="Friday"; saturday.ptrday="Saturday";

    sunday.next=&monday; monday.next=&tuesday; tuesday.next=&wednesday;
        wednesday.next=&thursday; thursday.next=&friday; friday.next=&saturday;
        saturday.next=(struct weekday *)0;
        head->next=&sunday;


    printf("This is a test to see if a day is in the list.\n");
    matchtest(ptreday, head, cursor);

    return 0;
}

(I will put a scan function in for "enteredday," for now it is set to Monday.) This program is nowhere near the most efficient one, but I am just testing out the different concepts that I have already learned. When I use breakpoints to pinpoint the issue of the program, I see that when I try to set the cursor to point to the next structure at the end of the first while statement in the "matchtest" function (cursor=cursor->next;), the cursor value for the day member of the structure is set to two quotation marks (""), instead of "Monday". How can I fix this issue?

For starters, you set isTrue to zero on the first mismatch and never set it non-zero anywhere else in the code. — Carey Gregory
– Carey Gregory, Commented Dec 14, 2013 at 16:36
bool variables should normally be assigned false (in preference to 0) or true (in preference to 1). A name isTrue boggles the mind a bit; what 'is true'? And a ptrIsTrue is more mind-bending still. Do you need to repoint it at isFalse sometimes? It is worrying nomenclature. — Jonathan Leffler
– Jonathan Leffler, Commented Dec 14, 2013 at 16:48
I agree with Jonathan. I just would add that defining global variables like can lead to future issues and here it does not make much sense to globally set them to the "all" program. Moreover, to be honest your code could be much more readable for you and the others, by avoiding the a;b;c;d;e; everything-in-one-line syntax. — Ervadac
– Ervadac, Commented Dec 14, 2013 at 17:24

Mohammad Jafar Mashhadi · Accepted Answer · 2013-12-14 16:39:01Z

2

This is because this line of code:

++cursor->ptrday;

You are incrementing ptrday till you reach NULL character, since C strings are implemented using arrays and the name of array is equivalent to a pointer to first member of array, when you increment the pointer till you reach \0 you ignore all characters before \0.

Memory is like this:

  _______________
  |M|o|n|d|a|y|\0|
  ________________
   ^ Where cursor->ptrday used to and should point to, 
               ^ Where cursor->ptrday points to after the second while statement

To solve this you can use strcmp function or change the while loop like this:

char* p = cursor->ptrday;
*ptrisTrue = 1;
while (*eday!='\0') {
    if (*eday != *p)
        *ptrisTrue=0;
    ++eday; ++p;
}

Also note that you forgot to reset *ptrisTrue to true.

answered Dec 14, 2013 at 16:39

Mohammad Jafar Mashhadi

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Comments

Armali · Accepted Answer · 2016-11-07 10:03:28Z

0

but why is the cursor=cursor->next; statement not working?

It is working - but by the assignment head->next=&sunday in main() you created an endless link loop, because *head is the object sunday and then sunday->next points back to sunday.

Just drop that head->next=&sunday line in main(); you already assigned sunday.next=&monday.

answered Nov 7, 2016 at 10:03

Armali

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