Function changes data on other variables - c

Question

i have structure Trip and i make linked list for this Trip, when i add trip to the list i dont have problem, but when add the second trip it overrides the first one.
The input I do is adding two trips with id 1 and 2 and when i iterate them i see there is two times id 2
#include <stdio.h>
#include <stdlib.h>
char date[10];
char date2[10];
char newdate[10];
char newdate2[10];
int i,t,i2,t2;
int ret;
int option;
typedef struct trip
{
int code;
char startdate[10];
int duration;
double price;
} Trip;
typedef struct list
{
Trip* Trip;
struct list* next;
} List;
List* create_List(Trip* trip)
{
List* newList = malloc(sizeof(List));
if (NULL != newList)
{
newList->Trip = trip;
newList->next = NULL;
}
return newList;
}
void delete_List(List* oldList)
{
if (NULL != oldList->next)
{
delete_List(oldList->next);
}
free(oldList);
}
List* add_List(List* wordList, Trip* trip)
{
List* newList = create_List(trip);
if (NULL != newList)
{
newList->next = wordList;
}
return newList;
}
void createTripData(Trip *trip);
int main(int argc, char *argv[])
{
List* trips;
int first = 1;
while(1)
{
printf("Select an option: \n 1-add new trip \n 2-Iterate\n");
scanf("%d", &option);
if (option == 1)
{
Trip newTrip;
createTripData(&newTrip);
if(first == 1)
{
first = -1;
trips = create_List(&newTrip);
}
else
{
trips = add_List(trips, &newTrip);
}
}
else if (option == 2)
{
system("#cls||clear");
List* iter;
for (iter = trips; NULL != iter; iter = iter->next)
{
printf("Id %d \n", iter->Trip->code);
}
}
}
return (0);
}
void createTripData(Trip *trip)
{
fflush(stdin);
printf("Enter id: ");
scanf("%d", &trip->code);//
}

Answer 1

A problem with your code is the newTrip variable. You can not add it to the list the way you are trying. It is a local variable in a part of main. Therefore it goes out of scope (i.e. becomes invalid) when you exit that part of main. I'll suggest that you rewrite createTripData like:
Trip *trip createTripData()
{
Trip* t = malloc(sizeof(*t));
if (!t)
{
// Ups... add error handling
exit(1);
}
printf("Enter id: ");
if (scanf("%d", &t->code) != 1)
{
// Ups... add error handling
exit(1);
}
return t;
}
and call it like:
if (option == 1)
{
Trip* newTrip = createTripData();
if(first == 1)
{
first = -1;
trips = create_List(newTrip);
}
else
{
trips = add_List(trips, newTrip);
}
}
Besides that you can simplify your code by handling the empty list case inside add_List.
List* trips = NULL; // Notice this initialization
while(1)
{
printf("Select an option: \n 1-add new trip \n 2-Iterate\n");
scanf("%d", &option);
if (option == 1)
{
Trip* newTrip = createTripData();
trips = add_List(trips, &newTrip);
}

Answer 2

the function: createTripData() calls scanf() but fails to check the returned value (not the parameter values) so if the user enters just a return key or enters anything but digit(s) then the code accepts the nothing input, This is an error
The function: main() has two parameters, but neither is being used. the function main() has two valid signatures:
int main( int argc, char *argv[] )
int main( void )
since the parameters are not used, the code should use the signature:
int main( void )
The function: main() is getting the menu selection from the user via:
scanf( "%d", &option );
but not checking the returned value (not the parameter values) so when the user only enters a 'return' or any character other than a digit, the prior value in the field option is used. if the user entered any digit besides 1 or 2 then the code 'screams' through the while() loop, doing either nothing or per what ever value was previously in the variable option.
I.E. ALWAYS check the returned value from calls to any of the scanf() family of functions and handle errors.
in the function: main(), a assumption is made that the user entered either 1 or 2. That is NOT a valid assumption. (never trust the user to do the right thing) Strongly suggest the if( option == 1 and else if(option == 2) be replaced with a switch() statement that includes adefault` case for when the user enters any number other than 1 or 2 I.E.
switch( option )
{
case 1:
...
break;
case 2:
...
break;
default:
printf( "INVALID option: %d, valid options are 1, 2\n", option );
break;
}
this field, in the definition of struct list
Trip* Trip;
It is very poor programming practice to name a variable the same as the type of the variable. A possible fix:
Trip *myTrip;
Then update the other references in the code to use myTrip
It is very poor programming practice to name a variable the same as the type of the variable, with the only difference being the capitalization. Such practice leads to confusion in the human reading the code.
In function: create_list() what happens when the call to malloc() fails? A NULL pointer is returned to function: add_List() which returns that NULL pointer to function: main(), which (successful or not) overlays the value in the variable trips.
the statement:
fflush(stdin);
while being allowed in visual studio, is NOT portable. suggest using something like:
int ch;
while( (ch = getchar()) != EOF && '\n' != ch );
regarding the calls to scanf(), here is one way to handle the error events:
if( 1 != scanf( "%d", &trip->code ) )
{ // then error occurred as 'scanf' returns number of successful input/conversions
perror( "scanf for code failed" );
exit( EXIT_FAILURE );
}
// implied else, scanf successful
The above suggested method immediately exits the program (leaving the OS to cleanup all the allocated memory, which would be poor programming practice) You might want to, instead code a loop that informs the user of the problem, empties stdin and allowing the user to try again.
this statement:
printf("Select an option: \n 1-add new trip \n 2-Iterate\n");
will become very 'difficult' to read and understand if there are several choices in the menu. suggest making use of the C feature of bring all the consecutive strings together and writing the statement like this:
printf("Select an option: \n"
" 1-add new trip \n"
" 2-Iterate\n");
Note: the suggested formatting also pays attention to the right edge of the printed page, making for a much nicer layout.
the function: delete_List() is never called, so the 'menu' in main() is missing an option.
BTW: the function: delete_List() uses recursion, which is OK if the linked list is rather short. However, if there are a lot of entries in the linked list then (especially on windows) there is a high probability of overflowing the stack. I suggest using a loop to walk the linked list, passing each entry to free() after saving the ->next pointer Then it does not matter how many entries are in the linked list
for ease of readability and understanding:
1. separate code blocks (for, if, else, while, do... while, switch, case, default) via a single blank line
2. separate functions by 2 or 3 blank lines (be consistent)
3. follow the axiom: only one statement per line and (at most) one variable declaration per statement.
Now, why are you seeing the problem you mention in the question:
the second pointer passed to add_List() is to a uninitialized local pointer on the stack in main(), not to the actual pointer to the linked list that was declared by: List* trips;
Suggest using unique names so YOU can keep track of what is pointing to what.
Also,in function: create_List(), the new node is being set to point to that uninitialized pointer in main(), rather than to the desired entry in trips in main.
Note, in function: main(), in modern C compilers, if the returned value is always 0, then the return statement is not needed.
Note: the statement return is not a function, so there is no need (unless wrapping multiple expression into a single result value) to have parens around the value passed back by return.
for simplification, suggest elimination of the variable first, changing List* trips; to List *trips = NULL; and replacing: if(first == 1) with: if( !trips )
I.E. always try to write the simplest code that still performs the functionality.