Thread: Accessing next entry in a queue.

Originally Posted by peripatein

Forgive me, what I meant to write was:could accessing the next entry be accomplished via - driversQueue->next++?

The typical purpose of using a queue in the 1st place is to hide away the internal implementation and define at least the enqueue() and dequeue() operations for handling elements in FIFO order (First In First Out).

You enqueue elements at the back and you dequeue them from the front of the queue.

You can also define additional operations, say peek() for example in order to check the element at the front without removing it.

That being said, since internally your queue is implemented as a linked list, and you don't mind violating its classic usage principles, then you could set a pointer p at the back and iterate through until the front element, by ...

Code:

p = p->next;

Incrementing pointer types as you are doing would attempt to change the value pointer itself by sizeof(Driver) bytes. It would not allow you to "move within the queue". This would invalidate the next pointer causing it to point to an incorrect address. Subsequent operations on this mangled pointer would result in weirdness.

This brings up the issue of const correctness. Your compiler would (should if it is a decent one) tell you you were doing something wrong in the isDriverInQueue function had you used the appropriate const qualifiers. Since the function should logically not mess with either of the pointers passed into it, the function should be declared as follows:

Code:

	
int isDriverInQueue(const Driver * const driversQueue, const char* const name);

Now if you tried to do any kind of naughty next++ business, the compiler would object... and rightfully so with a slap upside yo' head.

What you were trying to do does work in the world of C++ container objects where you can increment/decrement iterators to items in those containers (the iterators have this behavior coded for them). But this is not the case using pointers in C.

migf1's solution is the one to use here. Your function should have some sort of temp pointer that you can use to "iterate" through the queue/list.

I was instructed to thus declare my function.
As I am quite new to using structs and pointers, I am not certain how to approach this and would by all means appreciate some more punctual guidance.
Ergo, would/could the following allow me to determine whether a driver with a given name is listed as available?

Code:

int isDriverInQueue(Driver * driversQueue, char* name){
    for (; driversQueue->next[i] != NULL; i++)
    if strcmp(driversQueue->next->name, name) return 1;
    return 0;

}

Post #6 assumes an array of "next" pointers with some trailing NULL marking the last array element. The strcmp operation does not change at all repeatedly testing the same single name element in the passed in pointer and not checking any of the ones.

Post #7 again makes the mistake of attempting to increment a pointer (an error which again would be caught if you were employing proper const-correctness) by some fixed amount equal to sizeof(Driver) instead of actually walking through the array from element to element.

Your if statement has a syntax error (missing parenthesis).

You also need to remember that strcmp returns 0 when the strings match and a non-zero value when the strings do not match. Assuming you simply added the aforementioned parenthesis to correct that syntax error, your current comparison would immediately return 1 (true) the first time it came across a non-matching name and would return 0 (false) only if all names matched the test string.

Try this:

Code:

int isDriverInQueue(const Driver * const driversQueue, const char* const name)
{
    Driver * currDriver = driversQueue;
    while ( currDriver != NULL )
    {
        if ( !strcmp(currDriver->name, name) ) return 1;
        currDriver = currDriver->next;
    }
    return 0;
}

If you wanted to, you could forgo the const bits here since at least the rest of the code isn't attempting to modify the input parameters.