How do I when to extend a dynamically allocated array?

Question

I need to add elements to a dynamically allocated Flight array to my program. As initialization of the Flight* allFlights, I set the size of the array as 1 and want to enlarge its size as I need to add more.

Such as:

//Flight.cpp
Flight::Flight(int flightNumber, int rowNumber, int seatNumber)
{
    flightNo = flightNumber;
    rowNo = rowNumber;
    seatNo = seatNumber;
}

//main.cpp
Flight* allFlights = new Flight[1];

I could not find a proper way to extend it as needed, so I tried to approach the problem by having 2 separate integer values as numberOfFlights and capacityOfFlights. I initialized numberOfFlights as 0 and capacityOfFlights to 1, and incremented numberOfFlights by 1 each time I added a new Flight and compared it to capacityOfFlights at each addition. If they were equal, I followed the steps below to extend my array.

if (numberOfFlights != flightCapacity)
{
    Flight newFlight(flightNo, rowNo, seatNo); //Creating new flight object
    allFlights[numberOfFlights] = newFlight;
    numberOfFlights = numberOfFlights + 1;
}
else
{
    Flight newFlight(flightNo, rowNo, seatNo); //Creating new flight object
    //Reallocating dynamic array of allFlight to fit new flights
    flightCapacity = flightCapacity * 2;
    auto* temp = new Flight[flightCapacity];
    copy(allFlights, allFlights + numberOfFlights, temp);
    delete[] allFlights;
    allFlights = temp;

    //Adding the new flight to array
    allFlights[numberOfFlights] = newFlight;

    numberOfFlights = numberOfFlights + 1;
}

I know using std::vector could easily solve this problem, but in this case I'm not allowed to use it.

Is this a good way to solve the problem? I personally find it wasteful of resources by using 2 int values and constantly checking the size by if statements.

*" I personally find it wasteful of resources"* - why? You need to know the size of the array one way or another. — UnholySheep, Oct 22 '20 at 17:35
Explicit use of `new` and `delete` [is not recommended](https://www.modernescpp.com/index.php/c-core-guidelines-allocating-and-deallocating). Just use `std::vector` and inserting items will be trivial. — Marek R, Oct 22 '20 at 17:37
@MarekR I guess one of the requirements is no vectors, like the question says. — Anonymous1847, Oct 22 '20 at 17:44
You can use [`std::deque`](https://en.cppreference.com/w/cpp/container/deque) instead of `std::vector`. — MikeCAT, Oct 22 '20 at 17:47
*Is this a good way to solve the problem?* -- Create your own generic vector class, where you actually could learn how to properly put together such classes, instead of creating one-off classes such as `Flight`. Then anytime you get the requirements that you "can't use vector", you have a ready-made vector class. — PaulMcKenzie, Oct 22 '20 at 17:52

score 1 · Answer 1 · answered Oct 22 '20 at 17:35

1

Your code have duplicate parts. Increasing of array and putting data there can be separated.

       if (numberOfFlights == flightCapacity)
       {
          //Reallocating dynamic array of allFlight to fit new flights
          flightCapacity = flightCapacity * 2;
          auto* temp = new Flight[flightCapacity];
          copy(allFlights, allFlights + numberOfFlights, temp);
          delete[] allFlights;
          allFlights = temp;
       }
       Flight newFlight(flightNo, rowNo, seatNo); //Creating new flight object
       //Adding the new flight to array
       allFlights[numberOfFlights] = newFlight;
       numberOfFlights = numberOfFlights + 1;

answered Oct 22 '20 at 17:35

MikeCAT

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If the OP can't use `std::vector` then he likely can't use `std::copy()` either. If so, use a manual loop instead: `for(int i = 0; i < numberOfFlights; ++i) { temp[i] = allFlights[i]; }` – Remy Lebeau Oct 22 '20 at 17:41
2

What if they can't use `for`? – Asteroids With Wings Oct 22 '20 at 17:41
Then how is the OP expected to *ever* iterate through the available `Flight`s? Recursion? Ug. – Remy Lebeau Oct 22 '20 at 17:42
2

@AsteroidsWithWings `for(A; B; C) D;` can be written as `A; while(B) {D; C;}` expect for some difference like scopes and effect of `continue;`. – MikeCAT Oct 22 '20 at 17:42
2

@MikeCAT What if they can't use `while`? – Asteroids With Wings Oct 22 '20 at 17:43
@AsteroidsWithWings There are some choices: recursion, `goto`, and [`setjmp / longjmp`](https://stackoverflow.com/questions/64175221/is-it-allowed-to-do-longjmp-multiple-times-for-one-setjmp-call). – MikeCAT Oct 22 '20 at 17:44
@MikeCAT What if they can't use branches at all? Completely linear code? – Anonymous1847 Oct 22 '20 at 17:45
Think we might need an example of these fallback solutions Mike. Just in case. There are clearly some constraints involved in the assignment! Who knows where they might end... – Asteroids With Wings Oct 22 '20 at 17:50

How do I when to extend a dynamically allocated array?

1 Answers1