How to create and increase the size of an array at runtime in C++

Question

I want to create an array whose size I will only know at runtime, and then further increase that size during execution of the program.
This is from an /r/dailyprogrammer challenge which can be found here https://www.reddit.com/r/dailyprogrammer/comments/3twuwf/20151123_challenge_242_easy_funny_plant/
MSVisual gives me the error std::badd_array_new_length which means that it's having trouble instantiating the array?
I'm so tired with oftentimes copying code letter for letter from websites where it works and I constantly get errors. Is Visual a bad platform for learning C++? Should I try QT?

#include <iostream>
#include <string>
void main(int argc, char* argv[]) {

    int currentPlants = std::stoi(argv[2]), targetPeople = std::stoi(argv[1]), currentProduce = 0, week = 0;
    int * plants;
    plants = new int[currentPlants];
    for (int i = 0; i < currentPlants; i++) {
        plants[i] = 0;
    }

    if (plants == nullptr) EXIT_FAILURE;

    while (currentProduce < targetPeople) {
        currentProduce = 0;
        for (int i = 0; i < currentPlants; i++) {
            currentProduce += plants[i];
            plants[i]++;
        }
        if (currentProduce >= targetPeople) break;
        else {
            plants = new int[currentProduce];
            for (; currentPlants < currentProduce; currentPlants++) {
                plants[currentPlants] = 0;
            }
        }

        week++;
    }
    std::cout << week;    
}

Answer 1

You should use an std::vector.

As a summary :

// Create an array of size 10    
std::vector<int> my_vector(10);

// Add '3' to my_vector
my_vector.push_back(3);

// Remove the last element
my_vector.pop_back();

Explanation and example here : www.cplusplus.com/reference/vector/vector/

Edit : you don't need to specify the array size when you construct your object.

// Create an array    
std::vector<int> my_vector;

Answer 2

You can't increase the size of an array at runtime. You can create a new bigger array, and copy the contents of the old array to the new array.

Answer 3

Outside of using std::vector, you would need to allocate a new array on the heap, copy the contents over, and delete the old one. Then point your int* to the newly allocated array.

This wouldn't technically change the array size, but those accessing the object would see it as though it was changing.

Answer 4

The problem with your code is that on the first pass through plants all of your plants[x] are zero. You add all of these together and get zero => currentProduce == 0. You then try to new plants[currentProduce aka 0] which is illegal.

Your second problem is that each time you new you create a new array discarding the old values; new creates a new array, it doesn't know anything about the old one.

I rewrote your code using std::vector, which fixes the crash but produces an endless loop because on the first pass, currentProduce comes out to zero so the array is truncated.

#include <iostream>
#include <string>
#include <vector>

int main(int argc, const char* argv_real[])
{
    const char* argv[] = { "programname", "5", "25" };
    int currentPlants = std::stoi(argv[2]), targetPeople = std::stoi(argv[1]), currentProduce = 0, week = 0;
    std::cout << "targetPeople = " << targetPeople
              << ", currentPlants = " << currentPlants
              << "\n";

    std::vector<int> plants;
    // Option 1:
    // plants.resize(currentPlants);
    // Option 2:
    for (auto i = 0; i < currentPlants; ++i) {
        plants.push_back(0);
    }

    while (currentProduce < targetPeople) {
        std::cout << "cp: " << currentProduce
                  << ", tp: " << targetPeople
                  << "\n";
        currentProduce = 0;

        // plantCount is a reference to plants[i] for each i
        for (auto& plantCount : plants) {
            std::cout << plantCount << ", ";
            currentProduce += plantCount;
            plantCount++;
        }
        std::cout << " cp: " << currentProduce << "\n";

        if (currentProduce >= targetPeople)
            break;

        // Option 1:
            plants.resize(currentProduce);
        // Option 2:
        //  while (currentPlants < currentProduce) {
        //      plants.push_back(0);
        //  }

        week++;
    }
    std::cout << week;    
}

Live demo: http://ideone.com/xGpoF6

Answer 5

This is dangerous:

int * plants;
plants = new int[currentPlants];
for (int i = 0; i < currentPlants; i++) {
    plants[i] = 0;
}

if (plants == nullptr) EXIT_FAILURE;

This is what happens (if you are lucky):

• the program attempts to create some memory and returns nullptr if it can't
• the program then uses the memory in a loop even if nullptr was returned. (If nullptr was returned this will crash the program, silently corrupt the memory so you get the wrong results or otherwise doing something you don't want)
• the program then checks to see if nullptr was returned.

If you are unlucky the compiler does time travel and destroys the entire universe. I am not kidding, have a look at:

• https://blogs.msdn.microsoft.com/oldnewthing/20140627-00/?p=633
• Undefined behavior causing time travel