Assigning element values in an array without knowing the size of the array

Question

During my high school c++ course we were assigned an assignment to write a program that allows the user to input values that would be stored in an array without prompting the user for the array size. So if the user decides to enter 1,2,3 the array size would go from being unknown to three.

[`std::vector`](https://en.cppreference.com/w/cpp/container/vector) — NathanOliver, Dec 17 '19 at 14:52
Although more than likely they wont let you use that and you need something like: https://stackoverflow.com/questions/1350630/how-to-expand-an-array-dynamically-in-c-like-in-vector — NathanOliver, Dec 17 '19 at 14:54
My question is if there any ways of taking an unknown amount of inputs without declaring the size of the array — Xpseraph, Dec 17 '19 at 14:55
Can you explain what the assignment allows you to use? Can you use standard library containers? Can you use pointers? Can you use the new and delete keywords? What operating system are you using? What IDE and compiler are you using? — Filip, Dec 17 '19 at 15:00
For the assignment we have to use arrays, we aren't allowed to utilize pointer or vectors, we weren't given any restrictions for library containers, i am new to programming so i am not sure what the new and delete keywords are, for my IDE i am using visual studio, and for my operating system i am using windows 10. — Xpseraph, Dec 17 '19 at 15:13
"_we aren't allowed to utilize [...] vectors_" and "_we weren't given any restrictions for library containers_" seems to be conflicting, but just to be sure, you could use `std::list`. It's a library container and it's not a vector. — Ted Lyngmo, Dec 17 '19 at 15:34

score 2 · Answer 1 · answered Dec 17 '19 at 15:10

the array size would go from being unknown to three.

It is not possible to change the size of an array, nor is it possible to create an array of unknown size.

However, if the array is allocated dynamically then what you can do is allocate another, bigger array and copy elements of the smaller array onto the bigger one. Such data structure is called a "resizable array". The standard library has a container that implements such data structure: std::vector. You can use it to implement your program.

score 0 · Answer 2 · answered Dec 17 '19 at 15:39

use std::vector if you're allowed to, otherwise something like this might help

#include <cstdlib>
#include <iostream>

template <typename T>
struct list final {
  T* values;
  std::size_t capacity, size;

  list() : values{nullptr}, capacity{0u}, size{0u} {}
  ~list() { std::free(values); }
  void push_back(T value) {
    if (size == capacity) {
      capacity = (capacity + 1) * 2;
      if (void* mem = std::realloc(values, capacity * sizeof(T))) {
        values = reinterpret_cast<T*>(mem);
      } else {
        throw std::bad_alloc();
      }
    }
    values[size++] = value;
  }

  void pop_back() { --size; }
};

int main() {
  list<int> integers;
  for (int i = 0; i < 10; ++i) {
    integers.push_back(i);
  }
  for (int i = 0; i < integers.size; ++i) {
    std::cout << integers.values[i] << std::endl;
  }
}

You should describe what the program is doing for the answer to be useful - but I think the "_we aren't allowed to utilize pointer_" comment makes this approach problematic anyway. — Ted Lyngmo, Dec 17 '19 at 15:48

Assigning element values in an array without knowing the size of the array

2 Answers2