Filling Dynamically Allocated Array in C++ - c++

Question

I am trying to fill up the multi-dimensional array using dynamic memory allocation and running into problem of how to determine the size of each array.
The sizes are also dynamically generated in the for loop, not sure how i can transport this knowledge into looping construct to tell the compiler when to stop.
Please dont answer the problem directly, just a direction needed so i can figure out on how to solve the problem of determining this,
for (int v = 0; v < sizeof(a[y]); v++)
int** a = new int*[n];
for (int i = 0; i < n; i++) {
int colcount;
cout << "Enter Size of Array for " << i << " : ";
cin >> colcount;
a[i] = new int[colcount];
}
// How to fill the matrix now
for (int y = 0; y < n; y++) {
for (int v = 0; v < sizeof(a[y]); v++) {
cout << "Enter Array Content [" << y << "][" << v << "] :";
cin >> a[y][v];
}
}
Update:
Got it working by bringing the for loop inside.
int** a = new int*[n];
for (int i = 0; i < n; i++) {
int colcount;
cout << "Enter Size of Array for " << i << " : ";
cin >> colcount;
a[i] = new int[colcount];
for (int v = 0; v < colcount; v++) {
cout << "Enter Array Content [" << i << "][" << v << "] :";
cin >> a[i][v];
}
}
Last Update:
To better track contents and privileges of template class, i ended up using vectors and with the help of community, here is how i came up.
int n;
cout << "Enter Num of Vectors: ";
cin >> n;
vector<vector <int> > mult_arr(n);
for (int i = 0; i < n; i++) {
int k;
cout << "Enter size for vec " << i << ":";
cin >> k;
mult_arr[i].resize(k);
for (int x = 0; x < k; x++) {
cout << "Enter Array Contents [" << i << "][" << x << "] :";
cin >> mult_arr[i][x];
}
}

Answer 1

You cannot know or find the size of a dynamic allocated array without actually holding the size in a variable. You can not use the classic sizeof(arr)/sizeof(type) to find the size because the sizeof() a pointer is just its type size and not the total array size. Use std::vector or keep tracking the sizes of each dynamic array.

Answer 2

Instead of using pointers to pointers to int, consider using a single allocation. It is easier and faster!
int * matrix = new int[width * height];
matrix[y * width + x] = 0;
delete [] matrix;

Answer 3

I took the liberty to rewrite your program to a more domatic C++ program (including a main function, so you can test it). Technically, this does solve the same problem, but instead of using new and delete, it uses std::vector, which should always be the first thing you should reach for.
Unless you have a very very good reason not to use vector, I advice you forget about new/delete. Know that it's there and get on with your life.
I realize this doesn't actually answer your question. I think it's too long for a comment so I'm providing it as answer instead.
#include <iostream>
#include <vector>
#include <string>
int get_an_integer(std::string message)
{
std::cout << message;
int n;
std::cin >> n;
return n;
}
int main()
{
std::vector<std::vector<int>> matrix;
matrix.resize(get_an_integer("Number of vectors:"));
for (auto& vector : matrix)
{
vector.resize(get_an_integer("Vector size:"));
for (auto& element : vector)
element = get_an_integer("Element value:");
}
for (auto v : matrix)
{
for (auto e : v)
std::cout << e << " ";
std::cout << '\n';
}
}
Notice how I don't have to keep track of anything and I don't really care about the explicit size of anything. Even the values of the elements aren't interesting.
I hope this helps you. If you have any questions, please ask.

Answer 4

You can memset to initialize an array. like
int *arr = new int[5];
fill(c, c + 5, 3); //fill(array, array+size, intialvalue)
cout<<c[4];
I hope this will help you.