How to fix this code while using dynamic 2D array - c++

Question

I'm using dynamic 2D array and need the value of particular index but it is not printing the correct value.
```int u=5;//No. of elements
int S[u];
int i=0;
while(i<u)//elements in universal set
{
cin>>S[i];
i++;
}
int n;
cin>>n;//no. of subset
i=0;
int subcost[n];
int **subset;
subset=new int*[n];
while(i<n)
{
int l,c;
cin>>l;//size of ith subset
subset[i]=new int[l];
int j=0;
while(j<l)//Elements in each subset
{
cin>>subset[i][j];
j++;
}
cin>>c;//cost for each subset
subcost[i]=c;
i++;
}
i=0;
while(i<n)
{
int j=0;
int s=*subset[i];
while(j<s)
{
cout<<subset[i][j]<<"\n";
j++;
}
i++;
}```
I expect the output to be value of each subset, but the actual output is totally different.

Answer 1

arr[i]=new int[n1];
There's a misunderstanding of what new does. (Perhaps you come from Java?) This doesn't store an integer with the value of n1. It instead creates an array with a size of n1.
Just one pointer level should be enough for an array:
int n = 5;
int i = 0;
int *arr;
arr = new int[n];
arr[i] = 100;
cout << arr[i] << endl; // output: 100
delete[] arr; // remember to deallocate – otherwise memory leaks will haunt your system!
If you're looking for a 2D array, a pointer to a pointer (**) will work.
int **arr;
arr = new int[n]; //
arr[0] = new int[n]; // allocate first array
arr[0][0] = 100; // set first element of first array
delete[] arr[0];
delete[] arr; // deallocate

Answer 2

Here
arr[i]=new int[n1]; /* allocating memory for arr[i], equal to n1*sizeof(int) bytes & arr[i] gets points to address returned by new */
cout<<"Value of "<<i<<"th row is :- "<<arr[i]<<"\n";
I expect the output to be value of n1, but the actual output is some
random address ?
yes, arr[i] is dynamically created array & printing it will prints its base address only.
Try this version where I tried to explain code changes in comments.
int main(void) {
std::cout<<"Enter the value of n"<<"\n";
int n;
std::cin>>n;
int **arr;
/* allocate memory for arr. arr holds base address of array of n int ptr */
arr=new int*[n];
int i=0;
int n1;
std::cout<<"Enter the value of n1"<<"\n";
std::cin>>n1;
/* allocate memory for arr[0], arr[1] .. */
while(i < n1) {
arr[i]=new int[n1];
i++;
}
/* put the data into dynamically allocated array */
for(int row = 0; row < n; row++) {
for(int col = 0; col < n1; col++) {
std::cin>>arr[row][col];
}
}
/* printh te data */
for(int row = 0; row < n; row++) {
for(int col = 0; col < n1; col++) {
std::cout<<"Value of "<<i<<"th row is :- "<<arr[row][col];
}
std::cout<<"\n";
}
return 0;
}
And since you created dynamic array using new you need to free the dynamically allocated memory to avoid memory leakage, use delete operator accordingly.
As pointed by others std::vector is better option than above one.

Answer 3

While you have a good answer from #Achal for dynamically allocating, as mentioned, you really should use the container vector provided by C++ to make things much easier and more robust. All C++ containers provided automatic memory management freeing you from having to allocate manually (and with much less chance of getting it wrong)
When using containers, such as a vector of vectors to store your data, you can simply read and discard your "//elements in universal set", "//no. of subset" and "//Elements in each subset". Those values are not required when using containers. You simply read the value you want and add it to your container, the container will grow as needed to accommodate.
While it is not 100% clear what your input data looks like, we can deduce from your file it looks something like:
Example Input File
Where your first integers is your "//elements in universal set" which isn't needed to read the data. Likewise the second line, your "//no. of subset" is irrelevant for reading the data. Finally the 1st element on each data row, your "//Elements in each subset" is also not needed. Each of these values is simply read and discarded to arrive at your final data set.
$ cat dat/universal_sub.txt
5
4
5 1 2 3 4 5
3 1 2 3
4 1 2 3 4
6 1 2 3 4 5 6
The Final Dataset You Want To Store
From the full file, these appear to be the actual data values you want to store (there can be an even number, but there is no requirement for that)
1 2 3 4 5
1 2 3
1 2 3 4
1 2 3 4 5 6
There are many different ways you can put the pieces together. A short example of how to get your final dataset from your input file could be:
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <limits>
int main (void) {
std::string line; /* string for reading each line */
std::vector<std::vector <int>> subset; /* vector<vector<int>> */
int universal, nsubsets; /* two int to read/discard */
if (!(std::cin >> universal >> nsubsets)) { /* read/dicard 2 values */
std::cerr << "error: failed to read universal.\n";
return 1;
}
/* read/discard '\n' (any chars) left in input buffer line by cin above */
std::cin.ignore (std::numeric_limits<std::streamsize>::max(), '\n');
while (getline (std::cin, line)) { /* read each remaining data line */
int unneeded, i; /* 1st value and i for rest */
std::vector<int> tmp; /* vector<int> for each line */
std::stringstream ss (line); /* stringstream to read from */
if (ss >> unneeded) { /* read/discard 1st value */
while (ss >> i) /* read rest from stringstream */
tmp.push_back(i); /* add to tmp vector */
subset.push_back(tmp); /* add tmp vector to subset */
}
}
for (auto& i : subset) { /* loop over subsets */
for (auto& j : i) /* loop over each value in subset */
std::cout << " " << j; /* output value */
std::cout << '\n'; /* tidy up with newline */
}
}
(note: the output loops make use of the Range-based for loop (since C++11) but you are free to use the .begin() and .end() container functions with a traditional for loop if your compile does not support std=c++11)
Exaple Use/Output
Reading your data into a vector of vectors allows accessing each element:
$ ./bin/vector_2d_subset < dat/universal_sub.txt
1 2 3 4 5
1 2 3
1 2 3 4
1 2 3 4 5 6
Look things over and let me know if you have further questions or if I interpreted your data file format incorrectly.