How to read from a file multiple integer arrays c++ - c++

Question

I have a file with multiple arrays of variable length arrays like this:
15
1 5 2 7
8 4 9
53 21 60 4 342 4321
...
Let's say the first number(15) gives the number of arrays,so that it would be easier to understand(everything is random though).
How can I read from the file all the numbers in c++ and put them into a variable,let's say x[100][100], so when I code x[1][1] + x[2][2] it will give me 14 (5 + 9).I thought about reading until the end of the line,but I don't know how to keep track of columns

Answer 1

If you have e.g. int x[100][100] and only need a few of those elements you have quite a large mount of wasted memory that won't be used (but still exists and must be initialized).
The solution to that is pointers and dynamic allocation. Allocating the correct amount of "sub-arrays" is easy once you have read the first line. The problem comes with how to handle the sub-arrays since they all seem to be of variable number of elements. You can allocate a fixed amount of elements for each sub-array, and hope you will not need more (which brings back the issue of wasted memory that needs to be initialized). Some of the problems can be mitigated if you do two passes over the input: One to get the maximum number of elements in any line, and the second to actually read the data.
A second option is to read and dynamically allocate just enough elements for each line. This requires you to parse the input so you know when the line ends, and also to use reallocation as you add new numbers. You also need to keep track of the number of elements for each sub-array so you don't risk go out of bounds.
To keep track of the number of elements for each line you can either use a second array with the count. Or you can use an array of structures instead, where each structure contains the number of elements and the sub-array for each line.
A better solution (now that I noticed this was a C++-tagged question and not C) you should use std::vector. Or rather a vector of vectors (of int).
When you have read the first line and parsed its number, you know how many sub-vectors you need and can preallocate them.
Then it's just a matter of reading the rest of the data, which is very easy in C++ with std::getline and std::istringstream and std::istream_iterator.
Perhaps something like this:
std::string line;
// Get the first line, the amount of extra lines to read
std::getline(input_file, line);
// Create the vector (of vectors)
std::vector<std::vector<int>> data;
size_t number_of_sub_vectors = std::stoi(line);
// Preallocate memory for the sub-vectors
data.reserve(number_of_sub_vectors);
// Now read the data for each line
for (size_t i = 0; i < number_of_sub_vectors; ++i)
{
// Get the data for the current line
std::getline(input_file, line);
// And put into an input string stream for parsing
std::istringstream iss(line);
// Create the sub-vector in-place, and populate it with the data from the file
data.emplace_back(std::istream_iterator<int>(iss),
std::istream_iterator<int>());
}
Of course the above example doesn't have any kind of error handling, which is really needed.