Reading variable size 2D-STL vector from file - c++

Question

2
1 3
2 4 8 13
3 5 6 13
4
4
8
3 7 9 10 13
8 10 11
8 9 11 12
9 10
10 15
3 4 8 14
13
12 16
15 17 18
16
18 16
I want to read this these values from file into two dimensional STL vector. Note that the size of inner vector is not uniform and not known in adance so I need to detect '\n'. So far I have been unnsuccessful. My code is as follows. Please help. What is wrong?
int main()
{
ifstream Lin("A.txt");
double d;
vector<vector<double> > A;
vector<double> dummy;
if (Lin.is_open()){
while (Lin >> d) {
if (Lin.peek() == '\n'){
A.push_back(dummy);
dummy.clear();
}else{
dummy.push_back(d);
}
}
Lin.close();
}
...
return 0;
}
When I iterate over the vector using the following code
, it reveals what is stored:
for(int i = 0; i< A.size(); i++){
for(int j = 0; j< A[i].size() ; j++){
cout << A[i][j] << " ";
}
cout << endl;
}
1
2 4 8
3 5 6
3 7 9 10
8 10
8 9 11
9
10
3 4 8
12
15 17
18
The intended output is same as the way it is stored in the file

Answer 1

Make sure that there is no empty space after the last integer in each line in the data file.
In the code currently you are not adding the last integer to the dummy vector. Modify it like this:
while (Lin >> d)
{
dummy.push_back(d); // Add the number first
if (Lin.peek() == '\n') // Then check if it is end of line
{
A.push_back(dummy);
dummy.clear();
}
}

Answer 2

Less is more. This replaces your entire loop. Note: You don't need to check is_open or call close for this to work safely.
for(std::string s; std::getline(Lin, s);)
{
A.emplace_back(std::istream_iterator<double>(std::istringstream(s)),
std::istream_iterator<double>());
}

Answer 3

Consider using getline.
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
int main() {
std::string line;
std::vector<std::vector<double> > v;
while(std::getline(cin, line)) {
std::stringstream ss(line);
double value;
std::vector<double> numbers;
while(ss >> value) {
numbers.push_back(value);
std::cout << value << std::endl;
}
v.push_back(numbers);
}
return 0;
}

Answer 4

There could be other whitespace characters after the number at the end of line, other than \n. Like spaces:
1234 445 445 \n
^^^^
So your approach is not safe. Better would be to read entire line (getline) to some string, then construct istringstream on this string containing line, then just parse this istringstream to your vector for one line.
Like this:
for (string line; getline(filein, line); ) {
vector<double> dd;
istringstream is(line);
for (double d; is >> d; )
dd.push_back(d);
if (not dd.empty())
A.push_back(dd);
}