Read text file by column and show all array of column values - c++

Question

I am trying to do file read by column. Main function is that my file should be show one of columns all values.
I am trying to do it with vectors.
void search(){
const int COLUMNS = 4;
vector< vector <int> > data;
string filename = "bla.txt";
ifstream ifile(filename.c_str());
if (ifile.is_open()) {
int num;
vector <int> numbers_in_line;
while (ifile >> num) {
numbers_in_line.push_back(num);
if (numbers_in_line.size() == COLUMNS) {
data.push_back(numbers_in_line);
numbers_in_line.clear();
}
}
}
else {
cerr << "There was an error opening the input file!\n";
exit(1);
}
//now get the column from the 2d vector:
vector <int> column;
int col = 2;//example: the 2nd column
for (int i = 0; i < data.size(); ++i) {
column.push_back(data[i][col - 1]);
cout << column[i] << endl;
}
ifile.close();
}
my file looks like:
John 1990 1.90 1
Peter 1980 1.88 0
...
This code compiles, but I am not getting any value shown in console. When I try to debug last line it wont get cached, so I guess they do nothing?

Answer 1

while (ifile >> num) {
The loop is never entered because num is an int and the first element of the input line is John, which cannot be interpreted as an int, so ifile is set to an error state and the loop condition is immediately false.
The clean fix is to first read the entire line with std::getline and then tokenise the resulting std::string, for example with an std::istringstream.
Individual std::string tokens resulting from that tokenisation can then be converted to appropriate types with functions like std::stoi.

Answer 2

Do step by step and make sure each step is correct.
Read each line, then print out to make sure you are doing it correctly.
You need to split each line. After this step, you will have John, 1990 etc as strings. My favorite split method
Now convert 2-4th columns into integers.
There are good solutions for each step that you can easily find.