Is it possible to 'throw away' read value from input stream? - c++

Question

I process some data with data in columns like,
1 -0.004002415458937208 0.0035676328502415523
2 -0.004002415796209478 0.0035676331876702957
....
I am only interested in the last two values. I usually find it convenient to read the values as:
std::ifstream file(file_name);
double a, b;
for (lines) {
// | throwing away the first value by reading it to `a`
file >> a >> a >> b;
store(a, b);
}
I'm not sure how readable this is for others, and it might be thought of as an error when the structure of the data is not known. Could I somehow make it look more explicit that I really want to throw away the first read value?
I wanted something in the line of this, but nothing worked:
file >> double() >> a >> b; // I hoped I could create some r-value kind of thing and discard the data in there
file >> NULL >> a >> b;

Answer 1

If you don't want to create a variable explicitly to be ignored, and you feel explicitly ignoring the value with calls to manipulate the stream are too verbose, you could take advantage of the operator>> overload for std::istream that takes a std::istream&(*)(std::istream&) function pointer:
template <typename CharT>
std::basic_istream<CharT>& ignore(std::basic_istream<CharT>& in){
std::string ignoredValue;
return in >> ignoredValue;
}
to be used like:
std::cin >> ignore >> a >> b;
and if you wanted to verify that it was of a form that could be read into a type you could supply an additional template argument specifying the type of ignored value:
// default arguments to allow use of ignore without explicit type
template <typename T = std::string, typename CharT = char>
std::basic_istream<CharT>& ignore(std::basic_istream<CharT>& in){
T ignoredValue;
return in >> ignoredValue;
}
to be used like:
std::cin >> ignore >> a >> b;
// and
std::cin >> ignore<int> >> a >> b;
demo on coliru

Answer 2

You can use std::istream::ignore.
E.g.:
file.ignore(std::numeric_limits<std::streamsize>::max(), ' '); //columns are separated with space so passing it as the delimiter.
file >> a >> b;

Answer 3

You can use file::ignore(255, ' ') to ignore characters until the next space.
std::ifstream file(file_name);
double a, b;
for (lines) {
// skip first value until space
file.ignore(255, ' ');
file >> a >> b;
store(a, b);
}
or you can use an auxiliary variable to store the first value:
std::ifstream file(file_name);
double aux, a, b;
for (lines) {
// skip first value
file >> aux >> a >> b;
store(a, b);
}