C++ read from istream until newline (but not whitespace)

I have a std::istream which refers to matrix data, something like:

0.0 1.0 2.0 3.0 4.0 5.0 

Now, in order to assess the number of columns I would like to have some code like:

std::vector<double> vec; double x; while( (...something...) && (istream >> x) ) { vec.push_back(x); } //Here vec should contain 0.0, 1.0 and 2.0 

where the ...something... part evaluates to false after I read 2.0 and istream at the point should be at 3.0 so that the next

istream >> x; 

should set x equal to 3.0.

How would you achieve this result? I guess that the while condition

Thank you very much in advance for your help!

6 Answers

Use the peek method to check the next character:

while ((istream.peek()!='\n') && (istream>>x)) 

Read the lines into a std::string using std::getline(), then assign the string to a std::istringstream object, and extract the data from that rather than directly from istream.

std::vector<double> vec; { std::string line; std::getline( ifile, line ); std::istringstream is(line); std::copy( std::istream_iterator<double>(is), std::istream_iterator<double>(), std::back_inserter(vec) ); } std::cout << "Input has " << vec.size() << " columns." << std::endl; std::cout << "Read values are: "; std::copy( vec.begin(), vec.end(), std::ostream_iterator<double>( std::cout, " " ) ); std::cout << std::endl; 

You can use std::istream::peek() to check if the next character is a newline. See this entry in the cplusplus.com reference.

Read the number, then read one character to see if it's newline.

I had similar problem
Input is as below:

1 2 3 4 5 

The 1st two were N1 and N2
Then there is a newline
then elements 3 4 5, i dont know how many these will be.

// read N1 & N2 using cin int N1, N2; cin >> N1; cin >> N2; // skip the new line which is after N2 (i.e; 2 value in 1st line) cin.ignore(numeric_limits<streamsize>::max(), '\n'); // now read 3 4 5 elements int ele; // 2nd EOF condition may required, // depending on if you dont have last new-line, and it is end of file. while ((cin_.peek() != '\n') && (cin_.peek() != EOF)) { cin >> ele; // do something with ele } 

This worked perfect for me.