how to use getline correctly

c++, getline

Solution

The code mixes formatted input (i.e. using the `>>` operator for input) with unformatted input (in this case using `std::getline()`). Unformatted input does not skip leading whitespace while formatted input does skip leading whitespace. In both cases reading stops as soon as the input is the next character wouldn't fit the format.

For example, reading a `double` when the input is `3000.00\n...` stops after reading the last `0` because the '`\n` doesn't fit the `double`s format. The next character in the stream is, thus, a `\n`. The function `std::getline()` reads until it finds the first `\n` which is immediately: it extracts that one character and [successfully] produces an empty line. With your input the sequence `Dantooine` is next but doesn't fit the format of a `double` and the stream goes into fail state.

The simple fix is to always skip leading white space when switching from formatted input to unformatted input, e.g., using the `std::ws` manipulator:

std::getline(std::cin >> std::ws, s);

Whether this is suitable depends a bit on what you are trying to read. If the strings you want to read can start with a whitespace, you would need to be a bit more careful and, e.g., only extract whitespace up to the next newline:

std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

(just ignoring one character may not work, e.g., if the input of the line ends in `3000.00 \n`).

Reading `3000.00` as `long` actually stops at the `.`, i.e., the first change to your program is to correct the type of values read to be `double` rather than `long`. Note that you should also always check that your input was successful before processing it. That is, I would actually use something like this:

double      x0, y0, z0, x1, y1, z1;
std::string l0, l1;
while (0 < n--
       && std::getline(std::cin >> std::ws, l0)
       && std::cin >> x0 >> y0 >> z0
       && std::getline(std::cin >> std::ws, l1)
       && std::cin >> x1 >> y1 >> z1) {
    // do something with the input
}

Problem

I wrote a program that uses `getline()` function, but I don't know how should I use `getline()` in the correct way. I've searched it on the Internet but I just found something about about using `getchar()` with `getline()` or `cin.ignore()`. I tried and none of them worked and I don't know what is the problem and why `getline()` function needs `getchar()` or `cin.ignore()` to work correctly? My Input ``` 2 Alderaan 1000.00 2000.00 3000.00 Dantooine -1000.00 1000.00 1000.00 Circarpous Major -500.00 500.00 -500.00 Y’Toub -500.00 -500.00 500.00 ``` here is my code: ``` #include <cmath> #include <string> #include <cstdio> #include <iostream> using namespace std; int main() { long long int x, y, z, x1, y1, z1; int n; cin >> n; string s, p; while (n--) { getline(cin, s); cin >> x; cin >> y; cin >> z; cin.ignore(); getline(cin, p); cin >> x1; cin >> y1; cin >> z1; cin.ignore(); printf("%s to %s: %.2f\n", s.c_str(), p.c_str(), sqrt(pow(x - x1, 2) + pow(y - y1, 2) + pow(z - z1, 2))); } } ```

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