How to clear the contents read by cin

Question

After typing an integer and pressing ENTER,the newline is getting stored.
So when i do a char c = cin.get() , the previously entered newline is being assigned to variable c.The following loop gets skipped because of this.
.How to clear the contents of cin? Or how to avoid reading the newline?

program:

char s;
int T;
cin >> T; // pressing ENTER here.
while(T--)
{
  s = cin.get(); // storing the previously pressed ENTER.('\n')
  while(s != '\n') // does not execute.
  {
   .
   .
   .
   .
   .
   .
  }
}

@utnapistim i need to read one character at a time and perform some tasks on the character read. — Pradeep, Mar 03 '15 at 15:49

Emil Laine · Accepted Answer · 2015-03-03T15:30:12.270

2

std::cin.ignore();

to ignore one character.

std::cin.ignore(n);

to ignore n characters.

std::cin.ignore(std::numeric_limits<std::streamsize>::max());

to ignore all characters.

Optionally, add a second parameter to define a delimiting character. The default is EOF.

edited Mar 03 '15 at 15:30

answered Mar 03 '15 at 15:24

Emil Laine

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... up to a delimiter, which is by default EOF. You might want to mention this, or even better, suggest using `'\n'` as the delimiter. – Angew is no longer proud of SO Mar 03 '15 at 15:28
@Angew Is EOF a character or a state of `std::istream` actually (or both)? – πάντα ῥεῖ Mar 03 '15 at 15:31
@zenith Which character value actually represents EOF then? The link refers to the state only. – πάντα ῥεῖ Mar 03 '15 at 15:34
This only works if you know that a newline is next. It also doesn't work for CRLF on the input, and is very platform / format specific. – Jason C Mar 03 '15 at 15:35
1

@πάνταῥεῖ Actually [it's not a character](http://stackoverflow.com/questions/24991803/where-is-hex-code-of-the-eof-character). I have been living all my life thinking there's an EOF character... – Emil Laine Mar 03 '15 at 15:36
@zenith Exactly! It's a stream state. – πάντα ῥεῖ Mar 03 '15 at 15:36
1

@πάνταῥεῖ To be exact, the default delimiter is `Traits::eof()`, which for `std::char_traits` is "an integer value not equivalent to any value of `T`." So it's not a character, but an implementation-defined value. – Angew is no longer proud of SO Mar 04 '15 at 08:11

Jason C · Answer 2 · 2015-03-03T15:45:33.380

You have a few options.

You could just call cin.getline(), which will retrieve the remainder of the line including the newlines and leave the pointer at the start of the next line. Works as long as you know the remainder of the input line is ignorable.

You can also use cin.ignore(1) or cin.ignore(2) if you know a newline is next. For input streams that produce CR+LF instead of just one, you'll have to ignore two characters, and it's up to you to figure out if you're getting CR or CR+LF.

Another option is to use cin.peek() to check what the next character is ahead of time. That way you can use skip newlines without consuming non-newline characters. For example:

void skipNewlines (istream &in) {
    int ch;
    while ((ch = in.peek()) == '\r' || ch == '\n')
       in.ignore();
    // and now newlines skipped and next in.get() will read the next char.
}

You could modify that to skip all whitespace. That option probably works the smoothest with your use of cin >> T.

You could also consider writing your own character input function that reads the next character but ignores line breaks (or whatever). For example:

int getWithoutNewline (istream &in) {
    int ch;
    do {
        ch = in.get();
    } while (ch == '\n' || ch == '\r');
    return ch;
}

Then use getWithoutNewline(cin) instead of cin.get() to read characters when parsing your integers. You have flexibility there; you could e.g. translate the newlines to spaces. This has the advantage of being able to operate without you having to know anything about the location or form of the line breaks. However, it may not work for all situations.

Personally, I wouldn't take the approach of just blindly discarding all contents. I'm not even sure if that's reliably possible given that there's no end-of-file after each line. The closest approximation to what you're asking for, as far as this goes, is to just call getline() to read and consume the rest of the line (including the newlines).

Pick whichever one of those, or some variant of one of those, suits your application and usage the best.

score 0 · Answer 3 · answered Mar 03 '15 at 15:32

0

The answer from @zenith is correct. In addition to that, also look at std::cin.peek().

Here's some code:

char s;
int T;
cin >> T; // pressing ENTER here.
while(T--)
{
  while(std::isspace(cin.peek())) // peek used here
    cin.ignore(); // ignore spaces, tabs and enter

answered Mar 03 '15 at 15:32

utnapistim

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You could also `getline()` instead (it's not equivalent in general but it's equivalent for the OP's case of reading an integer and ignoring the rest of the line). – Jason C Mar 03 '15 at 15:47

How to clear the contents read by cin

3 Answers3