cin.clear() doesn't reset cin object

Question

I have the following loop. It should read numbers until EndOfFile, or the user input -999

int arr[100];

int index;

for (index = 0; index < 100; index++)
{
 cin >> arr[index];
 if (!cin)
 {
  cin.clear();
  index--;
  continue;
 }
 if (arr[index] == -999)
 {
     break;
 }
}

When the user input an invalid thing, such as some chars, this loop is being repeated for ever without clearing the error state or stopping.

score 11 · Accepted Answer · edited Mar 04 '19 at 21:01

11

After calling clear, you must also somehow remove the invalid input from the stream. Here is one way:

 cin >> arr[index];
 if (!cin)
 {
  cin.clear();
  std::string ignoreLine; //read the invalid input into it
  std::getline(cin, ignoreLine); //read the line till next space
  index--;
  continue;
 }

It's because when cin fails to read the invalid input, it remains there in the stream. It has to be removed, by some means. I just read and ignore it.

You can also use ignore as:

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

which is better in my opinion provided inputs are space separated (and if you don't want to inspect the invalid input). The online doc says:

istream::ignore

istream& ignore( streamsize n = 1, int delim = EOF );

Extract and discard characters

Extracts characters from the input sequence and discards them.

The extraction ends when n characters have been extracted and discarded or when the character delim is found, whichever comes first. In the latter case, the delim character itself is also extracted.

edited Mar 04 '19 at 21:01

scohe001

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answered Sep 14 '11 at 08:22

Nawaz

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@Mr.DDD: What invalid input your provide? – Nawaz Sep 14 '11 at 08:43
Your second version doesn't work. You are igonring `1024` characters or when `' '` is found. However, a user may not input space at the end, and the input will be contiued to 1024 characters! – Sep 14 '11 at 08:46
@Mr.DDD: You can put some bigger value, instead of `1024`. – Nawaz Sep 14 '11 at 08:49
1

@Mr.DDD: That's good. But what if you've valid input on the same line just after the invalid input?By the way, you could use `std::numeric_limits::max()` instead of `1024`. – Nawaz Sep 14 '11 at 08:52
You've to edit your answer, since the space delim won't work until the user input space. User must press ENTER so that \n is found – Sep 14 '11 at 08:53
@Mr.DDD: I just added `[...] provided inputs are space separated`. – Nawaz Sep 14 '11 at 08:56

cin.clear() doesn't reset cin object

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