Why would we call cin clear and cin ignore after reading input

Question

Google Code University s C   tutorial used to have this code      Description  Illustrate the use of cin to get input    and how to recover from errors    include  lt iostream gt  using namespace std   int main       int input var   0       Enter the do while loop and stay there until either      a non-numeric is entered  or -1 is entered   Note that      cin will accept any integer  4  40  400  etc    do       cout  lt  lt   Enter a number  -1   quit             The following line accepts input from the keyboard into        variable input var         cin returns false if an input operation fails  that is  if        something other than an int  the type of input var  is entered      if    cin  gt  gt  input var           cout  lt  lt   Please enter numbers only    lt  lt  endl        cin clear          cin ignore 10000   n              if  input var    -1          cout  lt  lt   You entered    lt  lt  input var  lt  lt  endl              while  input var    -1     cout  lt  lt   All done    lt  lt  endl     return 0      What is the significance of cin clear   and cin ignore    Why are the 10000 and  n parameters necessary

User · Accepted Answer

The cin clear   clears the error flag on cin  so that future I O operations will work correctly   and then cin ignore 10000    n   skips to the next newline  to ignore anything else on the same line as the non-number so that it does not cause another parse failure    It will only skip up to 10000 characters  so the code is assuming the user will not put in a very long  invalid line

User · Answer

You enter the  if    cin  gt  gt  input var     statement if an error occurs when taking the input from cin  If an error occurs then an error flag is set and future attempts to get input will fail  That s why you need  cin clear      to get rid of the error flag  Also  the input which failed will be sitting in what I assume is some sort of buffer  When you try to get input again  it will read the same input in the buffer and it will fail again  That s why you need  cin ignore 10000   n      It takes out 10000 characters from the buffer but stops if it encounters a newline   n   The 10000 is just a generic large value

User · Answer

use cin ignore 1000   n   to clear all of chars of the previous cin get   in the buffer and it will choose to stop when it meet   n  or 1000 chars first

User · Answer

Why do we use   1  cin ignore  2  cin clear     Simply   1  To ignore  extract and discard  values that we don t want on the stream  2  To clear the internal state of stream  After using cin clear internal state is set again back to goodbit  which means that there are no  errors    Long version   If something is put on  stream   cin  then it must be taken from there  By  taken  we mean  used    removed    extracted  from stream  Stream has a flow  The data is flowing on cin like water on stream  You simply cannot stop the flow of water     Look at the example   string name    line 1 cout  lt  lt   Give me your name and surname   lt  lt endl   line 2 cin  gt  gt  name   line 3 int age   line 4 cout  lt  lt   Give me your age    lt  lt endl   line 5 cin  gt  gt  age   line 6   What happens if the user answers   Arkadiusz Wlodarczyk  for first question   Run the program to see for yourself   You will see on console  Arkadiusz  but program won t ask you for  age   It will just finish immediately right after printing  Arkadiusz    And  Wlodarczyk  is not shown  It seems like if it was gone       What happened   -   Because there is a space between  Arkadiusz  and  Wlodarczyk     space  character between the name and surname is a sign for computer that there are two variables waiting to be extracted on  input  stream   The computer thinks that you are tying to send to input more than one variable  That  space  sign is a sign for him to interpret it that way   So computer assigns  Arkadiusz  to  name   2  and because you put more than one string on stream  input  computer will try to assign value  Wlodarczyk  to variable  age       The user won t have a chance to put anything on the  cin  in line 6 because that instruction was already executed     Why  Because there was still something left on stream  And as I said earlier stream is in a flow so everything must be removed from it as soon as possible  And the possibility came when computer saw instruction cin    age   Computer doesn t know that you created a variable that stores age of somebody  line 4    age  is merely a label  For computer  age  could be as well called   afsfasgfsagasggas  and it would be the same  For him it s just a variable that he will try to assign  Wlodarczyk  to because you ordered instructed computer to do so in line  6    It s wrong to do so  but hey it s you who did it  It s your fault  Well  maybe user  but still       All right all right  But how to fix it    Let s try to play with that example a bit before we fix it properly to learn a few more interesting things  -   I prefer to make an approach where we understand things  Fixing something without knowledge how we did it doesn t give satisfaction  don t you think      string name  cout  lt  lt   Give me your name and surname   lt  lt endl  cin  gt  gt  name  int age  cout  lt  lt   Give me your age    lt  lt endl  cin  gt  gt  age  cout  lt  lt  cin rdstate      new line is here  -    After invoking above code you will notice that the state of your stream  cin  is equal to 4  line 7   Which means its internal state is no longer equal to goodbit  Something is messed up  It s pretty obvious  isn t it  You tried to assign string type value   Wlodarczyk   to int type variable  age   Types doesn t match  It s time to inform that something is wrong  And computer does it by changing internal state of stream  It s like   You f     up man  fix me please  I inform you  kindly   -    You simply cannot use  cin   stream  anymore  It s stuck  Like if you had put big wood logs on water stream  You must fix it before you can use it  Data  water  cannot be obtained from that stream cin  anymore because log of wood  internal state  doesn t allow you to do so   Oh so if there is an obstacle  wood logs  we can just remove it using tools that is made to do so   Yes   internal state of cin set to 4 is like an alarm that is howling and making noise   cin clear clears the state back to normal  goodbit   It s like if you had come and silenced the alarm  You just put it off  You know something happened so you say   It s OK to stop making noise  I know something is wrong already  shut up  clear     All right let s do so  Let s use cin clear     Invoke below code using  Arkadiusz Wlodarczyk  as first input   string name  cout  lt  lt   Give me your name and surname   lt  lt endl  cin  gt  gt  name  int age  cout  lt  lt   Give me your age    lt  lt endl  cin  gt  gt  age  cout  lt  lt  cin rdstate    lt  lt  endl   cin clear      new line is here  -  cout  lt  lt  cin rdstate   lt  lt  endl     new line is here  -    We can surely see after executing above code that the state is equal to goodbit   Great so the problem is solved   Invoke below code using  Arkadiusz Wlodarczyk  as first input   string name  cout  lt  lt   Give me your name and surname   lt  lt endl  cin  gt  gt  name  int age  cout  lt  lt   Give me your age    lt  lt endl  cin  gt  gt  age  cout  lt  lt  cin rdstate    lt  lt  endl    cin clear     cout  lt  lt  cin rdstate    lt  lt  endl   cin  gt  gt  age   new line is here  -    Even tho the state is set to goodbit after line 9 the user is not asked for  age   The program stops   WHY    Oh man    You ve just put off alarm  what about the wood log inside a water   Go back to text where we talked about  Wlodarczyk  how it supposedly was gone   You need to remove  Wlodarczyk  that piece of wood from stream  Turning off alarms doesn t solve the problem at all  You ve just silenced it and you think the problem is gone      So it s time for another tool   cin ignore can be compared to a special truck with ropes that comes and removes the wood logs that got the stream stuck  It clears the problem the user of your program created   So could we use it even before making the alarm goes off   Yes   string name  cout  lt  lt   Give me your name and surname   lt  lt  endl  cin  gt  gt  name  cin ignore 10000    n      time to remove  Wlodarczyk  the wood log and make the stream flow int age  cout  lt  lt   Give me your age    lt  lt  endl  cin  gt  gt  age    The  Wlodarczyk  is gonna be removed before making the noise in line 7   What is 10000 and   n    It says remove 10000 characters  just in case  until   n  is met  ENTER   BTW It can be done better using numeric limits but it s not the topic of this answer     So the main cause of problem is gone before noise was made     Why do we need  clear  then   What if someone had asked for  give me your age  question in line 6 for example   twenty years old  instead of writing 20   Types doesn t match again  Computer tries to assign string to int  And alarm starts  You don t have a chance to even react on situation like that  cin ignore won t help you in case like that   So we must use clear in case like that   string name  cout  lt  lt   Give me your name and surname   lt  lt  endl  cin  gt  gt  name  cin ignore 10000    n      time to remove  Wlodarczyk  the wood log and make the stream flow int age  cout  lt  lt   Give me your age    lt  lt  endl  cin  gt  gt  age  cin clear    cin ignore 10000    n      time to remove  Wlodarczyk  the wood log and make the stream flow   But should you clear the state  just in case    Of course not   If something goes wrong  cin    age   instruction is gonna inform you about it by returning false   So we can use conditional statement to check if the user put wrong type on the stream  int age  if  cin  gt  gt  age    it s gonna return false if types doesn t match     cout  lt  lt   You put integer   else     cout  lt  lt   You bad boy  it was supposed to be int     All right so we can fix our initial problem like for example that   string name  cout  lt  lt   Give me your name and surname   lt  lt  endl  cin  gt  gt  name  cin ignore 10000    n      time to remove  Wlodarczyk  the wood log and make the stream flow  int age  cout  lt  lt   Give me your age    lt  lt  endl  if  cin  gt  gt  age    cout  lt  lt   Your age is equal to    lt  lt  endl  else    cin clear     cin ignore 10000    n      time to remove  Wlodarczyk  the wood log and make the stream flow  cout  lt  lt   Give me your age name as string I dare you    cin  gt  gt  age      Of course this can be improved by for example doing what you did in question using loop while   BONUS   You might be wondering  What about if I wanted to get name and surname in the same line from the user  Is it even possible using cin if cin interprets each value separated by  space  as different variable   Sure  you can do it two ways   1   string name  surname  cout  lt  lt   Give me your name and surname   lt  lt  endl  cin  gt  gt  name  cin  gt  gt  surname   cout  lt  lt   Hello     lt  lt  name  lt  lt       lt  lt  surname  lt  lt  endl    2  or by using getline function   getline cin  nameOfStringVariable     and that s how to do it   string nameAndSurname  cout  lt  lt   Give me your name and surname   lt  lt  endl  getline cin  nameAndSurname    cout  lt  lt   Hello     lt  lt  nameAndSurname  lt  lt  endl    The second option might backfire you in case you use it after you use  cin  before the getline   Let s check it out   a   int age  cout  lt  lt   Give me your age    lt  lt endl  cin  gt  gt  age  cout  lt  lt   Your age is   lt  lt  age  lt  lt  endl   string nameAndSurname  cout  lt  lt   Give me your name and surname   lt  lt  endl  getline cin  nameAndSurname    cout  lt  lt   Hello     lt  lt  nameAndSurname  lt  lt  endl    If you put  20  as age you won t be asked for nameAndSurname   But if you do it that way   b   string nameAndSurname  cout  lt  lt   Give me your name and surname   lt  lt  endl  getline cin  nameAndSurname    cout  lt  lt   Hello     lt  lt  nameAndSurname  lt  lt  endl  int age  cout  lt  lt   Give me your age    lt  lt endl  cin  gt  gt  age  cout  lt  lt   Your age is   lt  lt  age  lt  lt  endll   everything is fine   WHAT    Every time you put something on input  stream  you leave at the end white character which is ENTER    n   You have to somehow enter values to console  So it must happen if the data comes from user   b  cin characteristics is that it ignores whitespace  so when you are reading in information from cin  the newline character   n  doesn t matter  It gets ignored   a  getline function gets the entire line up to the newline character    n    and when the newline char is the first thing the getline function gets   n   and that s all to get  You extract newline character that was left on stream by user who put  20  on stream in line 3   So in order to fix it is to always invoke cin ignore    each time you use cin to get any value if you are ever going to use getline   inside your program   So the proper code would be   int age  cout  lt  lt   Give me your age    lt  lt endl  cin  gt  gt  age  cin ignore       it ignores just enter without arguments being sent  it s same as cin ignore 1    n    cout  lt  lt   Your age is   lt  lt  age  lt  lt  endl    string nameAndSurname  cout  lt  lt   Give me your name and surname   lt  lt  endl  getline cin  nameAndSurname    cout  lt  lt   Hello     lt  lt  nameAndSurname  lt  lt  endl    I hope streams are more clear to you know   Hah silence me please   -

[c++] Why would we call cin.clear() and cin.ignore() after reading input?

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