When and why do I need to use cin ignore in C

Question

I wrote a very basic program in C   which asked the user to input a number and then a string  To my surprise  when running the program it never stopped to ask for the string  It just skipped over it  After doing some reading on StackOverflow  I found out that I needed to add a line that said    cin ignore 256    n      before the line that gets the string input  Adding that fixed the problem and made the program work  My question is why does C   need this cin ignore   line and how can I predict when I will need to use cin ignore      Here is the program I wrote    include  lt iostream gt   include  lt string gt   using namespace std   int main         double num      string mystr       cout  lt  lt   Please enter a number     lt  lt    n       cin  gt  gt  num      cout  lt  lt   Your number is     lt  lt  num  lt  lt    n       cin ignore 256    n       Why do I need this line      cout  lt  lt   Please enter your name   n       getline  cin  mystr       cout  lt  lt   So your name is    lt  lt  mystr  lt  lt     n       cout  lt  lt   Have a nice day   n

User · Answer

As pointed right by many other users  It s because there may be whitespace or a newline character    Consider the following code  it removes all the duplicate characters from a given string    include  lt bits stdc   h gt  using namespace std   int main         int t      cin gt  gt t      cin ignore      Notice that this cin ignore   is really crucial for any extra whitespace or newline character     while t--           vector lt int gt  v 256 0           string s          getline cin s           string s2          for int i 0 i lt s size   i                 if  v s i    continue              else                  s2 push back s i                    v s i                                      cout lt  lt s2 lt  lt endl            return 0      So  You get the point that it will ignore those unwanted inputs and will get the job done

User · Answer

Ignore is exactly what the name implies   It doesn t  throw away  something you don t need instead  it ignores the amount of characters you specify when you call it  up to the char you specify as a breakpoint   It works with both input and output buffers    Essentially  for std  cin statements you use ignore before you do a getline call  because when a user inputs something with std  cin  they hit enter and a   n  char gets into the cin buffer  Then if you use getline  it gets the newline char instead of the string you want  So you do a std  cin ignore 1000   n   and that should clear the buffer up to the string that you want   The 1000 is put there to skip over a specific amount of chars before the specified break point  in this case  the  n newline character

User · Answer

Ignore function is used to skip discard throw away  characters in the input stream  Ignore file is associated with the file istream  Consider the function below ex  cin ignore 120   n    the particular function skips the next 120 input character or to skip the characters until a newline character is read

User · Answer

Short answer  Why   Because there is still whitespace  carriage returns  tabs  spaces  newline  left in the input stream   When   When you are using some function which does not on their own ignores the leading whitespaces  Cin by default ignores and removes the leading whitespace but getline does not ignore the leading whitespace on its own   Now a detailed answer   Everything you input in the console is read from the standard stream stdin  When you enter something  let s say 256 in your case and press enter  the contents of the stream become 256 n  Now cin picks up 256 and removes it from the stream and  n still remaining in the stream  Now next when you enter your name  let s say Raddicus  the new contents of the stream is  nRaddicus   Now here comes the catch  When you try to read a line using getline  if not provided any delimiter as the third argument  getline by default reads till the newline character and removes the newline character from the stream  So on calling new line  getline reads and discards  n from the stream and resulting in an empty string read in mystr which appears like getline is skipped  but it s not  because there was already an newline in the stream  getline will not prompt for input as it has already read what it was supposed to read   Now  how does cin ignore help here   According to the ignore documentation extract from cplusplus com-     istream amp  ignore  streamsize n   1  int delim   EOF        Extracts characters from the input sequence and discards them  until   either n characters have been extracted  or one compares equal to   delim       The function also stops extracting characters if the end-of-file is   reached  If this is reached prematurely  before either extracting n   characters or finding delim   the function sets the eofbit flag    So  cin ignore 256    n     ignores first 256 characters or all the character untill it encounters delimeter  here  n in your case   whichever comes first  here  n is the first character  so it ignores until  n is encountered    Just for your reference  If you don t exactly know how many characters to skip and your sole purpose is to clear the stream to prepare for reading a string using getline or cin you should use cin ignore numeric limits lt streamsize gt   max     n     Quick explanation  It ignores the characters equal to maximum size of stream or until a   n  is encountered  whichever case happens first

User · Answer

When you want to throw away a specific number of characters from the input stream manually   A very common use case is using this to safely ignore newline characters since cin will sometimes leave newline characters that you will have to go over to get to the next line of input   Long story short it gives you flexibility when handling stream input

User · Answer

You re thinking about this the wrong way  You re thinking in logical steps each time cin or getline is used  Ex  First ask for a number  then ask for a name  That is the wrong way to think about cin  So you run into a race condition because you assume the stream is clear each time you ask for a input  If you write your program purely for input you ll find the problem  void main void        double num      string mystr       cin  gt  gt  num      getline cin  mystr        cout  lt  lt   quot num  quot   lt  lt  num  lt  lt   quot  mystr    quot   lt  lt  mystr  lt  lt   quot    quot   lt  lt  endl     In the above  you are thinking   quot first get a number  quot  So you type in 123 press enter  and your output will be num 123 mystr     Why is that  It s because in the stream you have 123 n and the 123 is parsed into the num variable while  n is still in the stream  Reading the doc for getline function by default it will look in the istream until a  n is encountered  In this example  since  n is in the stream  it looks like it  quot skipped quot  it but it worked properly  For the above to work  you ll have to enter 123Hello World which will properly output num 123 mystr  Hello World   That  or you put a cin ignore between the cin and getline so that it ll break into logical steps that you expect  This is why you need the ignore command  Because you are thinking of it in logical steps rather than in a stream form so you run into a race condition  Take another code example that is commonly found in schools  void main void        int age      string firstName      string lastName       cout  lt  lt   quot First name   quot       cin  gt  gt  firstName       cout  lt  lt   quot Last name   quot       cin  gt  gt  lastName       cout  lt  lt   quot Age   quot       cin  gt  gt  age       cout  lt  lt   quot Hello  quot   lt  lt  firstName  lt  lt   quot   quot   lt  lt  lastName  lt  lt   quot   You are  quot   lt  lt  age  lt  lt   quot  years old  quot   lt  lt  endl     The above seems to be in logical steps  First ask for first name  last name  then age  So if you did John enter  then Doe enter  then 19 enter  the application works each logic step  If you think of it in  quot streams quot  you can simply enter John Doe 19 on the  quot First name  quot  question and it would work as well and appear to skip the remaining questions  For the above to work in logical steps  you would need to ignore the remaining stream for each logical break in questions  Just remember to think of your program input as it is reading from a  quot stream quot  and not in logical steps  Each time you call cin it is being read from a stream  This creates a rather buggy application if the user enters the wrong input  For example  if you entered a character where a cin  gt  gt  double is expected  the application will produce a seemingly bizarre output

User · Answer

It is better to use scanf       n   str  in c   than cin ignore   after cin   statement To do that first you have to include  lt  cstdio    header

[c++] When and why do I need to use cin.ignore() in C++?

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