Could seem an old question, but the problem here isn't the use of TWO cin.get(), but of more than two! if I write (in DEV C++)
I get just one input request (s) and then end program. Now, I expected having at least two request of cin, because I expected:
char s[50];
char t[100];
char r[100];
char f[100];
cin.get(s,49);
cin.get(t,99);
cin.get(r,99);
cin.get(f,99);
I expeted at least 2 input request, because:
first cin: buffer empty,I insert the string s and \n
second cin: I have in buffer \n still, then t=\n without input request
third cin: buffer empty, I insert the string r and \n
fourth cin: I have in buffer \n still, then f=\n without input request
But I have just the input request for s string!
why have I just one input request?the buffer didn't clean with second cin.get, letting third cin.get work properly? Thanks
t does NOT equal '\n'. It's empty. .get(char*,int) will never remove the '\n' from the buffer.
Worse, the attempt to read to t will set cin to a fail state since nothing could be read, which will cause all subsequent reads of any sort from cin to fail immediately without even trying until you .clear() the fail state.
This is surprising behavior, but you seem to have already guessed at most of it as per your last sentence in the question, so, Good Job! You're learning!
http://en.cppreference.com/w/cpp/io/basic_istream/get
Related
I have a program that takes two numbers and shows them on the screen.
However, when I hit "enter" after I input the first number, my program shows the answers before letting me input the second number.
Why does this happen?
int main()
{
int n1;
float n2;
cin>>n1;
cin>>n2;
cout<<"int n:"<<n1<<endl<<"float n:"<<n2;
return 0;
}
I wanna input 0.25 and 35 but when I write 0.25 and hit enter suddenly shows the answer "int: n:0 float n:0.25" it doesn't let me write second num. my os is Win10 and this program compiled with DevCpp
It works when both variables are ints.
There is no difference between cin>>n1; cin>>n2; and cin >> n1 >>n2. Enter key only serves as signal to sychronize input buffer and stream buffer. cin doesn't input per line, it parses buffer when there is available volume of data. If parse incomplete, it waits. If parse can't be done, it stops and state bit changes. To continue parsing you have either ignore or clear part or whole buffer content.
Something wrong was entered in first line, causing cin to go into bad() state. Edge case might happen if you're running program through a remote terminal, some incorrect character could slip in, e.g. ^M generated by new line from Windows would break cin stream on Linux. That's also case if you input from a file which was saved on different platform. On Windows line ends consist of two characters, #10 and #13. On linux steams expect only #13 as a new line and buffer flush signal, #10 is an unexpected character.
Edit (after OP gave information about input data):
"0.25" would be parsed as "0" and ".25", that expected and documented stream behavior. Parsing for n1 had stopped as soon as stream encountered character which doesn't fit int pattern, which could be space, end of line, alphabetic or punctuation. Period considered a punctuation in this case
Then it tries to get a float from stream input and buffer contains ".25". It's a legal float notation and it gets assigned to n2.
When you have both "int", you cannot get second value at all with same input, it always will be 0, because cin locks up in bad state, i.e. method its istream::good() returns false. You have to check state of stream after reading variables. Any further formatted reading that wouldn't be able to parse .25 wouldn't advance stream past that point.
If you want to read from stream exclusively line by line, you have to use istream::getline() method to get the string. There is also method get which can acquire content of stream and ignore which allows to discard part of stream.
I was learning how to read strings with getline function.
I know that getline function read string as far as we don't hit enter or the size value in the getline parameter go cross. As far as I tried getline function to read one line of string I had not faced any problem.
But when I was trying to read two line of string one after another in two different char array i got the output that was not expected to me.
To understand my question follow bellow lines
#include <iostream>
using namespace std;
int main()
{
char line1[10];
char line2[10];
cin.getline(line1,7);
cin.getline(line2,7);
cout << "\nline1 =" << line1 <<endl;
cout << "line2 =" << line2 <<endl;
}
When I ran the above program it ask me for input then I gave orange as first input and hit the enter button.
Next it ask me to give the second input .then i gave banana and hit the enter button .in this case it produce the result i expected .But if enter oranges for the first input it does not wait for me to enter the second input.
As a result line1 store orange but line2 remains blank.
Now my question is that there is no wrong with line1 storing orange. But I don't understand why the line2 remains blank should not it contain the data that remains after line1 take input I mean should not line2 contain s as value.
Because orange is a 6 digit word so getline will stores the first six digit after then a null character will be added as I set the size of geline 7.
Then other remaing data will be assigend in the next call of getline function.So should not s stored in line2 as after s a new_line character is read for the first time.
Why will be line2 remain blank and why the screen doesn't stop for taking input after giving the first input?
std::istream::getline is being overloaded with data.
According to cppreference,
Behaves as UnformattedInputFunction. After constructing and checking the sentry object, extracts characters from *this and stores them in successive locations of the array whose first element is pointed to by s, until any of the following occurs (tested in the order shown):
end of file condition occurs in the input sequence (in which case setstate(eofbit) is executed)
the next available character c is the delimiter, as determined by Traits::eq(c, delim). The delimiter is extracted (unlike basic_istream::get()) and counted towards gcount(), but is not stored.
count-1 characters have been extracted (in which case setstate(failbit) is executed).
Emphasis mine.
cin.getline(line1,7);
// ^ This is count
can read only 6 characters with the 7th reserved for the null terminator. "oranges" is seven characters, and this places cin in a non-readable error state that must be cleared before reading can be continued. Reading of the second line
cin.getline(line2,7);
instantly fails and no data is read.
The obvious solution is
cin.getline(line1, sizeof(line1));
to take advantage of the whole array. But...
Any IO transaction should be tested for success, so
if (cin.getline(line1, sizeof(line1)))
{
// continue gathering
}
else
{
// handle error
}
is a better option.
Better still would be to use std::getline and std::string to almost eliminate the size constraints.
Im using scanf because we must use it.
the problem is the following :
(thats just an example of the problem):
int main() {
char ch [10]={0};
scanf("%s",ch);
printf("%s",ch);
}
if i run the program and enter for example : word^Z
when ^Z is EOF.
the program stays in place, stuck in the scanf, althogh i did type word then ctrl+z then Enter. but somehow it stays in the scanf, its the same thing with redirection, like its not a problem with ctr+z or anything.
i hope that i can get some help
thanks in advance,
totally apprecaite it :)
scanf uses whitespace as a delimiter to store the read data into various fields. From the command line, entering ControlZ, then Enter only puts the EOF character into the input stream and scanf() continues waiting for whitespace. If you hit Enter again, scanf will receive the whitespace character, and everything including the EOF will be stored into the ch array.
Here's a sample run. The first line is the input, and the second line is the output.
Hello^Z
Hello→
For some reason in my program when I reach a certain spot, I have to press Enter twice in order to get it to submit. I added the clear to keep it from skipping input and the ignore() to keep it from keeping any extra characters in the buffer. I enter my input and then it drops down to a new line, I hit Enter again and it enter the input and continues the program no problem but I'm wondering why. Here's a code snippet:
cin.ignore();
cout << "Enter Student Major (ex. COSC): ";
cin.getline(student.major, 6);
for(int i = 0; i < sizeof(student.major); i++)
student.major[i] = toupper(student.major[i]);
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
Any suggestions?
It seems to me that you are tossing too many cin.ignore() around, not knowing exactly why they are needed and when to put them there.
There are two common circumstances where cin.ignore() is needed to "make input work right":
when mixing formatted and unformatted input;
to recover from a formatted input error.
In both cases, you want to get rid of spurious characters from the input buffer; if there isn't any such character (which is probably what happens in your program), cin.ignore() will pause the execution and wait for user input - after all, you asked it to ignore some characters, and dammit, it will obey to its orders.
(although ignore() by default would "eat" just one character, whatever it may be, the execution is paused until a newline is found because by default cin is line buffered - new input is not examined until a newline is recieved)
Case 1:
cin.ignore() calls are often needed if you are performing an unformatted input operation (like getline) after performing a formatted input operation (i.e. using the >> operator).
This happens because the >> operator leaves the newline in the input buffer; that's not a problem if you are performing only formatted input operations (by default they skip all the whitespace before trying to interpret the input), but it's a problem if afterwards you do unformatted input: getline by default reads until it finds a newline, so the "spurious newline" left will make it stop reading immediately.
So, here you will usually call cin.ignore(...) call to get rid of the newline just after the last formatted input operation you do in a row, guaranteeing that the input buffer is empty. Afterwards, you can call getline directly without fear, knowing that you left the buffer empty.
It's a bad idea, instead, to put it before any getline, as you seem to do in your code, since there may be code paths that lead to that getline that have the input buffer clean, so the ignore call will block.
Case 2:
when istream encounters an error in a formatted input operations, it leaves the "bad" characters in the buffer, so if you retry the operation you get stuck endlessly, since the offenders are still there. The usual clear()/ignore() idiom comes to the rescue, removing the whole offending line from the input buffer.
Again, you don't put the clear()/ignore() sequence at random, but only after you get an input error from a formatted input operation (which sets the failbit of the stream).
Now, aside from these cases, it's uncommon to use cin.ignore() (unless you actually want to skip characters); don't spread it around randomly "just to be safe", because otherwise you will encounter the problem you described.
The answer can be found here.
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.
So in your case, the program will not continue until a '\n' character is received.
I think cin.ignore(numeric_limits<streamsize>::max(), '\n'); is expecting a \n in the input and it doesn't find it, so you have to press Enter again for it to find it.
I mam trying to complete a college assignment in C++ and am having trouble with what should be a very basic operation. I am trying to read a string of characters from the keyboard. This is the relevant code:
string t;
cout << endl << "Enter title to search for: ";
getline(cin, t, '\n');
I understand, that the last line is supposed to read the input buffer (cin , in this instance) and store the character in the 't' string until it reaches a new line character and then continue the program flow.
However, when I run my code in XCode, it just sort of jumps over the getline function and treats 't' as an empty string.
What's going on? I tried using cin >> t but that just read characters forever - Why cant I get this to behave?
The reason that the input operation apparently is skipped, is most probably (that means, ignoring possible peculiarities of a bugsy XCode IDE) that you have performed some input earlier and left a newline in the input buffer.
To fix that, make sure that you have emptied the input buffer after each input operation that logically should consume a line of input.
One easy way is to always use getline into a string, and then use e.g. an istringstream if you want to convert a number specification to number type.
Cheers & hth.,
From the docs page it looks like you want
cin.getline(t,256,'\n');
or something similar.
This sounds like an issue with the way Xcode is running your program. Try running your program directly from the terminal, and see if this is sufficient to fix your issue.