I am learning c++, however, I can not understand what is the difference BTW:
std::cin.get();
and
std::cin.getline();
although;I know how to use each of them, but can't understand why there are two?
I've read this explanation :
getlinereads the newline character then discard it; whereas .get()reads it then leaves it in the input queue ..!! why each of them does what it does ?
sorry for bad English :(
std::cin.get(), when called without parameters, reads one single character from input and returns it.
std::cin.getline(char* str, std::streamsize count) reads one line of input and copies it into the buffer str, followed by an extra null character to form a C-string. count must be the size of that buffer, i.e. the maximal number of characters (plus the null byte) it can copy into it.
To read a line without caring about the buffer size it may be better to use std::getline:
#include <string>
std::string line;
std::getline(std::cin, line);
Reads a line from cin into line.
See http://en.cppreference.com/w/cpp/io/basic_istream and http://en.cppreference.com/w/cpp/string/basic_string/getline .
"get" only retrieves a character, "getline" gets all characters to a line terminator. Thats the main difference.
Related
I am using the getline() function in C++, but there is a problem that the input starts from the second character. I used the ignore() function to erase what remains in the buffer first, emptying the buffer and receiving input. How can I empty the buffer and receive input properly?
Above is the execution result. I previously used the ignore() function and the getline() function to empty the buffer and receive input because there may be some leftovers in the buffer before.
In other programs that write like that, it also receives integer input before.
void InputValue(string *st) {
//cin.clear();
cin.ignore();
getline(cin, *st);
}
int main(void) {
string str;
InputValue(&str);
cout << str;
return 0;
}
In the beginning, immediately after input, stdin (your input buffer) contains :
abcd
Those character are waiting to be extracted in whatever manner you choose. Since you are reading from stdin (using std::cin) using getline() from the stream into a std::string, getline() will consume all characters in the line, reading and discarding the trailing '\n' with all characters stored in your std::string. There are no characters left in stdin that need to be extracted using std::cin.ignore()
When you call std::cin.ignore() before getline() you are reading (and discarding) the first character waiting to be read in stdin. With the example characters above, after calling std::cin.ignore(), and extracting and discarding the character, the following is left in stdin:
bcd
So now when you do call getline (std::cin, *st); "bcd" are the only characters that are available. That is the reason why you miss the first character.
While you can std::cin.ignore() one character, the .ignore() member function is usually used to read/discard all remaining characters in a line using the form
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
Where #include <limits> provides the constants representing the maximum number of the type specified, above <std::streamsize>::max() (e.g. the maximum value that size_t can represent). A large constant is used to ensure the remainder of the line, no matter how long, is consumed and discarded leaving std::cin either empty or with the first character of the next line (in a multi-line buffer) waiting to be read.
std::basic_istream::ignore can also read up to a delimiter character reading/discarding less than the whole line. That can be useful when reading separated fields, or in cases where you use a delimiter with getline() to read less than an entire line.
You either perform your read operation first and then call ignore() to discard unwanted characters to the end of the line, or you can call ignore() first with either a specified number of characters (default 1) or a delimiter to discard the beginning portion of a line up to the first character you want to began your read with.
As you have used it above, it will simply chop off the first character of whatever the user enters -- which does not seem to be what you want. In fact, ignore() isn't even needed above. Remove it and see if the missing character problem doesn't go away.
I am new to C++ and I want to ask a question about how to find a line in a file using fstream.
I only found this and would someone explain to me what these parameters mean?
file.getline（char *,int sz）；
Thanks
If you mean std::basic_stream::getline(), you provide a pointer to character array and the size of that array. You have to create the array somewhere by yourself. If some line is longer than sz - 1, only part of it with length sz - 1 will be read.
If you don't know the maximum length of lines in the input file, it's better to use std::getline(), for example like this:
std::string line;
std::getline(file, line);
Directly from here:
The first variable:
Pointer to an array of characters where extracted characters are stored as a c-string.
The second variable:
Maximum number of characters to write to s (including the terminating null character).
If the function stops reading because this limit is reached without finding the delimiting character, the failbit internal flag is set.
streamsize is a signed integral type.
Why these two functions istream::get(char*, streamsize) and istream::get(char*, streamsize, char) set the cin.fail bit when they find '\n' as the first character in the cin buffer?
As can be seen here, that's the behavior of the two overloads mentioned above. I'd like to know what was the purpose in designing these functions this way ? Note that both functions leave the character '\n' in the buffer, but if you call any of them a second time, they will fail because of the newline character, as shown in the link. Wouldn't it make more sense to make these two functions not to leave the character '\n' in the buffer, as the overloads of the function istream::get() and istream::getline() do ?
With std::istream::getline, if the delimiting character is found it is extracted and discarded. With std::istream::get the delimiting character remains in the stream.
With getline you don't know, if the delimiting character was read and discarded or if just n - 1 characters where read. If you only want to read whole lines, you can use get and then peek for the next character and see if it is a newline or the given delimiter.
But if you want to read whole lines up to some delimiter, you might also use std::getline, which reads the complete line in any case.
char arr[100];
cin.get(arr,100);
Is this safe? Will the null-character be appended at the end even if I type more than 100 chars? or should I use cin.get(arr,99)?
When I type ENTER, is the end-of-line character part of the array or not?
The answers to both of your questions can be found here, but to reiterate:
The get method reads at most n - 1 characters. This means that the method expects the size of the buffer and not the number of characters to read. This method automatically appends a null character to the end.
The newline character is not extracted or stored in the array.
You may also want to consider using std::getline which you can use in conjunction with std::string.
1)is this safe. I mean will the null-character be appended
at the end. even if I typed more than 100 chars. or it must be cin.get(arr,99).
Taken from here.
The signature of get you are using looks like this:
basic_istream& get( char_type* s, std::streamsize count );
It will read at most count - 1 characters from the stream (in your case 99) or up until the delimiting character, which is by default \n. So if you type more than 100 characters, a call to get will read 99 of those characters and then append the null terminator \0 at the end.
2)also when I type ENTER, a newline get passed. so does this character is really a part of the array or not.
No, get read up until the delimiting character, so if you press ENTER, \n will be left in the stream as the next character to be read.
Advice:
Please use the site I linked to in order to understand how these functions work, and prefer std::string and std::getline if you are coding in C++.
I read istream::get and a doubt still hangs. Let's say my delimiter is actually the NULL '\0' character, what happens in this case? From what I read:
If the delimiting character is found, it is not extracted from the input sequence and remains as the next character to be extracted. Use getline if you want this character to be extracted (and discarded). The ending null character that signals the end of a c-string is automatically appended at the end of the content stored in s.
The reason I would prefer "get" over "readline" is because of the capability to extract the character stream into a "streambuf".
I dont' quite get your problem.
On the msdn website, for the get function, it says:
In all cases, the delimiter is neither extracted from the stream nor returned by the function. The getline function, in contrast, extracts but does not store the delimiter.
In all cases, the delimiter is neither extracted from the stream nor returned by the function. The getline function, in contrast, extracts but does not store the delimiter.
http://msdn.microsoft.com/en-us/library/aa277360(VS.60).aspx
I don't think your going to have a problem, since the msdn site tells that the delimiter is neither extracted from the stream, nor returned vy the function.
Or maybe I'm missing a point here?
If you have something like this, then delimiter will not get stuck in the input stream:
std::string read_str(std::istream & in)
{
const int size = 1024;
char pBuffer[size];
in.getline(pBuffer, size, '\0');
return std::string(pBuffer);
}
just an example if you have '\0' as delimiter and strings are not bigger than 1024 bytes.