There are some variations of this question but none exactly matches what I'm looking for.
Giving the following code:
string command="";
while (command.compare("quit")!=0)
{
os << prompt;
getline(is,command);
}
How may I detect if getline reached eof (end of file)
getline returns a reference to the stream you pass to it, and that stream will evaluate to false if it hits a failure state. Knowing that, you can leverage that to move getline into the condition for the while loop so that if it fails, then the condition will be false and the loop stops. You can combine that will your check for quit as well like
while (getline(is,command) && command != "quit")
{
// stuff
}
You can also add the prompt to the loop like
while (os << prompt && getline(is,command) && command != "quit")
while (command.compare("quit")!=0)
{
os << prompt;
getline(is,command);
if (is.eof())
do something at end of file
}
But note that is reaching end of file does not mean that there isn't something in command. You can read data and reach the end of file at the same time.
What you might be looking for instead is this
os << prompt;
while (getline(is,command) && command != "quit")
{
do something with command
os << prompt;
}
That code will quit the loop if you reach end of file and nothing has been input, or if 'quit' is input.
Related
I run the following snippet of code expecting that when i hit the new line (Enter key), the program will halt, but it does not do that, any idea what's the problem ? Thanks.
#include <bits/stdc++.h>
using namespace std;
int main() {
string s ;
while(getline(cin ,s)){
cout << s << endl ;
}
}
Hitting the Enter key ends a line. If there’s no other text on the line it’s an empty line, but a line nonetheless.
There are a couple of ways you can handle this.
First, depending on your OS, either ctrl-D or ctrl-Z will act like end-of-file, and the call to getline will fail, ending the loop.
Second, if you want an empty line to end the loop, just check for it:
while (getline(cin, s) && s.length() != 0)
std::cout << s << '\n';
Pressing enter is like entering an empty line, you didn't put any condition for your program to exit. It will stay in infinity unless you forcefully exit it. Implement an exit condition in the while(), so that when it is not met, the loop will exit, and obviously put the getline() inside to keep asking prompting for input.
I need to read lines from standard input, but I dont really know, how many it will be.
I tried to do it with getline() and cin combined with a while loop, but it led to an infinite loop:
string line;
while( getline(cin, string) ){...}
or
string word;
while( cin >> word ){...}
it doesnt stops at the end of the input( the lines are coming at one time, so the user is hitting just one time the Enter key ).
Thanks for your help.
Reading your comments you have a misunderstanding of "end of input".
When you start your program it waits for input from console, and if input is available it reads it. Initially your copy some strings to your console so your program takes this as input. But your program still keeps reading from the console because there was no "end of input". The program is still connected to the console.
You need to signal "end of input" to your program. On Windows you do this by pressing Ctrl+Z. On Linux you need to press Ctrl+D.
Your problem is reading from the console.
As your console does not put an EOF(end of file) when you enter an empty line.
You should try pipeing the input from a file to your program. This should end, when there is no more input.
Otherwise, just check if the string is empty, and break out of the loop, if it is empty.
The way you run your program, your input doesn't end, since the console can always provide more input. Your program behaves correctly, though perhaps not in the way you desire. That's because you have misunderstood your own desires.
What you are looking for is perhaps (but I can't be sure) for the program to end when either the input ends or when the input contains a blank line. This can be coded as follows:
int main()
{
for (std::string line; std::getline(std::cin, line); )
{
if (line.empty())
{
std::cout << "Got blank line, quitting.\n";
return 0;
}
// process line
}
std::cout << "End of input, quitting.\n";
return 0;
}
So I've run into the following problem. My goal is to create a loop that keeps taking user input over and over until the user doesn't enter anything into 'cin >>', leaves the line blank, and simply presses the ENTER key to move on, at which point the program is supposed to break out of the loop and continue on with the rest of program execution. Something like this:
do {
cout << "\nEnter a name: ";
cin >> input1;
if (input1.empty())
{
break;
}
else
{
user_name = input1;
}
} while (!input1.empty());
As you can see, I've already tried using the empty() function, but that didn't work, the program simply stays in the loop and doesn't break out, no matter how many times I press enter. It just keeps prompting me to enter a name. I've also tried using something like
if (input1 == "")
but that doesnt work either. Can anyone help? How do I break out of this loop?
UPDATE: OK guys, I've tried your recommendations, and it worked! Thank you so much! Unfortunately, although the getline function works, it has also created a new problem for me. Basically, in the first initial loop, the program prompts for a name, I type in a name, and the name is stored in user_name. However, in the SECOND loop, the program doesn't even give me the chance to enter any input, it simply prints "Enter a name: ", and then instantly exits out of the loop, and continues on with the rest of program execution. Why is this happening?
Use this getline(std::cin, input1):
while (getline(std::cin, input1))
{
if (input1.empty())
break;
username =input1;
std::cout << input1 << std::endl << "Enter Input : ";
}
Use std::getline(cin, input1); instead to read a line from the console.
Using cin directly reads exactly one word from stdin. If the user does not input anything, no word has been given and cin does not return yet (your empty check is not even executed).
After you use std::getline you can leave your empty-check as-is:
std::getline(cin, input1);
if(input1.empty())
break;
BTW: In C++ you should also check if the underlying stream has run into an error. So check the return code of cin or getline. This can be done with the following code:
if(!std::getline(cin, input1))
// I/O error
In general, looping until an empty line is entered would be:
while ( std::getline( line ) && !line.empty() ) ...
If you need a prompt: the prompt is part of the input logic, and
should be implemented as such:
std::string
getlineWithPrompt( std::string const& prompt )
{
std::cout << prompt;
std::string results;
return std::getline( std::cin, results )
? results
: std::string();
}
You then do something like:
std::string line = getlineWithPrompt( "prompt for first line" );
while ( !line.empty() ) {
// ...
getlineWithPrompt( "prompt for further line" );
}
(This is actually somewhat simplified, as it treats hard errors
on input, end of file, and empty lines identical, which is
rarely the right thing in professional software. But for
learning purposes, it should be sufficient.)
Cin won't read the whitespace that you call an empty line. Getline may do this, but I am not entirely sure. You could define an end character that the user would type and check for that. Gets would also work, it will just set the starting character to 0x0. Be careful with gets(), it is prone to allow buffer overflows.
This works as well:
char line[128];
do
{
cout << "Enter something: ";
gets(line);
} while (strcmp(&line[0], "\0") != 0);
#JamesKanze
So something like this to exit the while loop?
string str = "foo";
while (str == "foo"){
getline(cin, str);
}
str = "foo";
I'm starting with C++ in college (before used Modula2). I have problems with cin.
While interacting with the user, I need to recognize certain "commands".,
For example "addClient Rafael". I handle it the following way
cin >> command, strcoll (command, "addClient"), and then, if command equals addClient, y do
cin >> command2 (so I read Rafael),. and do the proper procedures...
But also, I have to recognize "deleteAll" which deletes all my database, so I dont have to read the second parameter.
When someone enters random things such as "skjdsjfnsdj" its supossed to say "Wrong command" for which, if command didnt equal none of my "known" commands it printf "wrong command".
The problem is, when some types "skajskajs jakasjkajs" it says "wrong command. worng command"... it should only say it once...
So, "noskip" i thing is no use, maybe if i could break the string.., maybe a simpler way, help anyone?
The most flexible and intuitive way to do this is as follows:
bool done = false;
while( !done ) {
string commandLine, cmd, value;
getline( cin, commandLine );
istringstream ss(commandLine);
ss >> cmd >> value;
if( cmd == "deleteAll" ) {
// BOOM
}
else if( cmd == "addClient" ) {
// Do something with 'value'. You could wait until here to read it
// if you want, instead of always attempting to read it.
}
else if( cmd == "quit" ) {
done = true;
}
else {
cout << "Wrong command\n";
}
}
Or edit to suit your purposes. I use this sort of approach for parsing simple key/value pair config files. Works a treat, and takes almost no effort to code.
You could simply try istream::getline() instead.
It will prevent the message appearing more than once for each command (separated by a \n).
I'm need to know if the next char in ifstream is the end of file. I'm trying to do this with .peek():
if (file.peek() == -1)
and
if (file.peek() == file.eof())
But neither works. There's a way to do this?
Edit: What I'm trying to do is to add a letter to the end of each word in a file. In order to do so I ask if the next char is a punctuation mark, but in this way the last word is left without an extra letter. I'm working just with char, not string.
istream::peek() returns the constant EOF (which is not guaranteed to be equal to -1) when it detects end-of-file or error. To check robustly for end-of-file, do this:
int c = file.peek();
if (c == EOF) {
if (file.eof())
// end of file
else
// error
} else {
// do something with 'c'
}
You should know that the underlying OS primitive, read(2), only signals EOF when you try to read past the end of the file. Therefore, file.eof() will not be true when you have merely read up to the last character in the file. In other words, file.eof() being false does not mean the next read operation will succeed.
This should work:
if (file.peek(), file.eof())
But why not just check for errors after making an attempt to read useful data?
file.eof() returns a flag value. It is set to TRUE if you can no longer read from file. EOF is not an actual character, it's a marker for the OS. So when you're there - file.eof() should be true.
So, instead of if (file.peek() == file.eof()) you should have if (true == file.eof()) after a read (or peek) to check if you reached the end of file (which is what you're trying to do, if I understand correctly).
For a stream connected to the keyboard the eof condition is that I intend to type Ctrl+D/Ctrl+Z during the next input.
peek() is totally unable to see that. :-)
Usually to check end of file I used:
if(cin.fail())
{
// Do whatever here
}
Another such way to implement that would be..
while(!cin.fail())
{
// Do whatever here
}
Additional information would be helpful so we know what you want to do.
There is no way of telling if the next character is the end of the file, and trying to do so is one of the commonest errors that new C and C++ programmers make, because there is no end-of-file character in most operating systems. What you can tell is that reading past the current position in a stream will read past the end of file, but this is in general pretty useless information. You should instead test all read operations for success or failure, and act on that status.
You didn't show any code you are working with, so there is some guessing on my part. You don't usually need low level facilities (like peek()) when working with streams. What you probably interested in is istream_iterator. Here is an example,
cout << "enter value";
for(istream_iterator<double> it(cin), end;
it != end; ++it)
{
cout << "\nyou entered value " << *it;
cout << "\nTry again ...";
}
You can also use istreambuf_iterator to work on buffer directly:
cout << "Please, enter your name: ";
string name;
for(istreambuf_iterator<char> it(cin.rdbuf()), end;
it != end && *it != '\n'; ++it)
{
name += *it;
}
cout << "\nyour name is " << name;
just use this code in macosx
if (true == file.eof())
it work for me in macosx!