I'm an just trying to fill an array with strings using a loop. My problem is that when it enters the loop to input names it will instantaneously input a blank line for the first slot in the vector. Why does this happen? And how can I fix it. Please don't mind my lack of code style, I'm a noob trying to regain my knowledge of programming before I start in a class this winter...
Here's some sample output:
How many people are in your family?
4
Please enter the names of all of your family members
check name:
Please enter the names of all of your family members
Matt
check name:Matt
Please enter the names of all of your family members
Evan
check name:Evan
Please enter the names of all of your family members
Michelle
check name:Michelle
Matt
Evan
Michelle
and this is my code:
vector<string> Names;
bool complete=0;
while(!complete)
{
int number;
cout << "How many people are in your family?" << endl;
cin >> number;
for(int i=0; i<number; i++)
{
string names;
cin.clear();
cout << "Please enter the names of all of your family members" << endl;
getline(cin,names);
Names.push_back(names);
cout << "check name:" << names << endl;
}
complete = 1;
}
for (int i=0; i< Names.size(); i++)
{
cout << Names[i] << endl;
}
The reason you see this behavior is mixing the >> reads with getline. When you read the count, the input pointer is advanced to the end of the numeric input, i.e. 4, and stops right before the new line character is read.
This is when you call getline; the new line character is read, and the new line is returned right away.
To fix this problem, add a call to getline right after the cin >> number call, and discard the result.
May I suggest you try
std::cin >> names;
instead of
getline(std::cin, names);
getline takes in std::endl or the \n from your std::cout print string. The idea is that getline will read until the \n character (which is an indication of an endline), but it will also consume the endline character. This is why it is consuming the newline character into your vector.
Consider doing this . . .
std::cin.get();
which will read std::endl character and then use the getline function.
The problem is mixing fonatted input (std::cin >> number) with unformatted input (std::getline(std::cin, names)). The formatted input stops at first non-integer character, most likely the newline you entered after the count. The easiest fix is to skip leading whitespace explicitly:
std::getline(std::cin >> std::ws, names);
Note, that you also need to check after each input that it was successful:
if (std::cin >> number) {
// do something after a successful read
}
Related
I'm attempting to get a few user-input parameters from the console, two strings, two ints and a double. The relevant code I'm trying to use is this:
#include <string>
#include <iostream>
using namespace std;
// ...
string inputString;
unsigned int inputUInt;
double inputDouble;
// ...
cout << "Title: ";
getline(cin, inputString);
tempDVD.setTitle(inputString);
cout << "Category: ";
getline(cin, inputString);
tempDVD.setCategory(inputString);
cout << "Duration (minutes): ";
cin >> inputUInt;
tempDVD.setDuration(inputUInt);
cout << "Year: ";
cin >> inputUInt;
tempDVD.setYear(inputUInt);
cout << "Price: $";
cin >> inputDouble;
tempDVD.setPrice(inputDouble);
However, when running the program, instead of waiting for the first inputString to be entered, the code doesn't stop until the second getline() call. Thus the console output looks like this:
Title: Category:
with the cursor appearing after category. If I input now, the program then jumps ahead to the year input, not allowing me to enter more than one string. What's happening here?
The problem is you are mixing calls to getline() with the use of the operator >>.
Remember that operator >> ignored leading white space so will correctly continue across lines boundaries. But stops reading after the input has successfully been retrieved and thus will not swallow trailing '\n' characters. Thus if you use a getline() after a >> you usually get the wrong thing unless you are careful (to first remove the '\n' character that was not read).
The trick is to not use both types of input. Pick the appropriate one and stick to it.
If it is all numbers (or objects that play nice with operator >>) then just use operator >> (Note string is the only fundamental type that is not symmetric with input/output (ie does not play nicely)).
If the input contains strings or a combination of stuff that will require getline() then only use getline() and parse the number out of the string.
std::getline(std::cin, line);
std::stringstream linestream(line);
int value;
linestream >> value;
// Or if you have boost:
std::getline(std::cin, line);
int value = boost::lexical_cast<int>(line);
You need to flush the input buffer. It can be done with cin.clear(); cin.sync();.
You can use
cin.ignore();
or as #kernald mentioned use
cin.clear();
cin.sync();
before using getline()
Use cin.clear() as mentioned and use proper error handling:
cin.clear();
cin.sync();
cout << "Title: ";
if (!getline(cin, inputString)) exit 255;
tempDVD.setTitle(inputString);
cout << "Category: ";
if (!getline(cin, inputString)) exit 255;
tempDVD.setCategory(inputString);
cout << "Duration (minutes): ";
if (!(cin >> inputUInt)) exit 255;
tempDVD.setDuration(inputUInt);
cout << "Year: ";
if (!(cin >> inputUInt)) exit 255;
tempDVD.setYear(inputUInt);
cout << "Price: $";
if (!(cin >> inputDouble)) exit 255;
tempDVD.setPrice(inputDouble);
Also works with ws. You can use getline(cin >> ws, inputString)) to eat the whitespaces or the newlines after reading data with cin command.
If user inputs a space before \n in previous cin before getline, only ignore itself wouldn't be enough so you have to use this code instead of ignore() alone. For example 12345 \t \n will not work anymore.
All unprocessed characters must be ignored.
#include <limits>
cin.ignore(numeric_limits<streamsize>::max(), '\n');
Use this between cin and getline.
Mixing getline() with input streams in generally a bad thing to do. It's theoretically possible to manually handle the dirty buffers left over by using streams, but it's an unnecessary pain that should definitely be avoided.
You are better off using a console library to grab your input, this way the dirty-work can be abstracted for you.
Take a look at TinyCon. You can just use the static method tinyConsole::getLine() in replace of your getline and stream calls, and you can use it as many times as you'd like.
You can find information here:
https://sourceforge.net/projects/tinycon/
cin.sync();
use this instead of cin.ignore(
works best.
The function_______ will return the last read character and will move the inside pointer one with -1 char.
Getline()
Peek()
3.flush()
4.putback()
#include <iostream>
#include <string>
using namespace std;
struct UserInfo{
string userPhoneNumber;
string userName ;
string userAddress ;
};
int main ()
{
cout << "How many Entries do you want to enter?";
int userAmountSelection;
cin >> userAmountSelection;
UserInfo userOne [userAmountSelection];
for (int i = 0; i < userAmountSelection; i++){
cout << "Please enter your first and last name: ";
cin.ignore(); // possible problem in code
getline (cin, userOne[i].userName, '\n');
cout << "Please Enter your address, " << userOne[i].userName << ":";
cin.ignore(); // possible problem in code
getline (cin, userOne[i].userAddress, '\n');
cout << "Please enter your phone#: ";
cin.ignore (); // possible problem in code
getline (cin, userOne[i].userPhoneNumber);
}
for (int i = 0; i < userAmountSelection; i++){
cout << userOne[i].userName << " " <<
userOne[i].userAddress << " " <<
userOne[i].userPhoneNumber << endl;
}
return 0;
}
As you can see its a simple code for learning structs and experimenting. The problem i run into appears to be from cin.ignore () code. it ignores the first characters of the input strings on output. The code does compile, however input and output are skewed.
For example when i enter a name of Terry, it will output erry.
I have tried removing the cin.ignore (), however when i do that, the output skips over the parts where the user needs to enter data and leaves areas blank.
I have scoured the forums and found suggestions such as adding an argument to the cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');. for example, however this does not solve the problem and only adds to the list of errors I'm experiencing.
The Problem
The problem is with the placement of ignores to prevent the bug outlined in Why does std::getline() skip input after a formatted extraction? The ignores have been placed before the getlines, and while this solves the getline skipping problem, it causes the problem the Asker has encountered. There isn't always something that needs to be ignored.
For example
cin >> userAmountSelection;
will leave a line ending in the stream if the user typed in the amount and then hit enter.
cout << "Please enter your first and last name: ";
cin.ignore(); // possible problem in code
getline (cin, userOne[i].userName, '\n');
inside the for loop Would trip over this line ending if not for the ignore. But getline does not leave a newline in the stream , so the second and subsequent iterations of the loop have no newline to ignore. Part of the requiured data is ignored instead.
After
cin >> userAmountSelection;
rather than before
getline (cin, userOne[i].userName, '\n');
would be a good place to place an ignore so the newline is removed from the stream only after it has been left in the stream, but...
The Solution
The best way to handle this is to always read entire lines with getline and then parse those lines (see option 2 of this answer) into the pieces you want.
std::string line;
std::getline(std::cin, line);
std::istringstream(line) >> userAmountSelection;
This always works (Note: Requires #include <sstream>) and now you only have one type of reading going on, not a game of mix-n-match where you may forget you need an ignore.
Feel free to stop reading now.
The ignore approach requires some extra smarts. It's not as simple as it looks, in addition to fallibility of the human memory. You should place ignores AFTER an operation that leaves unwanted stuff in the stream. If you ignore BEFORE an operation, you often find yourself losing data you wanted because there was nothing to ignore.
std::cin >> userAmountSelection; // read a number
std::cin.ignore(); // discard the next character
Works a lot of the time, but what if the user typed in the amount and then a space and then hit enter or typed in all of the input they needed to type because they new darn well what the next prompt was? You gotta get a bit craftier.
std::cin >> userAmountSelection;
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
This ignore annihilates everything until it hits the end of the line or runs out of space in the stream. (Note: Requires #include <limits>)
The problem i run into appears to be from cin.ignore () code. it ignores the first characters of the input strings on output.
Looking at the istream::ignore declaration
istream& ignore (streamsize n = 1, int delim = EOF);
It will discard the first character by default.
I have tried removing the cin.ignore (), however when i do that, the
output skips over the parts where the user needs to enter data and
leaves areas blank.
That's because of the std::cin performed which left a residual newline which was then consumed by the getline. You'll only need ignore between the std::cin and getline calls:
std::cin >> userAmountSelection;
std::cin.ignore(); //---
...
for (int i = 0; i < userAmountSelection; i++) {
...
getline (...);
...
getline (...)
}
Also, there may be instances where you don't know if an std::cin will precede the getline. In that case, you may check if the input stream contains new line before doing ignore:
...
// some I/O operations happening here
...
// ignore if there's white space
if (std::iswspace(std::cin.peek())) std::cin.ignore();
std::getline(std::cin, somestring);
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 <iostream>
#include <string>
using namespace std;
int main()
{
double num;
string mystr;
cout << "Please enter a number: " << "\n";
cin >> num;
cout << "Your number is: " << num << "\n";
cin.ignore(256, '\n'); // Why do I need this line?
cout << "Please enter your name: \n";
getline (cin, mystr);
cout << "So your name is " << mystr << "?\n";
cout << "Have a nice day. \n";
}
ignore does exactly what the name implies.
It doesn't "throw away" something you don't need. Instead, it ignores the number of characters you specify when you call it, up to the char you specify as a delimiter.
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 number of chars before the specified delimiter, in this case, the '\n' newline character.)
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:
int main()
{
double num;
string mystr;
cin >> num;
getline(cin, mystr);
cout << "num=" << num << ",mystr=\'" << mystr << "\'" << endl;
}
In the above, you are thinking, "first get a number." 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 "skipped" 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:
int main()
{
int age;
string firstName;
string lastName;
cout << "First name: ";
cin >> firstName;
cout << "Last name: ";
cin >> lastName;
cout << "Age: ";
cin >> age;
cout << "Hello " << firstName << " " << lastName << "! You are " << age << " years old!" << 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 "streams" you can simply enter John Doe 19 on the "First name:" 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 "stream" 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 >> double is expected, the application will produce a seemingly bizarre output.
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& 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<streamsize>::max(),'\n').
Quick explanation: It ignores the characters equal to maximum size of stream or until a '\n' is encountered, whichever case happens first.
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.
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.
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 <bits/stdc++.h>
using namespace std;
int main() {
int t;
cin>>t;
cin.ignore(); //Notice that this cin.ignore() is really crucial for any extra whitespace or newline character
while(t--){
vector<int> v(256,0);
string s;
getline(cin,s);
string s2;
for(int i=0;i<s.size();i++){
if (v[s[i]]) continue;
else{
s2.push_back(s[i]);
v[s[i]]++;
}
}
cout<<s2<<endl;
}
return 0;
}
So, You get the point that it will ignore those unwanted inputs and will get the job done.
It is better to use scanf(" %[^\n]",str) in c++ than cin.ignore() after cin>> statement.To do that first you have to include < cstdio > header.
I'm attempting to get a few user-input parameters from the console, two strings, two ints and a double. The relevant code I'm trying to use is this:
#include <string>
#include <iostream>
using namespace std;
// ...
string inputString;
unsigned int inputUInt;
double inputDouble;
// ...
cout << "Title: ";
getline(cin, inputString);
tempDVD.setTitle(inputString);
cout << "Category: ";
getline(cin, inputString);
tempDVD.setCategory(inputString);
cout << "Duration (minutes): ";
cin >> inputUInt;
tempDVD.setDuration(inputUInt);
cout << "Year: ";
cin >> inputUInt;
tempDVD.setYear(inputUInt);
cout << "Price: $";
cin >> inputDouble;
tempDVD.setPrice(inputDouble);
However, when running the program, instead of waiting for the first inputString to be entered, the code doesn't stop until the second getline() call. Thus the console output looks like this:
Title: Category:
with the cursor appearing after category. If I input now, the program then jumps ahead to the year input, not allowing me to enter more than one string. What's happening here?
The problem is you are mixing calls to getline() with the use of the operator >>.
Remember that operator >> ignored leading white space so will correctly continue across lines boundaries. But stops reading after the input has successfully been retrieved and thus will not swallow trailing '\n' characters. Thus if you use a getline() after a >> you usually get the wrong thing unless you are careful (to first remove the '\n' character that was not read).
The trick is to not use both types of input. Pick the appropriate one and stick to it.
If it is all numbers (or objects that play nice with operator >>) then just use operator >> (Note string is the only fundamental type that is not symmetric with input/output (ie does not play nicely)).
If the input contains strings or a combination of stuff that will require getline() then only use getline() and parse the number out of the string.
std::getline(std::cin, line);
std::stringstream linestream(line);
int value;
linestream >> value;
// Or if you have boost:
std::getline(std::cin, line);
int value = boost::lexical_cast<int>(line);
You need to flush the input buffer. It can be done with cin.clear(); cin.sync();.
You can use
cin.ignore();
or as #kernald mentioned use
cin.clear();
cin.sync();
before using getline()
Use cin.clear() as mentioned and use proper error handling:
cin.clear();
cin.sync();
cout << "Title: ";
if (!getline(cin, inputString)) exit 255;
tempDVD.setTitle(inputString);
cout << "Category: ";
if (!getline(cin, inputString)) exit 255;
tempDVD.setCategory(inputString);
cout << "Duration (minutes): ";
if (!(cin >> inputUInt)) exit 255;
tempDVD.setDuration(inputUInt);
cout << "Year: ";
if (!(cin >> inputUInt)) exit 255;
tempDVD.setYear(inputUInt);
cout << "Price: $";
if (!(cin >> inputDouble)) exit 255;
tempDVD.setPrice(inputDouble);
Also works with ws. You can use getline(cin >> ws, inputString)) to eat the whitespaces or the newlines after reading data with cin command.
If user inputs a space before \n in previous cin before getline, only ignore itself wouldn't be enough so you have to use this code instead of ignore() alone. For example 12345 \t \n will not work anymore.
All unprocessed characters must be ignored.
#include <limits>
cin.ignore(numeric_limits<streamsize>::max(), '\n');
Use this between cin and getline.
Mixing getline() with input streams in generally a bad thing to do. It's theoretically possible to manually handle the dirty buffers left over by using streams, but it's an unnecessary pain that should definitely be avoided.
You are better off using a console library to grab your input, this way the dirty-work can be abstracted for you.
Take a look at TinyCon. You can just use the static method tinyConsole::getLine() in replace of your getline and stream calls, and you can use it as many times as you'd like.
You can find information here:
https://sourceforge.net/projects/tinycon/
cin.sync();
use this instead of cin.ignore(
works best.
The function_______ will return the last read character and will move the inside pointer one with -1 char.
Getline()
Peek()
3.flush()
4.putback()
This question already has answers here:
Need help with getline() [duplicate]
(7 answers)
Closed 7 years ago.
This is probably a very simple problem but forgive me as I am new.
Here is my code:
#include <iostream>
#include <string>
#include <sstream>
using namespace std;
int main ()
{
string name;
int i;
string mystr;
float price = 0;
cout << "Hello World!" << endl;
cout << "What is your name? ";
cin >> name;
cout << "Hello " << name << endl;
cout << "How old are you? ";
cin >> i;
cout << "Wow " << i << endl;
cout << "How much is that jacket? ";
getline (cin,mystr);
stringstream(mystr) >> price;
cout << price << endl;
system("pause");
return 0;
}
The problem is that when asked how much is that jacket? getline does not ask the user for input and just inputs the initial value of "0". Why is this?
You have to be careful when mixing operator>> with getline. The problem is, when you use operator>>, the user enters their data, then presses the enter key, which puts a newline character into the input buffer. Since operator>> is whitespace delimited, the newline character is not put into the variable, and it stays in the input buffer. Then, when you call getline, a newline character is the only thing it's looking for. Since that's the first thing in the buffer, it finds what it's looking for right away, and never needs to prompt the user.
Fix:
If you're going to call getline after you use operator>>, call ignore in between, or do something else to get rid of that newline character, perhaps a dummy call to getline.
Another option, and this is along the lines of what Martin was talking about, is to not use operator>> at all, and only use getline, then convert your strings to whatever datatype you need. This has a side effect of making your code more safe and robust. I would first write a function like this:
int getInt(std::istream & is)
{
std::string input;
std::getline(is,input);
// C++11 version
return stoi(input); // throws on failure
// C++98 version
/*
std::istringstream iss(input);
int i;
if (!(iss >> i)) {
// handle error somehow
}
return i;
*/
}
You can create a similar function for floats, doubles and other things. Then when you need in int, instead of this:
cin >> i;
You do this:
i = getInt(cin);
Its because you have a '\n' left lying on the input stream from a previous call.
cin >> i; // This reads the number but the '\n' you hit after the number
// is still on the input.
The easiest way to do interactive user input is to make sure each line is processed independently (as the user will hit enter after each prompt).
As a result always read a line, then process the line (until you get familiar with the streams).
std::string line;
std::getline(std::cin, line);
std::stringstream linestream(line);
// Now processes linestream.
std::string garbage;
lienstream >> i >> garbage; // You may want to check for garbage after the number.
if (!garbage.empty())
{
std::cout << "Error\n";
}
Ignore some characters until line feed is reached.
cin.ignore(256, '\n')
getline (cin,mystr);