I know how to do this in C but have no idea for a C++ solution. I want the following to be fail safe, but after providing a string or even a char to the input, the program hangs. How to read input stream including \n to free it?
int main() {
int num;
do {
std::cin.clear();
std::cin >> num;
while ( std::cin.get() != '\n' );
} while ( !std::cin.good() || num > 5 );
return 0;
}
Once the stream is in an error state all read operations will fail. This means that, if the cin >> num read fails, the loop with the get() calls will never end: all those get()s will fail. Skipping to the end of the line can only be done after clearing the error state.
To build on top of R. Martinho Fernandes answer, here is a possible C++ alternative to your code:
std::string num;
std::getline(std::cin, num);
// Arbitrary logic, e.g.: remove non digit characters from num
num.erase(std::remove_if(num.begin(), num.end(),
std::not1(std::ptr_fun((int(*)(int))std::isdigit))), num.end());
std::stringstream ss(num);
ss >> n;
The std::getline function extracts characters from cin and stores to num. It also extracts and discards the delimiter at the end of the input (you can specify your own delimiter or \n will be used).
The string::erase function removes all characters but digits from the num string, using std::remove_if with a negative std::isdigit predicate.
The string is then represented as an integer using a std::stringstream (a boost::lexical_cast would have worked as well)
The logic here implemented by the erase function can be any other logic, but this code is probably much simpler to read than the one included in the question.
I would approach it using getline(cin,num) and then catch any fails using cin.fail(). I usually use cin.fail() with ints but theoretically should work with strings and chars also, for example :
string num;
getline(cin,num);
if(cin.fail())
{
cin.clear();
cin.ignore();
}
One way would be to check the state after every input and throw an exception if that happens
for example:
#include<iostream>
using namespace std;
int main(){
int a;
cout<<"Enter a number: ";
cin>>a;
//If a non number is entered, the stream goes into a fail state
try
{
if(cin.fail()){
throw 0;
cin.clear();
cin.ignore();
}
}
catch(int){
cin.clear();
cin.ignore();
}
return 0;
}
After that you can continue with whatever code you wish
To clear input stream, use cin.sync() .
no need to use cin.clear() or cin.ignore().
Related
When i write this code i could not enter two lines as input where each line contains 3 to 5 words by gets() function:
int main()
{
int t;
cin>>t;
char nm1[50],nm2[50];
while(t--)
{
gets(nm1);
gets(nm2);
puts(nm1);
puts(nm2);
}
}
But when i add a gets() function earlier before while() function now i can enter two line of strings like this :
int t;
cin>>t;
char nm1[50],nm2[50];
gets(nm1); //using gets() function here//
while(t--)
{
gets(nm1);
gets(nm2);
puts(nm1);
puts(nm2);
}
So, what is the logic behind this?
Don't use gets. See Why gets() is deprecated?.
Don't mix cin and functions from stdio.h. By default, cin is synchronized with stdin. However, it is possible to make them stay out of sync by using
std::ios_base::sync_with_stdio(false);
See http://en.cppreference.com/w/cpp/io/ios_base/sync_with_stdio for further details.
Real problem.
cin >> t;
leaves a newline character in the input stream. The next call to gets reads that. If you don't want gets to read that, you have to add code to read and discard the rest of the line.
Here's what I suggest:
int main()
{
int t;
cin >> t;
// Ignore the rest of the line.
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
char nm1[50],nm2[50];
while(t--)
{
cin.get(nm1, 50);
cin.get(nm2, 50);
puts(nm1);
puts(nm2);
}
}
Make sure to dd
#include <limits>
to be able to use std::numeric_limits.
in the cin>>t you entered a number and then pressed enter \n, and the end of line is still waiting in the buffer, if you read another integer cin>>another_integer, cin will ignore \n or ' ' (whitespaces), but gets will not. What you are really typing is something like follows
5\n ---- see the end of line?
my string\n
my bigger string\n
gets() reads until it find a \n or end of file
by the way, gets() is deprecated in c++11, you should use getline() or other function instead
I highly recommend performing safer I/O using C++ constructs:
int main()
{
unsigned int quantity;
cin >> quantity;
// Now input the words
std::vector<std::string> database;
while (cin >> word)
{
database.push_back(word);
}
return 0;
}
In my c++ book, there is this loop:
vector<double> temps; // temperatures
for (double temp; cin>>temp; ) // read and put into temps
temps.push_back(temp);
This appears to loop forever. I can't stop this loop. What purpose does this serve?
The operator>> will return the stream (cin) which itself can be converted to a bool that represents if the stream is still valid or not. That will loop until something causes cin's read to fail (end of file, a letters instead of numbers, etc).
This is similar to:
int num;
while(cin.good()) //could also use while(cin)
{
cin >> num;
cout << num << endl;
}
Where you continuously read a number and print it. Inputting a non-number causes the input stream to fail hence cin.good() becomes false.(stream is good no more)
This function keeps getting called in another function inside a while-loop while valid_office_num is false. The problem is that if the input begins with a digit but is followed by other invalid characters (e.g. 5t) it takes the digit part and accepts that as a valid input. I want it to consider the whole input and reject it so it can ask for another one. I thought I could use getline() but then I cannot use cin.fail(). How could I implement this behavior?
I forgot to mention I am very new to C++, I have only learnt the basics so far.
(To be clear the desired behavior is to reject anything that contains anything other than digits. This is not an integer range check question. If it is NOT an integer, discard it and request another one)
//Function to read a valid office number, entered by user
int read_office_num()
{
//Declaration of a local variable
int office_num;
//Read input
cin >> office_num;
//Check if input was valid
if (cin.fail())
{
//Print error message
cout << "\nInvalid office number, it should only consist of digits!! Enter another:\n";
//Clear error flags
cin.clear();
//Ignore any whitespace left on input stream by cin
cin.ignore(256, '\n');
}
else
{
//Office number entered is valid
valid_office_num = true;
}
return office_num;
}
From what I gather you want the whole line to be read as a number and fail otherwise?
Well, you can use std::getline(), but you have to follow the algorithm below (I will leave the implementation to you..)
use std::getline(cin, str) to read a line, and if this returns true
use std::stoi(str, &pos) to convert to integer and get the position of the last integer
if pos != str.size() then the whole line in not an integer (or if the above throws an exception), then it's not a valid integer, else return the value...
Read a line of input as a std::string using std::getline().
Examine the string and check if it contains any characters that are not digits.
If the string only contains digits, use a std::istringstream to read an integer from the string. Otherwise report a failure, or take whatever other recovery action is needed (e.g. discard the whole string and return to read another one).
You could use a stringstream
int read_office_num()
{
//Declaration of a local variable
int office_num;
string input = "";
while (true) {
getline(cin, input);
stringstream myStream(input);
if (myStream >> office_num)
break;
cout << "\nInvalid office number, it should only consist of digits!! Enter another:\n" << endl;
}
return office_num;
}
If you want to reject input like 123 xxx you could add an additional check to verify that the received string is indeed an integer:
bool is_number(const string& s)
{
string::const_iterator itr = s.begin();
while (itr != s.end() && isdigit(*itr)) ++itr;
return !s.empty() && itr == s.end();
}
int read_office_num()
{
//Declaration of a local variable
int office_num;
string input = "";
while (true) {
getline(cin, input);
stringstream myStream(input);
if (is_number(input) && myStream >> office_num)
break;
cout << "\nInvalid office number, it should only consist of digits!! Enter another:\n" << endl;
}
return office_num;
}
You should probably just look at the number of input characters that are left in cin. You can do that with in_avail
Your function will probably end up having a body something like this:
//Declaration of a local variable
int office_num;
//Read input and check if input was valid
for (cin >> office_num; cin.rdbuf()->in_avail() > 1; cin >> office_num){
//Print error message
cout << "\nInvalid office number, it should only consist of digits!! Enter another:\n";
//Ignore any whitespace left on input stream by cin
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
//Office number entered is valid
valid_office_num = true;
return office_num;
Points of interest:
There is always at least 1 character in cin otherwise the cin would be marked as bad and that would not be good
You don't need valid_office_num if read_office_num is implemented this way, cause valid_office_num will always be set to true before returning
Hm. I may be missing something, but why not read a line, trim it, use regular expressions to validate a number and then exploit strstream's facilities or just atoi if you must? In all reality I'd probably just let users get away with extraneous input (but discard it if I'm sure I'm always running interactively). Following the motto "be lenient in what you accept."
The "interactive" caveat is important though. One can generally not assume that cin is a terminal. Somebody may get cocky and let your program run on a text file or in a pipeline, and then it would fail. A robust approach would separate data processing (portable) from means of input (possibly machine specific and therefore also more powerful and helpful than stdin/stdout via a console).
Here's how to do it using Boost Lexical Cast:
#include <boost/lexical_cast.hpp>
#include <iostream>
#include <vector>
#include <string>
int read_office_num()
{
using boost::lexical_cast;
using boost::bad_lexical_cast;
using namespace std;
int office_num;
while (true)
{
try
{
string input = cin.getline();
office_num = lexical_cast<int>(*argv));
break;
}
catch(const& bad_lexical_cast)
{
cout << "\nInvalid office number, it should only consist of digits!! Enter another:\n";
}
}
return office_num;
}
Ok, I'm trying to get good at using pointers so I'm trying to write a input validation for the user input to make sure that anything that isn't a number is handled correctly. When I use isdigit() isn't working for me. I still get an exception when I enter a alphabet. Any suggestions? Thanks. Check this out:
#include<iostream>
#include<algorithm>
#include<string>
#include<cctype>
using namespace std;
void EnterNumbers(int * , int);
int main()
{
int input = 0;
int *myArray;
cout << "Please enter the number of test scores\n\n";
cin >> input;
//Allocate Array
myArray = new int[input];
EnterNumbers(myArray,input);
delete[] myArray;
return 0;
}
void EnterNumbers(int *arr, int input)
{
for(int count = 0; count < input; count++)
{
cout << "\n\n Enter Grade Number " << count + 1 << "\t";
cin >> arr[count];
if(!isdigit(arr[count]))
{
cout << "Not a number";
}
}
}
If you test if (!(cin >> arr[count])) ... instead - isdigit(arr[digit]) tests if the value of arr[digit] is the ASCII code of a digit [or possibly matches Japanese, Chinese or Arabic (that is, as an Arabic script typeface, not that it's a 0-9 like our "Arabic" ones) digit]. So if you type in 48 to 57, it will say it's OK, but if you type 6 or 345, it's complaining that it is not a digit...
Once you have discovered a non-digit, you will also need to either exit or clean out the input buffer from "garbage". cin.ignore(1000, '\n'); will read up to the next newline or a 1000 characters, whichever happens first. Could get annoying if someone has typed in a million digits, but otherwise, should solve the problem.
You will of course also need a loop to read the number again, until a valid number is entered.
The way I do this kind of input validation is that I use std::getline(std::cin, str) to get the whole line of input and then I parse it using the following code:
std::istringstream iss(str);
std::string word;
// Read a single "word" out of the input line.
if (! (iss >> word))
return false;
// Following extraction of a character should fail
// because there should only be a single "word".
char ch;
if (iss >> ch)
return false;
// Try to interpret the "word" as a number.
// Seek back to the start of stream.
iss.clear ();
iss.seekg (0);
assert (iss);
// Extract value.
long lval;
iss >> lval;
// The extraction should be successful and
// following extraction of a characters should fail.
result = !! iss && ! (iss >> ch);
// When the extraction was a success then result is true.
return result;
isdigit() applies to char not to int as you're trying. The cin >> arr[count]; statement already ensures an integer numeric digits format is given in the input. Check cin.good() (!cin respectively) for possible input parsing errors.
I understand that cin.eof() tests the stream format. And while giving input, end of character is not reached when there is wrong in the input. I tested this on my MSV C++ 2010 and am not understanding the strange results. No matter what I give the input, I am getting Format Error message that is present in the program.
#include <iostream>
using namespace std;
int main()
{
int i;
cin>> i;
if(!cin.eof())
{
cout<< "\n Format Error \n";
}
else
{
cout<< "\n Correct Input \n";
}
getchar();
return 0;
}
Results I expected:
Values for i =
10 => Correct Input but the output is Format Error
12a => Format Error
Could someone explain where I am going wrong. Thanks.
std::cin.eof() tests for end-of-file (hence eof), not for errors. For error checking use !std::cin.good(), the built-in conversion operator (if(std::cin)) or the boolean negation operator (if(!std::cin)).
Use a direct test of the status of the stream with:
while (cin >> i)
{
...
}
For an input stream to enter the EOF state you have to actually make an attempt to read past the end of stream. I.e. it is not enough to reach the end-of-stream location in the stream, it is necessary to actually try to read a character past the end. This attempt will result in EOF state being activated, which in turn will make cin.eof() return true.
However, in your case you are not only not doing that, you (most likely) are not even reaching the end of stream. If you input your 10 from the keyboard, you probably finished the input by pressing the [Enter] key. This resulted in a new-line character being added to the input stream. So, what you are actually parsing with >> operator in this case is actually a 10\n sequence. Since you requested an int value from the stream, it only reads the numerical characters from the stream, i.e. it reads 1 and 0, but it stops at \n. That \n remains in the stream. You never read it. So, obviously, your code never reaches the end-of-file position in the stream. You have to reason to expect cin.eof() to become true in such case.
#include <iostream>
int main() {
using namespace std;
int i;
if (cin >> i) {
cout << "Extracted an int, but it is unknown if more input exists.\n";
char c;
if (cin.get(c)) { // Or: cin >> c, depending on how you want to handle whitespace.
cin.putback(c);
cout << "More input exists.\n";
if (c == '\n') { // Doesn't work if you use cin >> c above.
cout << "But this was at the end of this line.\n";
}
}
else {
cout << "No more input exists.\n";
}
}
else {
cout << "Format error.\n";
}
return 0;
}
Also see Testing stream.good() or !stream.eof() reads last line twice.
Sample session with the above program, note that input lines are marked with comments not present in the actual output:
$ your-program
12 # input
Extracted an int, but it is unknown if more input exists.
More input exists.
But this was at the end of this line.
$ your-program
12a # input
Extracted an int, but it is unknown if more input exists.
More input exists.
$ echo -n 12 | your-program
Extracted an int, but it is unknown if more input exists.
No more input exists.
$ your-program
a # input
Format error.
Assuming your input is line based, I suggest that you read the whole line using std::getline(). Once you have the line, you can analyse it and decide whether it contains correct or wrong input. Put the line into std::istringstream and do something like the following:
Edit: Changed !! iss to static_cast<bool>(iss) for compatibility with C++0x.
std::istringstream iss (line);
char ch;
long lval;
// read the input
iss >> lval;
// result variable will contain true if the input was correct and false otherwise
result
// check that we have read a number of at least one digit length
= static_cast<bool>(iss)
// check that we cannot read anything beyond the value read above
&& ! (iss >> ch);
Adding to the previous answer:
After reading your input (like 10), you are not at end-of-file, as you can easily type some more. How is the system to know that you will not?
When reading your second input (12a), it correctly reads all the digits that can be part of an integer. The letter 'a' cannot, so it is left for some possible later input. For example, you can read all parts of 12a with this code
int i;
char c;
cin >> i >> c;
cin.eof() test if the stream has reached end of file which happens if you type something like Ctrl+C (on Windows), or if input has been redirected to a file etc.
To test if the input contains an integer and nothing but an integer, you can get input first into a string and then convert that with a stringstream. A stringstream indeed reaches eof if there's no more to be extracted from it.
#include <iostream>
#include <sstream>
#include <string>
int main() {
using namespace std;
int i;
string input;
cin >> input; //or getline(cin, input)
stringstream ss(input);
if (ss >> i && ss.eof()) { //if conversion succeeds and there's no more to get
cout<< "\n Correct Input \n";
}
else {
cout<< "\n Format Error \n";
}
return 0;
}