loop involving "cin.fail()" running multiple times - c++

I am having trouble with the following code. It is meant to keep asking for a valid input until an integer or double is inputted. It works as intended for characters, however when I input a string of length more than 1, it will run the loop multiple times. For example, the input "hello" with cause "Please enter a valid number" to be printed 5 times. Interestingly "h llo" will only print the sentence 4 times.
int gamenumber;
while(true)
{
cin >> gamenumber;
if(cin.fail())
{
cout << "Please enter a valid number" << endl;
cin.clear();
cin.ignore();
} else
break;
I did manage to fix this issue by replacing "cin.ignore()" with "cin.ignore(1000, '\n')".
But regardless, it's bugging me that I don't understand why "cin.ignore()" alone doesn't fix this? Is there a way to fix the above code without using "cin.ignore(1000, '\n')"? (This is part of a homework assignment, and we may not be allowed to use "cin.ignore(1000, '\n')")
Thank you!

You need use ignore with the overloaded one, see this anser here.
Or you can just need to run getline to drain the contents, but this way is slower and unnecessary.
#include <iostream>
#include <string>
int main()
{
double n;
while( std::cout << "Please, enter a number\n"
&& ! (std::cin >> n) )
{
std::cin.clear();
std::string line;
std::getline(std::cin, line);
std::cout << "I am sorry, but '" << line << "' is not a number\n";
}
std::cout << "Thank you for entering the number " << n << '\n';
}

Related

std::cin failure leading to looped if statement in while loop

So I figure I'll put this here since I had to traverse a lot of docs and forums to find the definitive answer. I was trying to get input from the user and check if the input was an integer using isdigit() in an if statement. If the if statement failed the program would output an error message. Although, when a nondigit character was entered the program would loop through the error message endlessly. Here's that code:
int guess = -1;
while (game.getCurQuestion() <= 4) {
std::cout << "Guess: " << game.getCurQuestion() + 1 << std::endl;
std::cin >> guess;
if(isdigit(guess))
{
game.guess(guess);
else
{
std::cout << "Error\n"; //this would be looped endlessly
}
}
std::cout << "You got " << game.getCorrect() << " correct" << std::endl;
return 0;
}
NOTE: Solved, only posted to include my solution. Feel free to correct if I stated anything incorrectly.
The posted way will fail sometimes and will cast the doubles to integers if any doubles are input.
Use something like the following
int getIntInput() {
try {
std::string input;
std::cout << "\nPlease Enter a valid Integer:\t";
std::cin >> input;
size_t takenChars;
int num = std::stoi(input, &takenChars);
if (takenChars == input.size()) return num;
} catch (...) {}
return getIntInput();
}
Problem: The program kept hold of the non-integer value stored in the cin buffer. This leads to the program never leaving the error message.
Solution:
Use std::cin.fail() to check if the input matches the variable data type. I.E. int was the expected input but the user entered a char. In this case std::cin.fail() would be true.
In the case of std::cin.fail(), use std::cin.clear() and std::cin.ignore(std::numeric_limits<int>::max(), 'n') std::cin.clear() will clear the error flag. The std::cin.ignore(std::numeric_limits<int>::max(), 'n') will ignore any other input that is not an integer and will skip to the new line. Effectively progressing the program.
The solution implemented in my code looks like this:
int guess = -1;
while (game.getCurQuestion() <= 4) {
std::cout << "Guess: " << game.getCurQuestion() + 1 << std::endl;
std::cin >> guess;
if (std::cin.fail())
{
std::cout << "Please enter a valid number\n";
std::cin.clear();
std::cin.ignore(std::numeric_limits<int>::max(), '\n');
}
game.guess(guess);
}
Hope this helps and that it saves some people the tedious research because of never learning std::cin error handling! Note: I'm aware my implementation skips the current move, call it punishment ;)

`cin.clear()` leaves the input stream in failure state

I am reading the book 'Accelerated C++', and I am unable to reproduce the results for their read homework problem even after exactly copying their code on my machine. The basic problem is about using cin.clear() to change the failure state of the inpute stream after the use of EOF to indicate that all the grades have been entered. Authors suggest Ctrl+D on linux systems for EOF. I already saw this and this but they couldn't solve my problem.
Here is my minimal working example:
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main()
{
string name, city;
vector<double> homework;
cout << "Enter your name : " ;
cin >> name;
cout << "Hello " + name + "!" << endl;
cout << endl;
cout << "Enter your grades" << endl;
if (cin)
{
homework.clear();
double x;
while (cin >> x)
{
homework.push_back(x);
}
cin.clear();
}
cout << endl;
cout << "Enter your city : " ;
cin >> city ;
cout << "You live in " + city << endl;;
return 0;
}
After entering all the homework, I hit Ctrl+D and then I expect that I would now be given chance to enter city name. But the program just ends after printing the two strings at the end of my code. What is wrong with my understanding of cin.clear()? I would also like to point out that using cin.ignore() after cin.clear() doesn't help either.
On fail you need to clear the flags and ignore all the bad input.
Include #include <limits> for std::numeric_limits.
if ( std::cin.fail( ) )
{
std::cin.clear( );
std::cin.ignore( std::numeric_limits<std::streamsize>::max( ), '\n');
}
If you want a way to exit the loop use a value that you can test for (like -1). When you receive that value, exit the loop.
double value{ 0 };
while( std::cin >> value && value != -1 )
homework.push_back( value );
if ( std::cin.fail( ) )
{
std::cin.clear( );
std::cin.ignore( std::numeric_limits<std::streamsize>::max( ), '\n');
}
Edit:
I didn't see that you were trying to end the stream with EOF.
Read this answer to figure out how to accomplish this.
How to resume input stream after stopped by EOF in C++?

Why does this cause in infinite loop with chars but not doubles?

I feel like im doing something really silly wrong. I just want the program to tell the user when they are entering non-doubles, and continue to loop back to the cin where you enter a value.
I want the user to input any number. Then essential do this trivial math and repeat. Its working fine in that regard, the problem comes when some unexpected input like a char gets entered. Then the input somehow sends it into a loop where it loops the math problem, instead of just telling the user that they must type a number and looping back to cin type in a new number.
#include <iostream>
#include <cstdlib>
using std::cout; using std::cin; using std::endl;
long double domath(long double i)
{
cout << i << "/" << 2 << "=" << i/2 << endl;
cout << i/2 << "*" << 10 << "=" << (i/2)*10 << endl << endl;
cout << 5 << "*" << i << "=" << 5*i << "\n\n";
return 0;
}
int main()
{
long double in = 0;
while(true)
{
cin >> in;
if (cin.fail()) {
in = char(int(in));
}
domath(in);
}
system("pause>nul");
return 0;
}
You don't clear the cin in case of fail, and it infinitely tries to parse wrong input to double, failing every time. You need to clear the buffer in case of error:
if (cin.fail()) {
cin.clear();
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
in = char(int(in));
}
Also, can't understand what you're trying to achieve with
in = char(int(in));
in is a long double variable and will hold the last value you assigned to it, no need to "convert" it to do math.
Couldn't you try doing something like this?
int x;
if(std::cin >> x)
doSomethingCool(x);
else
std::cout << "Error, not a valid integer!" << std::endl;
Exit your loop on bad input.
I think this just feels more natural/looks cleaner than clearing the buffer and all the other jazz. Just my opinion.
if (cin >> x) - Why can you use that condition?
edit: Bul's answer is still a good one though.

Why is exit(0); giving me a std:string... error?

I'm new to C++. I decided to not watch the next tutorial and put my skills to use, by making a funny Mind Reader application. I'm pleased with myself, however, even though I've ironed out most bugs, I still have one concerning the exit function. I read the C++ documentation for it, and I'm not sure what I did wrong. I did exit(0);. I have a very weird error, which is:
no match for call to '(std::string {aka std::basic_string<char>}) (int)
I have searched online, however I am still unaware of what the problem is. My error is on line 59 (marked in the code):
#include <iostream>
#include <string>
#include <cstdlib>
using namespace std;
int main()
{
//declaring variables to be used later
string name;
string country;
int age;
//header goes below
cout << "#######################################";
" ############ MIND READER ############"
"#######################################\n\n";
//asks if the user would like to continue and in not, terminates
cout << "Would like you to have your mind read? Enter y for yes and n for no." << endl;
cout << "If you do not choose to proceed, this program will terminate." << endl;
string exitOrNot;
//receives user's input
cin >> exitOrNot;
//deals with input if it is 'y'
if (exitOrNot == "y"){
cout << "Okay, first you will need to sync your mind with this program. You will have to answer the following questions to synchronise.\n\n";
//asks questions
cout << "Firstly, please enter your full name, with correct capitalisation:\n\n";
cin >> name;
cout << "Now please enter the country you are in at the moment:\n\n";
cin >> country;
cout << "This will be the final question; please provide your age:\n\n";
cin >> age;
//asks the user to start the sync
cout << "There is enough information to start synchronisation. Enter p to start the sync...\n\n";
string proceed;
cin >> proceed;
//checks to see if to proceed and does so
if (proceed == "p"){
//provides results of mind read
cout << "Sync complete." << endl;
cout << "Your mind has been synced and read.\n\n";
cout << "However, due to too much interference, only limited data was aquired from your mind." << endl;
cout << "Here is what was read from your mind:\n\n";
//puts variables in sentence
cout << "Your name is " << name << " and you are " << age << " years old. You are based in " << country << "." << endl << "\n\n";
cout << "Thanks for using Mind Reader, have a nice day. Enter e to exit." << endl;
//terminates the program the program
string exit;
cin >> exit;
if (exit == "e"){
exit(0); // <------------- LINE 59
}
}
}
//terminates the program if the input is 'n'
if (exitOrNot == "n"){
exit(0);
}
return 0;
}
Thanks
The local variable exit shadows other identifiers from outer scopes with the same name.
To illustrate with a smaller example:
int main()
{
int i;
{
int i;
i = 0; // assign to the "nearest" i
// the other i cannot be reached from this scope
}
}
Since the only exit visible is an object of type std::string, the compiler sees exit(0) as a call to operator()(int) and throws a hissy fit when it doesn't find one among std::string members.
You can either qualify the name (std::exit(0);) or rename the variable. And since all of your code is in main you can simply say return 0; instead.
Try using return 0; or return EXIT_SUCCESS;. It's the exact same thing. Also, you can only input one word into a cin. Instead, use getline(cin, string name); If it still doesn't work, add a cin.ignore(); before your getline(cin, string name);, like this:
//stuff
string country;
cout << "Now please enter the country you are in at the moment:\n\n";
cin.ignore();
getline(cin, country);
//stuff
return 0;
The problem is arrising because you declared a standard keyword as the name of a local variable.
Now as the local variable is of type sting it is not able to take it as its value.

Writing a first and last name readable over 2 file matching programs

I'm writing a file matching program for a project for school. The idea is that one program allows you to enter info as follows: 1000 (acct number) Jane Doe 54.50 (balance). Then allow you to enter the account number and a transaction amount for the second program to combine and update a new master file.
The programs are working together just fine (the second one takes information from the first, including any transactions and updates the new balance - searching by account number) but the problem I am running into is with the name.
---Wasn't clear here. When I ask for a name and I put in a single string of characters, the program works fine, if I try to put in a full name, like Jane Doe I go into the loop mentioned below.
I've tried char name[20] which puts me into an infinite loop and I have to 'x' out of the program and I've tried assigning first and lastName to string. That worked for the writing but the program that takes the input file oldMaster and the transaction file inTransaction then outputs a new file newMaster, doesn't recognize the name.
I've tried getline also which isn't working for me, probably programmer error.
Should this be done as an array, if that's possible for this? I think I'm getting hung up on the fact that I am editing files. Answers are fine - but I like to figure it out on my own, just looking for a little guidance on where to go from here.
Hopefully this was fairly clear - if not I'll be happy to explain again in a different way. Just frustrated that I'm this close and can't solve it.
Thanks in advance!
#include "stdafx.h"
#include <iostream>
#include <fstream>
#include <cstdlib>
#include <iomanip>
using namespace std;
void createOldMaster()
{
ofstream oldMaster;
int accountNum;
double balance;
char name[15];
oldMaster.open("oldmast.dat", ios::out);
if(!oldMaster)
{
cout << "Unable to open the file." << endl;
exit(1);
} // end if
cout << "Enter the account number (0 to exit)." << endl;
while(true)
{
cout << "Account Number: ";
cin >> accountNum;
if(accountNum == 0)
break;
else
{
\\ This is where it hangs up if I use a first and last name
cout << "\nName: ";
cin >> name;
cout << "\nBalance : " << endl;
cin >> balance;
oldMaster << accountNum << " " << name << " " << balance << endl;
}
}
} //end createOldMaster
void createTransaction()
{
ofstream inTransaction;
int accountNum;
double balance;
inTransaction.open("trans.dat");
if(!inTransaction)
{
cout << "Unable to open the transaction file." << endl;
exit(1);
}
cout << "Enter the account number and balance (0 to exit): " << endl;
while(true)
{
cout << "Account Number: " << endl;
cin >> accountNum;
if(accountNum == 0)
break;
else
{
cout << "Balance: " << endl;
cin >> balance;
inTransaction << accountNum << " " << balance << endl;
}
}
} //end createTransaction
int main()
{
createOldMaster();
createTransaction();
return 0;
}
Your best bet is to use as much of the standard C++ library as you can. Have a reference handy, maybe even a copy of the C++ standard if you're so inclined, and look for shortcuts to make your work easier and your code shorter.
Avoid primitive arrays and primitive strings wherever possible. Instead of primitive arrays try to use std::vector. Instead of primitive strings try to use std::string. Instead of C's FILE* try to use std::ofstream and std::ifstream. If you need to prohibit two accounts with the same account number then choose a C++ container that guarantees unique elements. If you need to find an element in a container try to use a member function of the container for the search, and if that doesn't exist then a standard search function from the standard C++ algorithms.
Reuse and steal mercilessly.