I'm currently working on a small project to learn (and for fun) and need a while-loop to check if a user's input is one of the integers 1, 2, 3, 4, or 5. What is the best way to do this? Here's my basic idea in code, but it's not quite working:
std::cin >> input;
while (cin.fail() == true || input != 1 && input != 2 && input != 3 && input != 4 && input != 5){
std::cout << std::endl "The valid choices are 1, 2, 3, 4, and 5. Please choose: ";
std::cin >> input;
std::cout << std::endl;
}
This only works if it's a digit above 5, but fails if I enter a letter. How can I use cin.fail() to validate correctly?
First
#include <limits>
to get max streamsize. Then flip some of the logic around
while (!std::cin >> input || // didn't read valid input
input < 1 || // number too small
input > 5) // number too large
{
std::cout << std::endl "The valid choices are 1, 2, 3, 4, and 5. Please choose: ";
Then clear the stream error and any other crap the user typed in
std::cin.clear();
std::cin.ignore(std::numeric_limits<streamsize>::max(), '\n');
Finally ask for a redo
std::cin >> input;
std::cout << std::endl;
}
What this won't catch:
1.23
cin will stop reading at the '.' because it's not found in an integer and happily return 1. Ooops.
1a
Same problem
1 holy bad input, Batman!
Similar problem. cin stops at the space.
What you really want to do is something that gets the whole input line from the user with std::getline and then uses std::stoi to make sure that it is all an int
Assuming that input is an integer, failures will happen when it reads in a string/char. What you need is
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
in the body of your while loop since when it fails the stream closes, so you need to clear it.
To expand a bit, the cin.clear() just clears the stream, and then the rest of the line is ignored in the cin.ignore(). It will ignore the maximum possible line size so it's the most correct, but most programs can get away with just putting some huge number in there instead.
Related
I am currently trying to write a program at school involving a random number generator for rolling a dice. I have written the code successfully, but I am trying to improve it so that the size of the dice and the number the user is trying to roll can be chosen by the user.
I have written code that does this, and I have also added code that repeats the request for input if the wrong value (ie not one of the dice sizes offered or trying to roll a number outside the range of the dice) or input type (ie var instead of int) is entered. However, if a floating point number is entered, and the number to the left of the floating point is in the correct range of values, it is using that number.
For example:
int size = 0;
cout << "Please choose the size of the dice to roll (6, 12 or 20): ";
cin >> size;
while (size != 6 && size != 12 && size != 20 || !cin)
{
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
cout << "Invalid number entered. Choose the size of the dice to roll (6, 12 or 20): ";
cin >> size;
}
This will correctly ask to repeat the input if any letters or any numbers that aren't 6, 12 or 20 are entered, but if 20.125 (or any floating point number that is 6.- 12.- or 20.-) is entered it will take the number and move on to the next bit of code. This also happens if I enter a valid number followed by letters (ie 6f).
I tried modifying the condition of the while loop to:
while (size != 6 && size != 12 && size != 20 || !(cin >> size))
{
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
cout << "Invalid number entered. Choose the size of the dice to roll (6, 12 or 20): ";
cin >> size;
}
And that fixes the problem, and it asks me for another input if I enter 12.5 or 20R etc, but then when I enter a correct input (6, 12 or 20) it just takes me to a new line in the debugger without moving to the next line of code. If I then enter a correct input again, it reads it at takes me to the next line of code.
I don't quite understand why one works 99% how I want it to with one error, and the other fixes that error but then gives me another one.
Thanks in advance, any advice guidance is much appreciated!
The way cin >> some_int_variable will interpret the input is character by character, until it stops making sense as an int. For instance, when it encounters . or f, cin is done reading that int.
If you want a different behavior, you will have to implement it yourself. Specifically, how do you stop processing one input, and starts processing the next?
cin >> some_int_variable will stop when it is no longer a valid it, cin >> some_std_string_variable will stop when it encounters an white-space character (new lines included). How about your problem? How do you want to separate one input from the next?
If white-space is a sensible approach, you can do so:
std::string word;
std::cin >> word;
int value;
bool error = false;
try {
size_t pos;
value = std::stoi(word, &pos);
// Was the full string used to build the int?
if(pos != word.size()) error = true;
} catch(std::logic_error&) {
error = true;
}
if(error) { /* something went wrong */ }
See the docs for std::stoi().
You could read a float, assign it to an integer variable and then compare the two. Something along these lines:
int integerSize;
float floatSize;
cin >> floatSize;
integerSize = floatSize; //Your program will perform a cast from float to integer, it will essentially truncate whatever appears after the floating point.
if (integerSize != floatSize) //Your program will now perform a cast from integer to float in order to compare the two, but if floatSize was not a whole number, this comparison should return false.
{
//code to ask for another value
}
That being said, floats (and doubles and long doubles) do experience rounding errors, so as another user suggested, the safest bet is to read a string and parse it yourself, or using a built-in function like std::stoi() (only for integers) and std::stof() (for floats).
template<typename T>
T typed_read_line(const std::string& prompt)
{
T result{};
std::string l;
std::cout << prompt << ":";
while (std::getline(std::cin, l)) {
std::istringstream line{l};
line >> result >> std::ws;
if (line.eof()) break;
//line contains more then expected
std::cout << "Again: " << prompt << ":";
}
return result;
}
https://godbolt.org/z/dfYKe3
Or version with extra validation or v3.
I'm new to C++ and can't figure out what's going on here...
Basically I have a function with a bunch of Log("...") statements (text-based adventure)
and then I want to basically create a try-catch block to see if the user tries to print anything that isn't 1 or 2 as those are the only 2 choices. I noticed when you input strings into the console it converts it to 0 so basically my code is-
void myfunciton()
{
top:
Log("Choose...");
Log("(1) ...");
Log("(2) ...");
std::cout << std::string(11, '\n') << std:endl;
std::cout << "Enter a number: ";
std::cin >> userchoice; // Userchoice is a global variable defined above and set to 100.
if (userchoice == 0) {
userchoice = 100;
goto top;
}
Instead of the line goto top; I also tried just calling myFunction(); but it gives me the same outcome... The code runs again and I see my story, but I can't enter another number. It prints out the line "Enter a number: " inside my console, but than immediately prints "exited with code 0 press any key to close..."
I just don't understand why it isn't letting me input another number again. Any help would be appreciated :)
Because the numerical extractor leaves non-digits behind in the input buffer.
Try this:
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
and avoid using goto-s.
Also you can use:while (std::cin >> x) (x is for your input) for better flow of code.
I am trying to determine if the user enters something other than a character, and specifically, that they only enter m or s. See code below.
Note:
filingStatus is a char
do
{
cout << "Please enter your filing status (s for single, m for married)" << '\n';
cin >> filingStatus;
if (cin.fail())
{
cin.clear();
dataTest = 1;
cout << "Error, please enter a valid input!" << '\n';
double dummyDouble;
cin >> dummyDouble;
if (cin.fail())
{
cin.clear();
cin >> dummyString;
}
}
else
{
if (filingStatus == 'm' || filingStatus == 's')
{
dataTest = 0;
}
else
{
cout << "Error, please enter either m or s!" << '\n';
dataTest = 1;
}
}
} while (dataTest == 1);
Here is the problem, if I enter 1000 for example, the input doesn't fail. It instead stores the 1 in the char, and since 1 is neither m or S, it loops again, then it puts the 0, loops again, puts another 0, etc.
My understanding was it would fail when it sees that a integer is being stored in a char, but obviously it isn't failing.
My question is:
Why isn't the input failing? How can I change it so if someone enters a string, or number that it fails?
The input isn't failing, because '1' is a character. Digits are a subset of characters.
Read into a std::string. Then test whether that string consists of a single character from your desired range.
Note however, that reading into a string using >> stops at the first white space. To prevent this and read the whole line instead, read using std::getline().
I am assuming that fillingStatus is of char type.
Now even if you enter a numeral say '1' or '0', it is read as a char. Hence cin does not fail. It just keeps on looping as per your code.
Also, while reading an invalid char, you should be careful of clearing the input buffer because the return character '\n' stays along with other characters in the input buffer.
I would do it something like the following:
while ( !(cin >> fillingStatus) || (filingStatus != 'm' && filingStatus != 's') ) {
cout << "Error, please enter either m or s!" << '\n'; // error message
cin.clear(); // clear the error flag
cin.ignore(numeric_limits<streamsize>::max(), '\n'); // ignore all invalid previous input
}
One way could be to change the fillingStatus to string and get only the first character of that string and see if it fails or not.
Alternatively, there used to be a method for getting a character input, getche() I think (it has been many years since I worked in C++ so don't exactly recall)...you maybe able to use that too.
Thanks
Since you are only reading the input one character at a time, your are essentially unable to tell that the user has input more and it is being held until you read more from the input stream.
Using a string to read a line of data at a time and having the program react to that string as a whole will solve your problem.
std::string filingStatus ;
while(!(cin >> filingStatus ) || ( filingStatus != "m" && filingStatus != "f") )
{
cin.clear();
std::cout << "Error, please enter either m or s!" << '\n';
};
I was wondering if there was anyways of stopping letters being entered for an integer. Here is the code which I have been using in my int main.
do
{
cout << "Player 1 please enter the value of the row you would like to take ";
cin >> row;
}while (row != 0 && row != 1 && row != 2 && row != 3);
My problem with this code is that if the user enters a letter it creates a never ending loop. Any help would be much appreciated.
Standard library doesn't provide anything that would filter characters that are entered through standard input. I believe you could use libraries like curses to do that.
What you can do, though, is check whether input suceeded. operator>> for int will set the stream's state to failbit if it couldn't extract an integer (for example, when it encountered an 'a' or something like that. You can use extraction operators in boolean context, something like this:
cout << "Player 1 please enter the value of the row you would like to take ";
while (!(cin >> row) || (row < 0 || row > 3)) {
cout << "Invalid input, try again!\n";
// clear the error flags and discard the contents,
// so we can try again
cin.clear();
cin.ignore(std:numeric_limits<std::streamsize>::max(), '\n');
}
Note that if you enter for example 1abc, the read will succesfuly read 1 and leave the abc in the stream. This might not be a desired behaviour. If you wish to treat that as an error you can say
if ((cin >> std::ws).peek() != EOF) { /* there's more input waiting */ }
and act accordingly, or just unconditionaly ignore everything from the stream once you've got a value.
Get characters one at a time and only add the number characters to the string. Use
cin.get();
in a loop.
I am having trouble using the cin method to acquire a variable. When the input is a number there is no problem, but when it is a special character like a dot [.],
the whileloop loops into infinity.
What am I doing wrong?
cout << "What is your race" <<endl<<"1.Human\n2.troll\n3.zombie"<<endl;
cin >> *race;
while(*race<1||*race>3)
{
system("cls");
cout << "Wrong choice"<<endl<< "What is your race" <<endl<<"1.Human\n2.troll\n3.zombie"<<endl;
cin >> *race;
}
I searched for the answer and i should have to flush the buffer but i don"t get how to do it. I'm rather new with c++. Thanx
Make race an char, then you will be able do to:
while (*race < '1' || *race > '3')
which is probably what you want to achieve.
Explanation:
When you cin >> into an int, it converts given ASCII string to integer value. . doesn't have an integer meaning, so it isn't read into race and failbit is set - further >>s are no-op, until you clear them. However, if you cin >> into char and compare it with other chars (well, their ASCII codes, actually), you will be able to check it without troubles.
This example exactly reproduces your problem:
#include <iostream>
int main()
{
int i = 5;
while (i < 1 || i > 3)
{
std::cin >> i;
}
}
Here's what happens: When operator>> fails to read an integer (e.g. when you type a dot and press enter), whatever you typed stays in the stream, including the newline character.
So in the next iteration of the while loop the next input is already there and since it's not a valid integer, the loop can never break.
You need to make sure that, when operator>> fails, you empty the stream and clear all the error flags that got set.
#include <iostream>
#include <limits>
int main()
{
int i = 5;
while (i < 1 || i > 3)
{
if (!(std::cin >> i))
{
// clear streams internal error flags
std::cin.clear();
// ignore what's left in the stream, up to first newline character
// or the entire content, whichever comes first
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
}
}
There are several problems with your code. The first is that you don't
verify that your input has succeeded; the correct condition for the
while should be:
while ( !cin || (*race < 1 || *race > 3) )
As written, if the input fails (which is what is happening when you
enter a '.', supposing that race has type int*), then *race
contains its previous value, whatever that was.
The second is that if you do get an error from cin, you don't clear
it. Once the stream is in an error state, it stays that way until you
explicitly clear it. If cin has failed, you need to execute:
cin.clear();
somewhere in the loop.
The third is that if cin fails, you don't extract the character which
made it failed, so that after clearing the error status, you need to
extract it. Given the way you've structured your dialog, you probably
want to ignore everything until the end of the line:
cin.ignore( INT_MAX, '\n' );
You may want to do this even if cin didn't fail, either in the loop
(if entered because of the *race < 1 || *race > 3 condition), or in
case of success. Alternatively, you may want to shift to reading lines,
and ensure that the line only contains whitespace after the character
you're interested in.
This last solution is the one I would adopt, since it handles pretty
much all of the above problems. So my code would look something like:
// return -1 on error in input,
// throw exception on (unexpected) end of file
int
getRace( std::istream& source )
{
std::string line;
if ( !std::getline( source, line ) ) {
throw std::ios_base::failure( "Unexpected end of file" );
}
std::istringstream tmp( line );
int results;
return tmp >> results >> std::ws && tmp.get() == EOF
? results
: -1;
}
// ...
int race = -1;
while ( race < 0 ) {
std::cout << "What is your race\n"
"1. Human\n"
"2. Troll\n"
"3. Zombie\n" << std::flush;
race = getRace( std::cout );
if ( race < 0 ) {
std::cout << "Wrong choice" << std::endl;
}
}
Note that by inputting through a line, you avoid any problems with
resetting format errors, skipping erroneous input or resynchronizing in
case of error.
The other solution besides the one accepted is to clear the cin's failbit and ignore the last input like below:
cout << "What is your race" <<endl<<"1.Human\n2.troll\n3.zombie"<<endl;
cin >> *race;
while(*race<1||*race>3)
{
// Clears the state of cin to be in good state
cin.clear();
// Ignores the last input read so that it's not read back again
cin.ignore();
system("cls");
cout << "Wrong choice"<<endl<< "What is your race" <<endl<<"1.Human\n2.troll\n3.zombie"<<endl;
cin >> *race;
}