In C++, how do you handle wrong inputs? Like, if the program asks for an integer, when you type a character it should be able to do something and then loop to repeat the input but the loop goes infinite when you input a character when an integer is need and vice versa.
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5 Answers
The reason the program goes into an infinite loop is because std::cin's bad input flag is set due to the input failing. The thing to do is to clear that flag and discard the bad input from the input buffer.
//executes loop if the input fails (e.g., no characters were read)
while (std::cout << "Enter a number" && !(std::cin >> num)) {
std::cin.clear(); //clear bad input flag
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); //discard input
std::cout << "Invalid input; please re-enter.\n";
}
See the C++ FAQ for this, and other examples, including adding a minimum and/or maximum into the condition.
Another way would be to get the input as a string and convert it to an integer with std::stoi or some other method that allows checking the conversion.
10 Comments
Zik
i have done some research before asking here. I saw that they put cin.ignore(1000, '\n'); what does this do? Also !(cin >> num) returns a boolean? i didn't know that
Qaz
@Marvin,
cin.ignore (1000, '\n') ignores/discards characters in the input buffer until either 1000 have been discarded, or a newline has been encountered, whichever comes first. It's a good way to get rid of a line. You'll see in the parashift example, they use the maximum size of a stream instead of 1000 to account for a line of max length. I use cin.sync() because when doing this, I want to be on equal footing with the user (not read the next line yet), so I discard everything. Finally, cin has an operator void *, so it cann convert to a bool.Qaz
@Marvin,
cin >> num fails if a user types, say 'a' when it was expecting an int. It provides a conversion operator to allow it to be implicitly converted to a void *. If cin is in a bad state, it will return NULL. If not, it will return the object. This can then be taken and converted to bool: true if not NULL, false if NULL. The loop can then use that to evaluate the bool expression it needs.Qaz
@VaisakMohan, That would be fine too; it's one way to do the last paragraph. However, do be aware of whether you want to count multiple inputs on one line. It would be a shame if you read a line of valid inputs, convert the first part of it, and toss the others.
getline works best for actual line-based input instead of token-based. You also have to be aware of whitespace, which could be handled differently in your conversion. Beyond that, reading line by line means you have to do the conversion and error checking (minus discarding bad input from the stream) instead of only the latter.Qaz
In addition, with
getline, you have to be careful not to leave a stray newline in the stream before using it. If using getline, it's generally recommended to go all-in instead of trying to mix and match it with token-based input. |
The top voted answer covers the solution really well.
In addition to that answer, this may help visualize what's going on a little better:
int main()
int input = 1;//set to 1 for illustrative purposes
bool cinState = false;
string test = "\0";
while(input != -1){//enter -1 to exit
cout << "Please input (a) character(s): ";//input a character here as a test
cin >> input; //attempting to input a character to an int variable will cause cin to fail
cout << "input: " << input << endl;//input has changed from 1 to 0
cinState = cin;//cin is in bad state, returns false
cout << "cinState: " << cinState << endl;
cin.clear();//bad state flag cleared
cinState = cin;//cin now returns true and will input to a variable
cout << "cinState: " << cinState << endl;
cout << "Please enter character(s): ";
cin >> test;//remaining text in buffer is dumped here. cin will not pause if there is any text left in the buffer.
cout << "test: " << test << endl;
}
return 0;
}
Dumping the text in the buffer to a variable isn't particularly useful, however it helps visualize why cin.ignore() is necessary.
I noted the change to the input variable as well because if you're using an input variable in your condition for a while loop, or a switch statement it may go into deadlock, or it may fulfill a condition you weren't expecting, which can be more confusing to debug.
Comments
Here is my approach to this matter.
Below is a C++ program that allows an end user to answer Yes or No to a simple "Yes" or "No" question.
This program will re-ask the same question, as long as the answer is neither Yes nor No.
Since C++ is case sensitive, I have also set up the program to accept both upper and lower case letters of the first letter of the allowed answers, since they both basically signify the same thing.
#include <iostream>
#include <string>
using namespace std;
int main() {
string input;
do {
cout << "Are you interested in learning how to solve problems through coding?" <<endl;
cout << "Choose Yes or No" <<endl;
cin >> input;
if ((input == "Yes") || (input == "yes"))
{
cout << "That is one of the ways of becoming a software engineer.";
break;
}
else if ((input == "No") || (input == "no"))
{
cout << "You have little possibility of becoming a computer engineer."<<endl;
break;
}
} while ((input != "Yes" && input != "No") || (input != "yes" && input != "no"));
return 0;
}
Comments
You can check it through the ASCII value if the ascii value s between 65 t0 90 or 97 to 122 the it would be character.
1 Comment
Toby Speight
How do you know the system is using ASCII?
