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C++ is a powerful and popular programming language and we use it widely in coding applications. It provides us with a number of powerful means to check the input given by users. In this article, we will discuss how we can check whether the input given by users is an integer or not.
How to Check Whether the Input Is an Integer in C++?
When we need to perform operations on integers, it is important to check the input so that the operation is performed smoothly and does not produce undesired results. We can have a number of such applications especially in applications like calculators.
Once the input is taken into a string or an array of characters we can check it for its type. For this, we can traverse the input from left to right and check whether each character is a digit or not. We can use two different methods for this.
In the first method, we can use the isdigit() function which is implemented in the C++ library to check whether the character is a digit or not.
In the second method, we can manually check each character by comparing the ASCII value of the character with the ASCII value of the digits. In this way, we will be able to check our input. I have given the code of both of the methods separately but both the approaches are similar so I have written the approach only once. Interestingly, the code works for both positive as well as negative integer inputs.
Further reading:
Check Whether the Input Is an Integer Using the isdigit() Function in C++
The isdigit() function is an inbuilt library function that takes a character as the input argument and returns a boolean value based on the character. If the character is found to be a digit it returns true otherwise it returns false. To check for integer input using the isdigit() function, we will follow the following steps.
- First, we will define a string variable to store the user input.
- Next, we will ask the user for the input through standard input.
- To check if the input is integer or not, we will define a function named
checkInteger(). It will take the string as an input argument, and check whether the input string is an integer or not. It returns true if the input is an integer otherwise returns false. - Inside the
checkInteger()function:- First, we will check if the input string is empty. If yes, It means that the input string is not an integer. Hence, we will return False.
- Otherwise, we will check if the first character is a minus sign (-) so that we can check if the input is a negative integer. If we find a negative sign we have to skip it.
- After checking for the negative sign, we will use a for loop and the
isdigit()function. For each character in the input string, we will check if it is a decimal digit or not using theisdigit()function. If there exists any character that isn’t an integer, theisdigit()function will return False. Hence, thecheckInteger()function will return False denoting that the input string is not an integer. - If there are only decimal digits in the string, the control reaches to the end of the
checkInteger()function out of the loop. In such a case, we return true denoting that the input string is an integer.
- After the function
checkInteger()finishes its execution, we return to themain()function. - In the
main()function, we check if the functioncheckInteger()returns True. If yes, we print output “Yes” denoting that the input string is an integer. Otherwise we print “No”.
Following is the implementation of the above approach to check if an input is an integer in C++.
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#include <iostream> using namespace std; bool checkInteger(string input) { bool isNeg=false; int itr=0; if(input.size()==0) return false; if(input[0]=='-') { isNeg=true; itr=1; } for(int i=itr;i<input.size();i++) { if(!isdigit(input[i])) return false; } return true; } int main() { string input; cout<<"Enter the input\n"; cin>>input; bool isInt = checkInteger(input); cout<<"Is the input an integer? "; if(isInt) cout<<"Yes\n"; else cout<<"No\n"; return 0; } |
Output:
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stark@stark:~/eclipse-workspace/Aditya/src$ g++ checkInt.cc stark@stark:~/eclipse-workspace/Aditya/src$ ./a.out Enter the input 1234 Is the input an integer? Yes stark@stark:~/eclipse-workspace/Aditya/src$ ./a.out Enter the input 123h Is the input an integer? No stark@stark:~/eclipse-workspace/Aditya/src$ g++ checkInt.cc stark@stark:~/eclipse-workspace/Aditya/src$ ./a.out Enter the input -123 Is the input an integer? Yes stark@stark:~/eclipse-workspace/Aditya/src$ ./a.out Enter the input 12.3 Is the input an integer? No |
Check Whether the Input Is an Integer Using the Ascii Values in C++
In this approach, instead of the isdigit() function, we can check whether a character is a digit or not by comparing the ASCII value of the character with the ASCII value of known digits. Here, we will use the ASCII value of the character and compare it with the known ASCII values of the integer characters (from ‘0’ till ‘9’). If the character is found to be outside the range of ASCII values from ‘0’ till ‘9’, the character is not an integer.
For implementing this approach, we can modify the checkInteger() function and insert the code for the above logic instead of the isdigit() function as follows.
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#include <iostream> using namespace std; bool checkInteger(string input) { bool isNeg=false; int itr=0; if(input.size()==0) return false; if(input[0]=='-') { isNeg=true; itr=1; } for(int i=itr;i<input.size();i++) { if(input[i]<'0' || input[i]>'9') return false; } return true; } int main() { string input; cout<<"Enter the input\n"; cin>>input; bool isInt = checkInteger(input); cout<<"Is the input an integer? "; if(isInt) cout<<"Yes\n"; else cout<<"No\n"; return 0; } |
Output:
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stark@stark:~/eclipse-workspace/Aditya/src$ g++ checkInt.cc stark@stark:~/eclipse-workspace/Aditya/src$ ./a.out Enter the input 1234 Is input integer? Yes stark@stark:~/eclipse-workspace/Aditya/src$ ./a.out Enter the input -1234 Is input integer? Yes stark@stark:~/eclipse-workspace/Aditya/src$ ./a.out Enter the input 1234h Is input integer? No stark@stark:~/eclipse-workspace/Aditya/src$ ./a.out Enter the input 12.34 Is input integer? No stark@stark:~/eclipse-workspace/Aditya/src$ |
Conclusion
We come across multiple instances where we need to check whether the input is an integer or not while programming in C++. Before performing operations, we need to check the type of input so that the operations can be performed efficiently and the code does not produce undesired results. In this article, we have seen two different approaches to check if an input is an integer or not in C++. Both methods have approximately the same performance. You can use any of them to check if the user input is an integer or not in C++.
I hope you enjoyed reading this article. Stay tuned for more informative articles.
Happy Learning!
