How to Check if Input Is Integer in C++

1. Use the std::find_if Algorithm to Check if Input Is Integer in C++
2. Use the std::string::find_first_not_of Function to Check if Input Is Integer in C++
3. Use the std::string::find_first_not_of Function to Check if Input Is Integer in C++

This article will demonstrate multiple methods about how to check if the input is an integer in C++.

Use the std::find_if Algorithm to Check if Input Is Integer in C++

std::find_if is part of the STL algorithms library defined in the <alogrithm> header file, and it can be utilized to search for the specific element in the range. Since the user input is most likely to be a string, we will assume that the input data is stored in a std::string object. Note that we implement a function called isNumber that takes a reference to std::string and returns the bool value.

The prototype of the std::find_if function that we used in the following example takes three arguments, the first two of which specify the range elements - [first, last]. The third argument is a unary predicate, which is a lambda function that returns the bool value from evaluating the inverted value of the isdigit function. On the outer layer, the std::find_if return value is compared to the str.end(), as the true value of the expression indicates that no non-digit character was found; thus it’s the number. Additionally, we logically AND the former expression with the !str.empty to indicate that string is empty and return false.

#include <algorithm>
#include <iostream>
#include <string>

using std::cin;
using std::cout;
using std::endl;
using std::string;

bool isNumber(const string& str) {
  return !str.empty() &&
         std::find_if(str.begin(), str.end(), [](unsigned char c) {
           return !std::isdigit(c);
         }) == str.end();
}

int main() {
  string str1 = "12345.";
  string str2 = "12312";
  string str3 = "123142.2";

  isNumber(str1) ? cout << "Number\n" : cout << "Not number\n";
  isNumber(str2) ? cout << "Number\n" : cout << "Not number\n";
  isNumber(str3) ? cout << "Number\n" : cout << "Not number\n";

  exit(EXIT_SUCCESS);
}

Output:

Not number
Number
Not number

Use the std::string::find_first_not_of Function to Check if Input Is Integer in C++

Alternatively, we can reimplement isNumber function using the find_first_not_of method that is built-in for the std::string object. find_first_not_of can take the string value and find the first character that equals none of the characters in the string sequence. If the function does not find such a character, string::npos is returned. Thus, we specify all ten decimal digits as find_first_not_of argument and check its equality to npos, as the expression value is returned from the function.

#include <algorithm>
#include <iostream>
#include <string>

using std::cin;
using std::cout;
using std::endl;
using std::string;

bool isNumber(const string& str) {
  return str.find_first_not_of("0123456789") == string::npos;
}

int main() {
  string str1 = "12345.";
  string str2 = "12312";
  string str3 = "123142.2";

  isNumber(str1) ? cout << "Number\n" : cout << "Not number\n";
  isNumber(str2) ? cout << "Number\n" : cout << "Not number\n";
  isNumber(str3) ? cout << "Number\n" : cout << "Not number\n";

  exit(EXIT_SUCCESS);
}

Output:

Not number
Number
Not number

Use the std::string::find_first_not_of Function to Check if Input Is Integer in C++

Note though, the previous method does not identify the real numbers and treats them as illegitimate. So, we can add the . character to the string and allow the function to recognize any sequence of digits with a dot symbol as a valid number. We need to eliminate two cases when the . character is the first and the last symbol in the input sequence, which by our convention would not be a valid real number. We can use string built-in methods front and back to verify that input does not begin/end with a dot symbol. Finally, we logically AND all three expressions with each other and return that value.

#include <algorithm>
#include <iostream>
#include <string>

using std::cin;
using std::cout;
using std::endl;
using std::string;

bool isNumber3(const string& str) {
  return str.find_first_not_of(".0123456789") == string::npos &&
         str.front() != '.' && str.back() != '.';
}

int main() {
  string str1 = "12345.";
  string str2 = "12312";
  string str3 = "123142.2";

  isNumber(str1) ? cout << "Number\n" : cout << "Not number\n";
  isNumber(str2) ? cout << "Number\n" : cout << "Not number\n";
  isNumber(str3) ? cout << "Number\n" : cout << "Not number\n";

  exit(EXIT_SUCCESS);
}

Output:

Not number
Number
Number