C++ Even or Odd Program
Checking whether a number is even or odd is one of the first real programs every C++ beginner writes. It’s small, but it teaches the modulo operator, conditionals, and reading input — three skills you’ll use forever. Let’s build it three ways.
Method 1: The Modulo Check
An even number divides by 2 with nothing left over. The modulo operator % gives the remainder, so number % 2 == 0 means “even”:
#include <iostream>
int main() {
int number = 7;
if (number % 2 == 0)
std::cout << number << " is even\n";
else
std::cout << number << " is odd\n";
return 0;
}7 % 2 is 1 (odd), so this prints “7 is odd.” Change number to 8 and the remainder becomes 0, so it prints “even.” That % 2 == 0 test is the heart of the whole program.
Method 2: Ask the User
A program is more useful when it works on any number the user types. Read it with std::cin:
#include <iostream>
int main() {
int number;
std::cout << "Enter a number: ";
std::cin >> number;
if (number % 2 == 0)
std::cout << number << " is even\n";
else
std::cout << number << " is odd\n";
return 0;
}std::cin >> number waits for the user to type a value and stores it. Everything else is the same — the logic doesn’t care where the number came from.
Method 3: The One-Line Ternary
Once the idea is clear, you can collapse the if/else into a single line with the ternary operator:
#include <iostream>
#include <string>
int main() {
int number = 13;
std::string result = (number % 2 == 0) ? "even" : "odd";
std::cout << number << " is " << result << "\n";
return 0;
}The ternary picks "even" when the test is true and "odd" when it’s false. It’s the same logic, just more compact — handy once you’re comfortable with the pattern.
Reusing It with a Function
If you check even/odd in several places, wrap it in a function so you write the logic once:
#include <iostream>
bool isEven(int n) {
return n % 2 == 0;
}
int main() {
for (int i = 1; i <= 5; ++i)
std::cout << i << ": " << (isEven(i) ? "even" : "odd") << "\n";
return 0;
}isEven returns a bool you can use anywhere a condition is expected. This is a great early example of why functions make code cleaner and easier to read.
Bonus: The Bitwise Shortcut
Experienced programmers sometimes use & 1 to test the last bit, which is 0 for even numbers:
#include <iostream>
int main() {
int number = 42;
// The last bit is 0 for even numbers, 1 for odd
if ((number & 1) == 0)
std::cout << "even\n";
else
std::cout << "odd\n";
return 0;
}This is slightly faster, but % 2 is just as efficient on modern compilers and far easier to read. Stick with modulo unless you have a specific reason not to.
Related Articles
- C++ Modulo Operator (%) — the operator behind this program
- C++ Conditionals Tutorial — if, else, and comparisons
- C++ cin User Input — reading values from the keyboard
- C++ Ternary Operator — the one-line if/else
- C++ Prime Number Program — another classic beginner build
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