Beginner’s Guide: Simple Even/Odd Test in C with Sample Code

1. Introduction

2. Definition and Detection of Even and Odd Numbers

2.1 Definition of Even and Odd Numbers

• Even: integers divisible by 2 (including 0, 2, 4, -2, -4, etc.)
• Odd: integers not divisible by 2 (such as 1, 3, 5, -1, -3, etc.)

2.2 The Modulo Operator “%” Used in C

2.3 Logic for Determining Even and Odd

• If the remainder (num % 2) is 0, it’s “even”
• If the remainder is 1 or -1, it’s “odd”

if num % 2 == 0 then even
else odd

3. Even/Odd Determination Program in C

3.1 Basic Sample Code

#include <stdio.h>

int main(void) {
    int num;

    printf("Please enter an integer: ");
    scanf("%d", &num);

    if (num % 2 == 0) {
        printf("%d is even.\n", num);
    } else {
        printf("%d is odd.\n", num);
    }

    return 0;
}

3.2 Detailed Explanation of the Program

• Variable Declaration int num; declares an integer variable num.
• Reading Input scanf("%d", &num); reads an integer from the keyboard.
• Conditional Check if (num % 2 == 0) checks whether the remainder of num divided by 2 is 0. If it is 0, the number is even; otherwise, it is odd.
• Displaying the Result Displays the result on the screen using printf.

3.3 Sample Run

Please enter an integer: 8
8 is even.

Please enter an integer: -3
-3 is odd.

4. Variations of Determination Methods

4.1 Determination Using if Statements

if (num % 2 == 0) {
    // Process for even numbers
} else {
    // Process for odd numbers
}

4.2 Determination Using switch Statements

switch (num % 2) {
    case 0:
        printf("%d is even.\n", num);
        break;
    case 1:
    case -1: // Handles negative odd numbers
        printf("%d is odd.\n", num);
        break;
}

• If you include both case 1 and case -1, you can safely handle negative numbers.

4.3 Concise Expression Using the Ternary Operator

printf("%d is %s.\n", num, (num % 2 == 0) ? "even" : "odd");

• The ternary operator is suitable for concise conditional expressions and is handy when you want a one-liner.

4.4 Pros and Cons of Each

5. Application Example: Combining Sign Determination

5.1 Sample Code for Determining Sign as Well

#include <stdio.h>

int main(void) {
    int num;
    printf("Please enter an integer: ");
    scanf("%d", &num);

    if (num == 0) {
        printf("0 is even.\n");
    } else if (num > 0) {
        if (num % 2 == 0) {
            printf("%d is a positive even.\n", num);
        } else {
            printf("%d is a positive odd.\n", num);
        }
    } else {
        if (num % 2 == 0) {
            printf("%d is a negative even.\n", num);
        } else {
            printf("%d is a negative odd.\n", num);
        }
    }
    return 0;
}

5.2 Explanation of the Program

• When num == 0 Zero is mathematically classified as an even number, so it is handled specially.
• When num > 0 For positive integers, determine even or odd accordingly.
• When num < 0 For negative integers, also determine even or odd similarly.

5.3 Practical Application Examples

• Differentiated use in data processing and control programs For example, you can apply it to cases where you process only even numbers or give special treatment to positive odd numbers.
• Enhancing program variations Changing output or processing based on combinations of sign and parity improves the program’s flexibility and practicality.

6. Common Errors and Their Solutions

6.1 Handling Non-Integer Input

#include <stdio.h>

int main(void) {
    int num;
    printf("Please enter an integer: ");
    if (scanf("%d", &num) != 1) {
        printf("Non-integer input was entered.\n");
        return 1;
    }
    // Continue the decision process here
    return 0;
}

6.2 Considerations for Handling Negative Numbers and Zero

• Handling Zero Zero is mathematically even, but it can be inadvertently omitted from the condition. Writing explicit handling for zero provides peace of mind.
• Remainder of Negative Numbers In C, the remainder of a negative number divided by % 2 can be -1. For odd-number checks, consider both num % 2 == 1 and num % 2 == -1.

6.3 Debugging Methods When the Program Doesn’t Behave as Expected

• Verify Output Using printf to check variable values and remainders lets you verify that the calculations are as expected.
• Review if Statements and Conditions Re-examine whether the decision expressions are set correctly and that no conditions are missing.
• Test Repeatedly with Sample Data For example, input values like “0”, “2”, “-3”, etc., and test to ensure they are judged as expected.

6.4 Summary

7. Summary

7.1 Recap of Article Points

• Definition and Determination of Even and Odd We learned that an even number is defined as a number divisible by 2, and an odd number as one that is not, and that in C you can easily determine this using the modulo operator (%).
• Basic Program Examples and Explanation We introduced various ways of writing code, focusing on if statements, as well as switch statements and the ternary operator.
• Advanced Usage We also covered practical examples combining even/odd checks with sign checks, error handling, and input validation, providing a more practical perspective.
• Common Errors and Troubleshooting We also discussed points on input validation and debugging.

7.2 How to Apply the Knowledge Learned

• Extract only even numbers from an array
• Calculate the sum of odd numbers among multiple input values
• Branch processing under specific conditions

7.3 Next Steps for Further Learning

8. FAQ (Frequently Asked Questions)

printf("%d is %s.\n", num, (num % 2 == 0) ? "even" : "odd");