Day 5 of C Language: Operators = Letting the Program Perform Calculations

Day 5 of C Language: Operators = Letting the Program Perform Calculations 🧮

Learn to calculate from scratch, master addition, subtraction, multiplication, and division in 5 minutes

In the first four days, we learned how to make the program “speak”, “remember”, and “ask questions”. Today, we will learn something more practical—how to make the program “calculate”! 🔢

By mastering operators, your program will be able to solve math problems! Addition, subtraction, multiplication, division, and more… the program will become smarter!

What are Operators?

Imagine you have a calculator:

A program without operators:

Can only store numbers: 18, 95, 3.14
But cannot perform calculations, it can only “look at” these numbers

A program with operators:

Can perform addition:18 + 5 = 23
Can perform subtraction:95 - 10 = 85
Can perform various calculations!

Operators are the “calculation buttons” in a program

+ = Addition button
– = Subtraction button
* = Multiplication button
/ = Division button

Operators are: symbols that allow the program to perform mathematical calculations! 🔢

Five Basic Arithmetic Operators

Similar to addition, subtraction, multiplication, and division in math class, with just a slight difference!

1. Addition (+)

int a = 10;
int b = 5;
int sum = a + b;  // 10 + 5 = 15
printf("Result: %d ", sum);
// Output: Result: 15

2. Subtraction (-)

int a = 10;
int b = 5;
int diff = a - b;  // 10 - 5 = 5
printf("Result: %d ", diff);
// Output: Result: 5

3. **Multiplication (*)**

⚠️ Note: It is not ×, but * (asterisk)

int a = 10;
int b = 5;
int product = a * b;  // 10 * 5 = 50
printf("Result: %d ", product);
// Output: Result: 50

4. Division (/)

⚠️ Note: It is not ÷, but / (slash)

int a = 10;
int b = 5;
int quotient = a / b;  // 10 / 5 = 2
printf("Result: %d ", quotient);
// Output: Result: 2

Special Note on Integer Division:

int a = 10;
int b = 3;
int result = a / b;  // 10 / 3 = 3 (not 3.33!)
printf("Result: %d ", result);
// Output: Result: 3

Why is it 3 instead of 3.33? Because when two int integers are divided, the result is also an integer, and the decimal part is automatically discarded!

Want a decimal result? Use float:

float a = 10.0;
float b = 3.0;
float result = a / b;  // 10.0 / 3.0 = 3.333...
printf("Result: %.2f ", result);
// Output: Result: 3.33

5. Modulus (%) 🎯

This is a new face!% does not mean “percent”, but “remainder”!

Example 1: Even or Odd?

int a = 10;
int b = 3;
int remainder = a % b;  // 10 divided by 3, remainder is 1
printf("Remainder: %d ", remainder);
// Output: Remainder: 1

Example 2: Determine Even or Odd

int number = 7;
int result = number % 2;
if (result == 0) {
    printf("Even ");
} else {
    printf("Odd ");  // Output: Odd
}

Memory Tips:

10 % 3 = 1 (10 apples, 1 left after grouping 3)
15 % 4 = 3 (15 oranges, 3 left after grouping 4)

Assignment Operators

Basic Assignment (=)

Do you remember what we learned on Day 2?= is not “equal to”, but “assignment”!

int age = 18;  // Put 18 into the box named age

Compound Assignment Operators (Shortcut Notation)

If you want to add a number to a variable, there are two ways to write it:

Traditional Notation:

int score = 80;
score = score + 10;  // score becomes 90

Shortcut Notation:

int score = 80;
score += 10;  // Equivalent to score = score + 10

Four Common Compound Operators

int a = 10;

a += 5;   // Equivalent to a = a + 5;  Result: 15
a -= 3;   // Equivalent to a = a - 3;  Result: 12
a *= 2;   // Equivalent to a = a * 2;  Result: 24
a /= 4;   // Equivalent to a = a / 4;  Result: 6

Why use compound operators?

Code is more concise
Faster to write
Clearer to read

Operator Precedence

Do you remember the “multiply and divide first, then add and subtract” rule from math class? C language follows the same!

Precedence Rules

1. Parentheses have the highest precedence()

int result = (2 + 3) * 4;  // First calculate 2+3=5, then calculate 5*4=20
printf("%d ", result);     // Output: 20

2. Multiplication and Division take precedence over Addition and Subtraction

int result = 2 + 3 * 4;  // First calculate 3*4=12, then calculate 2+12=14
printf("%d ", result);   // Output: 14

3. Operations of the same precedence are evaluated from left to right

int result = 10 - 5 - 2;  // From left to right: (10-5)-2 = 3
printf("%d ", result);    // Output: 3

Practical Advice 💡

If unsure about precedence, add parentheses!

int result = (2 + 3) * (4 + 5);  // Clear and straightforward!

Adding parentheses not only makes the program correct but also makes the code more readable!

Complete Example: Simple Calculator

Let’s write a complete program to perform four basic operations:

#include <stdio.h>

int main() {
    int a = 10;
    int b = 3;
    
    printf("=== Simple Calculator === ");
    printf("a = %d, b = %d  ", a, b);
    
    // Addition
    printf("a + b = %d ", a + b);
    
    // Subtraction
    printf("a - b = %d ", a - b);
    
    // Multiplication
    printf("a * b = %d ", a * b);
    
    // Division
    printf("a / b = %d ", a / b);
    
    // Modulus
    printf("a %% b = %d ", a % b);  // Note: To print %, write %% 
    
    // Compound Operations
    int score = 80;
    printf(" Initial Score: %d ", score);
    
    score += 10;
    printf("After adding 10 points: %d ", score);
    
    score -= 5;
    printf("After subtracting 5 points: %d ", score);
    
    return 0;
}

Running Result

=== Simple Calculator ===
a = 10, b = 3

a + b = 13
a - b = 7
a * b = 30
a / b = 3
a % b = 1


Initial Score: 80
After adding 10 points: 90
After subtracting 5 points: 85

⚠️ Common Mistakes for Beginners

Mistake 1: Integer division does not yield a decimal

int a = 10;
int b = 3;
float result = a / b;  // ❌ Result is still 3.0, not 3.33
printf("%.2f ", result);


// ✅ Correct approach: At least one is float
float result = 10.0 / 3;  // Or (float)a / b

Mistake 2: Incorrect multiplication symbol

int result = 5 × 3;   // ❌ Error! × is not a C language symbol
int result = 5 * 3;   // ✅ Correct! Use asterisk *

Mistake 3: Division by zero

int result = 10 / 0;  // ❌ The program will crash!
// Remember: Never divide by 0!

Mistake 4: Forgetting precedence

int result = 2 + 3 * 4;  // It is 14, not 20!
// Unsure? Add parentheses: (2 + 3) * 4

🎯 Today’s Takeaways

✅ Understood that operators are the “calculation buttons” in a program✅ Learned five arithmetic operators: + – * / %✅ Mastered compound assignment operators: += -= *= /=✅ Understood the operator precedence rules✅ Wrote a program that can perform calculations

That’s it for Day 5! By mastering operators, your program can solve math problems!

Now you have mastered:

✅ Making the program speak (printf)
✅ Making the program remember data (variables)
✅ Making the program ask questions (scanf)
✅ Making the program calculate (operators)

Next, we will learn how to make the program “make decisions”—the if statement! The program will do different things based on conditions! 🤔

🏆 Mini Challenge

Write a program that implements the following functionality:

Define two variables:price = 100 (original price) and discount = 20 (discount amount)
Calculate the discounted price:final_price = price - discount
Print the original price, discount, and final price
Advanced Challenge: Use scanf to let the user input the original price and discount, then calculate automatically!

Tips:

Use - operator for subtraction
Remember to use printf to display the result
Review the scanf knowledge from Day 4

Tomorrow we will learn about if statements, and the program will become smarter! Keep it up! 💪