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“Essential Knowledge Points for C Language Beginners: 100 Articles Series“

17. Arithmetic Operators: Addition, Subtraction, Multiplication, Division, and Modulus are Simple, but There are 2 Common Pitfalls for Beginners to Avoid

1. Basic Arithmetic Operators

The C language provides 5 basic arithmetic operators:

Operator	Name	Example	Mathematical Equivalent
+	Addition	a + b	a + b
–	Subtraction	a – b	a – b
*	Multiplication	a * b	a × b
/	Division	a / b	a ÷ b
%	Modulus	a % b	a mod b

2. Examples of Operator Usage

1. Basic Operations

int a = 10, b = 3;
printf("%d + %d = %d\n", a, b, a + b);  // 13
printf("%d - %d = %d\n", a, b, a - b);  // 7
printf("%d * %d = %d\n", a, b, a * b);  // 30
printf("%d / %d = %d\n", a, b, a / b);  // 3
printf("%d %% %d = %d\n", a, b, a % b); // 1

2. Mixed Type Operations

int x = 5;
double y = 2.0;
printf("%d / %.1f = %.1f\n", x, y, x / y);  // 2.5 (automatic type conversion, unified to floating-point calculation)

3. Common Issues with Integer Division

1. Truncation

int result = 5 / 2;  // Result is 2, not 2.5

2. Forced Type Conversion for Correct Results

double quotient = (double)5 / 2;  // 2.5 (forced type conversion)

4. Rules of Modulus Operation

1. Basic Properties

printf("%d %% %d = %d\n", 10, 3, 10 % 3);   // 1
printf("%d %% %d = %d\n", -10, 3, -10 % 3);  // -1 (sign same as dividend)

2. Common Uses

1. Check for Even or Odd:
```
if (num % 2 == 0) { /* Even number */ }
```

2. Loop Cycle:

int pos = i % array_size;  // Ensure index is within array bounds

5. Operator Precedence

The precedence of arithmetic operators from high to low:

1. Parentheses ()
2. Unary Plus and Minus +a, -a
3. Multiplication, Division, Modulus *, /, %
4. Addition, Subtraction +, -

Example:

int result = 5 + 3 * 2;   // 11 (not 16)
int correct = (5 + 3) * 2; // 16

6. Compound Assignment Operators

Operator	Equivalent Expression
a += b	a = a + b
a -= b	a = a – b
a *= b	a = a * b
a /= b	a = a / b
a %= b	a = a % b

Example:

int x = 10;
x += 5;  // x = 15
x /= 3;  // x = 5

7. Common Errors

1. Integer Overflow

int big = 2000000000;
printf("%d\n", big * 2);  // Overflow!

Correct approach: Use a larger type (long long)

long long big = 2000000000LL;
printf("%lld\n", big * 2);

2. Division by Zero

int a = 5, b = 0;
printf("%d\n", a / b);  // Runtime error!

Correct approach: Check if the divisor is zero.

if (b != 0) {
    printf("%d\n", a / b);
} else {
    printf("Divisor cannot be zero\n");
}

8. Some Calculation Tricks

1. Swap Variable Values (without Temporary Variable)

a = a + b;
b = a - b;
a = a - b;

2. Check for Divisibility

if (a % b == 0) { /* a is divisible by b */ }

3. Get Each Digit of an Integer

int num = 123;
int units = num % 10;      // 3
int tens = (num / 10) % 10; // 2
int hundreds = num / 100;   // 1

9. Floating Point Precision Issues

1. Example of Precision Loss

float f = 0.1f;
printf("%.20f\n", f);  // 0.10000000149011611938

2. Comparing Floating Point Numbers

#include <math.h>
if (fabs(a - b) < 1e-6) { /* Considered equal */ }

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Essential Knowledge Points for C Language Beginners: Arithmetic Operators

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