C Arithmetic Operators and Type Casting

Type Conversion & Casts

When you mix different types in an expression, C performs type conversion. Some conversions are automatic, others must be made explicit.

Implicit Conversion (Automatic)

When you mix int and double, the int is automatically promoted to double. This is called implicit conversion.

int a = 7;
double b = 2.0;
double result = a / b;  // a is auto-promoted to double → 7.0 / 2.0 = 3.5

int / int

→ int

7 / 2 = 3 (truncated)

int / double

→ double

7 / 2.0 = 3.5

double / int

→ double

7.0 / 2 = 3.5

Rule: if either side is double, the result is double. Only when both are int do you get integer (truncated) division.

Explicit Cast (Manual)

By writing (target type) before an expression, you can convert it temporarily. This is called a cast.
Even for int / int, casting one side gives you a floating-point result.

int a = 7, b = 2;

// No cast → integer division
printf("%d\n", a / b);           // 3 (truncated)

// With cast → floating-point
printf("%f\n", (double)a / b);   // 3.500000

Cast syntax: (type) expression

int → double

int x = 5;
double d = (double)x;  // 5.0

double → int

double d = 3.7;
int n = (int)d;       // 3 (fractional part truncated)

Computing an average (typical case)

int sum = 17, n = 3;
double avg = (double)sum / n; // 5.666...

Common mistake: (double)(sum / n) first performs the integer division sum / n = 5, then casts 5 to 5.0.
The cast must happen before the division.

Cast Experiment

Enter values and see how the result changes with and without a cast.

a = b =

a / b (int / int)

—

fractional part lost

(double)a / b (with cast)

—

exact result

Common Situations Where a Cast Is Needed

Situation	Code	Why
Integer division, want a float result	`(double)a / b`	Make one side double → floating-point division
Computing an average	`(double)sum / count`	Avoid int / int division
malloc return value	`(int *)malloc(...)`	Convert void* to the appropriate pointer type
Round a float toward zero	`(int)3.7` → `3`	Drop the fractional part
Treat a char as a number	`(int)'A'` → `65`	Get the ASCII code

Check Quiz

int a = 10, b = 3;
printf("A: %d\n", a / b);
printf("B: %f\n", (double)a / b);
printf("C: %f\n", (double)(a / b));

What do A, B, and C print?

A: 3, B: 3.333333, C: 3.333333

A: 3, B: 3.333333, C: 3.000000

A: 3.333333, B: 3.333333, C: 3.000000

A: 3, B: 3.000000, C: 3.333333

Explanation:
A: int / int = int → 10/3 = 3 (truncated)
B: (double)a / b → 10.0 / 3 = 3.333333 (cast before divide → float result)
C: (double)(a / b) → (double)(3) = 3.000000 (integer division happens first → cast position matters!)

Arithmetic Operators

Arithmetic Operators

Operator Calculator

Type Conversion & Casts

Implicit Conversion (Automatic)

Explicit Cast (Manual)

Cast Experiment

Common Situations Where a Cast Is Needed

Check Quiz

What do A, B, and C print?

Comparison & Logical Operators

Try the Operators

Related Lessons

Check Your Understanding

Q1. What is 7 / 2 ? (both operands are int)

Q2. What is 7 % 2 ?

Q3. What is (double)5 / 2 ?

Recommended Books for This Lesson