What is the difference between ++i and i++ in C?

The difference between ++i (prefix increment) and i++ (postfix increment) in C lies in when the increment is applied and the value returned by the expression. Here’s a breakdown:

1. Prefix Increment (++i)

• Increment First: The value of i is increased by 1 immediately.
• Return Value: The updated value (after increment) is returned. Example:

   int i = 5;
   int a = ++i; // i becomes 6, then a = 6

2. Postfix Increment (i++)

• Return First: The original value of i is returned.
• Increment After: i is incremented by 1 afterward. Example:

   int i = 5;
   int b = i++; // b = 5 (original value), then i becomes 6

Key Differences

Aspect	++i (Prefix)	i++ (Postfix)
Increment Timing	Increments i first.	Increments i after returning its value.
Returned Value	New value (i + 1).	Original value (i before increment).

Practical Examples

Example 1: Basic Usage

int i = 3;
printf("%d\n", ++i); // Output: 4 (i is now 4)
printf("%d\n", i++); // Output: 4 (i becomes 5 afterward)

Example 2: In Expressions

int i = 5;
int j = ++i + 2; // i = 6 → j = 6 + 2 = 8
int k = i++ + 2; // k = 6 + 2 = 8 → i = 7

Example 3: Array Indexing

int arr[] = {10, 20, 30};
int idx = 0;
printf("%d\n", arr[++idx]); // Output: 20 (idx = 1)
printf("%d\n", arr[idx++]); // Output: 20 (idx becomes 2 after)

Performance Consideration

• For primitive types (e.g., int), modern compilers optimize both to the same machine code.
• For complex types (e.g., structs/objects in C++), ++i is generally more efficient (avoids copying old values).

When to Use Which

• Use ++i when you need the updated value immediately (e.g., in loops).

  for (int i = 0; i < 10; ++i) { ... } // Efficient and clear intent

• Use i++ when you need the original value before incrementing.

  int current = i++; // Use current value, then increment

Undefined Behavior Warning

Avoid using ++i and i++ on the same variable in a single expression without sequence points, as it leads to undefined behavior:

int i = 1;
int j = i++ + ++i; // Undefined! Order of operations is ambiguous.

Summary

• ++i: Increment first, return new value.
• i++: Return original value, then increment.
• Choose based on whether you need the value before or after the increment.