Array size and bounds in C - Step-by-Step Execution

Concept Flow - Array size and bounds

Declare array with fixed size

↓

Access elements using index

↓

Check if index < size?

No→Error: Out of bounds

Yes↓

Read or write element

↓

Repeat for other indices or exit

This flow shows how arrays have a fixed size and how accessing elements must stay within valid index bounds to avoid errors.

Execution Sample

C

int arr[3] = {10, 20, 30};
int x = arr[1];
int y = arr[3];

This code declares an array of size 3, accesses a valid index 1, and then tries to access an invalid index 3.

Execution Table

Step	Action	Index Accessed	Index Valid?	Result	Notes
1	Declare array arr with size 3	-	-	arr = {10, 20, 30}	Array created with indices 0,1,2
2	Access arr[1]	1	Yes	Value = 20	Valid access within bounds
3	Access arr[3]	3	No	Undefined behavior	Index 3 is out of bounds (max index 2)

💡 Execution stops or causes error when accessing index 3 which is outside array bounds

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
arr	undefined	{10, 20, 30}	{10, 20, 30}	{10, 20, 30}
x	undefined	undefined	20	20
y	undefined	undefined	undefined	undefined or garbage

Key Moments - 2 Insights

Why is accessing arr[3] a problem?

Because the array size is 3, valid indices are 0,1,2. Index 3 is outside bounds, causing undefined behavior as shown in step 3 of the execution table.

Does declaring arr[3] mean indices 0 to 3 are valid?

No, arr[3] means size is 3, so valid indices are 0,1,2 only. Index 3 is one past the last element, which is invalid.

Visual Quiz - 3 Questions

Look at the execution table, what value is stored in x after step 2?

A 10

B 20

C 30

D Undefined

💡 Hint

Check the 'Result' column for step 2 in the execution table.

At which step does the program access an invalid index?

A Step 1

B Step 2

C Step 3

D No invalid access

💡 Hint

Look at the 'Index Valid?' column in the execution table.

If the array size was changed to 4, which index would be invalid?

A 4

B 3

C 2

D 1

💡 Hint

Array indices go from 0 to size-1, so index equal to size is invalid.

Concept Snapshot

Declare arrays with fixed size: int arr[3];
Valid indices: 0 to size-1
Accessing outside bounds causes undefined behavior
Always check index < size before access
No automatic bounds checking in C

Full Transcript

In C, arrays have a fixed size declared at creation. For example, int arr[3] creates an array with indices 0, 1, and 2. Accessing arr[1] is valid and returns the stored value. However, accessing arr[3] is invalid because it is outside the declared size. This causes undefined behavior, which can lead to errors or crashes. The execution table shows each step: declaring the array, accessing a valid index, and then an invalid index. Variables track the array contents and accessed values. Beginners often confuse the size with the maximum index; remember size 3 means indices 0 to 2 only. Always ensure your index is less than the array size before accessing elements.