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

Concept Flow - Array size and bounds

Declare array with fixed size

↓

Access elements by index

↓

Check if index < size?

No→Error: Out of bounds

Yes↓

Read or write element

↓

Repeat or end

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 element at index 1, then tries to access an invalid element at index 3.

Execution Table

Step	Action	Index Accessed	Index Valid?	Result	Notes
1	Declare array	-	-	arr = {10, 20, 30}	Array created with size 3
2	Access arr[1]	1	Yes	Value = 20	Index 1 is within bounds (0 to 2)
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 because it 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 (invalid access)

Key Moments - 2 Insights

Why is accessing arr[3] a problem?

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

Does declaring an array set all elements automatically?

No, only the elements you initialize are set. In this example, all three elements are initialized. Uninitialized elements can contain garbage values.

Visual Quiz - 3 Questions

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

A 10

B 30

C 20

D Undefined

💡 Hint

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

At which step does the program try to access an invalid index?

A Step 2

B Step 3

C Step 1

D No invalid access

💡 Hint

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

If the array size was changed to 4, what would happen at step 3?

A Access is valid and returns a value

B Access is still invalid

C Compilation error

D Array size does not affect access

💡 Hint

Refer to the 'Index Valid?' logic and array size in the concept flow.

Concept Snapshot

Array size is fixed at declaration.
Valid indices are from 0 to size-1.
Accessing outside this range causes undefined behavior.
Always check index before accessing array elements.
Example: int arr[3]; valid indices: 0,1,2.

Full Transcript

This lesson shows how arrays in C++ have a fixed size and how accessing elements must be done within valid index bounds. The example declares an array of size 3 with elements 10, 20, and 30. Accessing arr[1] is valid and returns 20. Accessing arr[3] is invalid because the maximum index is 2, leading to undefined behavior. The execution table traces these steps clearly. Remember, always keep array accesses within bounds to avoid errors.