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.

Practice

1. What is the valid range of indexes for an array declared as int arr[10]; in C?

easy

A. 0 to 9

B. 1 to 10

C. 0 to 10

D. 1 to 9

Solution

1. Step 1: Understand array size declaration

   The array arr is declared with size 10, meaning it has 10 elements.

2. Step 2: Determine valid index range

   In C, array indexing starts at 0, so valid indexes are from 0 up to size-1, which is 9.

3. Final Answer:

   0 to 9 -> Option A

4. Quick Check:

   Array indexes = 0 to size-1 = 0 to 9 [OK]

Hint: Array indexes start at 0 and end at size minus one [OK]

Common Mistakes:

• Assuming array indexes start at 1
• Using size as the last index
• Confusing size with maximum index

2. Which of the following is the correct way to declare an array of 5 integers in C?

easy

A. int arr(5);

B. int arr[5];

C. int arr{5};

D. int arr = 5;

Solution

1. Step 1: Recall C array declaration syntax

   In C, arrays are declared using square brackets with the size inside, like int arr[5];.

2. Step 2: Check each option

   int arr[5]; uses correct syntax. Options A and C use parentheses or braces which are invalid for arrays. int arr = 5; assigns an int, not an array.

3. Final Answer:

   int arr[5]; -> Option B

4. Quick Check:

   Array declaration uses square brackets [OK]

Hint: Use square brackets [] to declare array size in C [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using braces instead of brackets
• Assigning a single value instead of declaring array

3. What will be the output of the following code?

int arr[3] = {10, 20, 30};
printf("%d", arr[3]);

medium

A. Undefined behavior (may print garbage or crash)

B. 0

C. Compilation error

D. 30

Solution

1. Step 1: Analyze array size and indexing

   The array arr has size 3, valid indexes are 0, 1, 2. Accessing arr[3] is out of bounds.

2. Step 2: Understand consequences of out-of-bounds access

   Accessing outside array bounds in C causes undefined behavior. It may print garbage value, cause crash, or unpredictable output.

3. Final Answer:

   Undefined behavior (may print garbage or crash) -> Option A

4. Quick Check:

   Out-of-bounds access = undefined behavior [OK]

Hint: Accessing index >= size causes undefined behavior [OK]

Common Mistakes:

• Assuming arr[3] prints last element
• Expecting zero instead of garbage
• Thinking compiler catches out-of-bounds

4. Identify the error in the following code snippet:

int arr[4];
for(int i = 0; i <= 4; i++) {
    arr[i] = i * 2;
}

medium

A. Initialization of i is incorrect

B. Array size should be 5 instead of 4

C. Loop condition should be i < 4, not i <= 4

D. Multiplication by 2 is invalid

Solution

1. Step 1: Check loop bounds against array size

   The array size is 4, so valid indexes are 0 to 3. The loop runs while i <= 4, which includes i=4, an invalid index.

2. Step 2: Correct the loop condition

   Changing the loop condition to i < 4 ensures indexes 0 to 3 are used, avoiding out-of-bounds access.

3. Final Answer:

   Loop condition should be i < 4, not i <= 4 -> Option C

4. Quick Check:

   Loop index must be less than array size [OK]

Hint: Use < not <= for loop index to avoid out-of-bounds [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Assuming array size includes index equal to size
• Ignoring off-by-one errors

5. Given the array declaration int nums[5] = {1, 2, 3, 4, 5};, which code snippet correctly sums all elements without causing out-of-bounds errors?

hard

A. for(int i = 0; i < 6; i++) sum += nums[i];

B. for(int i = 1; i < 5; i++) sum += nums[i];

C. for(int i = 0; i <= 5; i++) sum += nums[i];

D. for(int i = 0; i < 5; i++) sum += nums[i];

Solution

1. Step 1: Understand array size and valid indexes

   The array nums has size 5, so valid indexes are 0 to 4.

2. Step 2: Evaluate each loop condition

   for(int i = 0; i <= 5; i++) sum += nums[i]; uses i <= 5, which accesses nums[5] (out-of-bounds). for(int i = 1; i < 5; i++) sum += nums[i]; starts at 1, missing nums[0]. for(int i = 0; i < 5; i++) sum += nums[i]; uses i < 5, correctly iterating 0 to 4. for(int i = 0; i < 6; i++) sum += nums[i]; uses i < 6, accessing nums[5] (out-of-bounds).

3. Final Answer:

   for(int i = 0; i < 5; i++) sum += nums[i]; -> Option D

4. Quick Check:

   Loop from 0 to size-1 to avoid errors [OK]

Hint: Loop from 0 to less than array size for safe access [OK]

Common Mistakes:

• Using <= size causing out-of-bounds
• Starting loop at 1 missing first element
• Looping beyond array size