Pointers and arrays in C - Step-by-Step Execution

Concept Flow - Pointers and arrays

Declare array

→Array memory allocated

↓

Pointer declared

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Pointer assigned to array address

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Access elements via pointer

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Modify elements via pointer

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Pointer arithmetic to move through array

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End of access

This flow shows how an array is declared and how a pointer can be assigned to its address to access and modify elements using pointer arithmetic.

Execution Sample

C

int arr[3] = {10, 20, 30};
int *p = arr;
int val = *(p + 1);

This code declares an array, assigns a pointer to its start, and accesses the second element using pointer arithmetic.

Execution Table

Step	Action	Pointer p	Expression	Value Accessed	Array State
1	Declare array arr with values {10,20,30}	N/A	N/A	N/A	[10, 20, 30]
2	Declare pointer p	Uninitialized	N/A	N/A	[10, 20, 30]
3	Assign p = arr (address of arr[0])	Address of arr[0]	N/A	N/A	[10, 20, 30]
4	Access *(p + 1)	Address of arr[0]	*(p + 1)	20	[10, 20, 30]
5	Modify *(p + 2) = 40	Address of arr[0]	*(p + 2) = 40	40	[10, 20, 40]
6	Increment pointer p++	Address of arr[1]	p++	N/A	[10, 20, 40]
7	Access *p	Address of arr[1]	*p	20	[10, 20, 40]
8	End of access	Address of arr[1]	N/A	N/A	[10, 20, 40]

💡 Pointer arithmetic and array access complete; program ends.

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 6	Final
arr	[10, 20, 30]	[10, 20, 30]	[10, 20, 40]	[10, 20, 40]	[10, 20, 40]
p	Uninitialized	Address of arr[0]	Address of arr[0]	Address of arr[1]	Address of arr[1]
val	Undefined	Undefined	20	20	20

Key Moments - 3 Insights

Why does *(p + 1) access the second element of the array?

Because p points to the first element, adding 1 moves the pointer to the next element in memory, so *(p + 1) dereferences the second element. See execution_table row 4.

What happens when we do p++ on the pointer?

The pointer moves to the next element in the array (address of arr[1]). It does not change the array contents. See execution_table row 6.

How does modifying *(p + 2) affect the array?

It changes the value at the third element of the array because (p + 2) points to arr[2]. The array updates accordingly. See execution_table row 5.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what value does *(p + 1) access?

A 10

B 20

C 30

D 40

💡 Hint

Check the 'Value Accessed' column at step 4 in execution_table.

At which step does the array value change from 30 to 40?

A Step 6

B Step 4

C Step 5

D Step 7

💡 Hint

Look at the 'Array State' column in execution_table to see when the last element changes.

After p++, what element does *p point to?

A arr[1]

B arr[0]

C arr[2]

D Undefined

💡 Hint

Check pointer p value in variable_tracker after step 6.

Concept Snapshot

Pointers and arrays:
- Array name is address of first element
- Pointer can point to array start
- *(p + i) accesses i-th element
- Pointer arithmetic moves by element size
- Modifying *p changes array content
- p++ moves pointer to next element

Full Transcript

This lesson shows how arrays and pointers work together in C. We start by declaring an array with three integers. Then we declare a pointer and assign it to the array's address, which points to the first element. Using pointer arithmetic, we access the second element with *(p + 1), which gives us 20. We also modify the third element by setting *(p + 2) to 40, changing the array's last value. Incrementing the pointer with p++ moves it to the second element. Accessing *p now gives 20. This step-by-step trace helps understand how pointers and arrays relate in memory.