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“C Language Beginner’s Essential Knowledge Points Note Series of 100 Articles”“

The following notes finally enter the practical series, which is also the most important and difficult part of C language. Keep it up! Young learners!

37. The Most Confusing Question for C Language Beginners: Understanding the Relationship Between Pointers and Arrays, Finally Understood!

1. The Relationship Between Pointers and Arrays

Pointers and arrays are inseparable in C language; the array name is usually converted to a pointer pointing to its first element:

int arr[5] = {10, 20, 30, 40, 50};
int *ptr = arr;  // Equivalent to int *ptr = &arr[0]

Key Features

• arr[i] is equivalent to *(arr + i)
• &arr[i] is equivalent to arr + i
• The array name is not an lvalue and cannot be assigned or modified, for example, arr = &addr is incorrect

2. Pointer Arithmetic Rules

1. Basic Operation Types

Operation	Effect	Example
ptr + n	Advance n elements (not bytes)	ptr + 2
ptr – n	Retreat n elements	ptr – 1
ptr++	Point to the next element	while(*ptr++)
ptr–	Point to the previous element	while(*ptr–)
ptr1 – ptr2	Calculate the number of elements between two pointers	int diff = ptr1 – ptr2

It is the difference in elements, not bytes; one element may occupy multiple bytes, pointer difference = element difference * element size.

2. Type Determines Step Size

char *cp;   // cp+1 advances 1 byte
int *ip;    // ip+1 advances sizeof(int) bytes
double *dp; // dp+1 advances sizeof(double) bytes

3. Pointer Access to Arrays

1. Accessing One-Dimensional Arrays

int arr[5] = {1,2,3,4,5};
for (int *p = arr; p &lt; arr + 5; p++) {
    printf("%d ", *p);
}

2. Accessing Two-Dimensional Arrays

int matrix[3][4] = {...};
// Using a single pointer to access a two-dimensional array
int *p = &matrix[0][0];
for (int i = 0; i &lt; 3*4; i++) {
    printf("%d ", *(p + i));
}

4. Differences Between Pointers and Arrays

Feature	Pointer	Array
Memory Usage	Fixed size (usually 4/8 bytes)	Number of elements × size of elements
Stored Content	Memory address	Actual data
sizeof	Returns pointer size	Returns total byte size of the array
Modifiability	Can be reassigned	Cannot be reassigned
As Function Parameters	Pass pointer value	Degenerates to pointer

5. Typical Applications of Pointer Arithmetic

1. Adding an Integer to a Pointer

int arr[5] = {0};
int *p = arr;
p = p + 2;  // Equivalent to p = &arr[2]

2. Subtracting Pointers

int *p1 = &arr[1];
int *p2 = &arr[4];
int n = p2 - p1;  // n = 3 (difference in number of elements)

3. Comparing Pointers

if (p1 &lt; p2) {  // Compare memory addresses
    // The element pointed to by p1 is before p2
}

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Essential Knowledge Points for C Language Beginners: Understanding the Relationship Between Pointers and Arrays

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