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“Series of 100 Essential Notes for C Language Beginners“

Persevere! We are finally entering the practical series, which is also the most important and challenging part of C language.

Keep it up! Young learners!

[~~As a beginner, please forgive any mistakes~~]

54. When Does the Array Name in C Language Degenerate into a Pointer Type

Description

Arrays and pointers are two data types in C language, but they are closely related. One of the key points is that the array name automatically degenerates into a pointer to its first element in most cases.

However, there are two exceptional cases where the array name does not degenerate into a pointer. These distinctions can often be very confusing, and I would like to share them here.

Cases of Degeneration into Pointer

Function Parameter Passing:

void func(int arr[]);  // Equivalent to void func(int* arr)

int main() {
    int a[5];
    func(a);  // a degenerates to int*
    return 0;
}

Assignment and Pointer Arithmetic:

int a[5];
int* p = a;      // degeneration occurs
int* p2 = a + 1; // degeneration occurs

Exceptions Where Degeneration Does Not Occur

1. sizeof Operator

int a[5];
printf("%zu\n", sizeof(a));  // Outputs 20, not the size of a pointer
// Returns the byte size of the entire array

2. & Address-of Operator

int a[5];
int* p1 = a;        // Pointer to the first element
int (*p2)[5] = &a;  // Pointer to the entire array

printf("%p\n", p1);     // Points to a[0]
printf("%p\n", p1 + 1); // Address +4 (advances by one int)
printf("%p\n", p2 + 1); // Address +20 (advances by the entire array)

Specific Application of Code

Getting the Number of Elements in an Array:

int a[10];
int count = sizeof(a) / sizeof(a[0]);  // Correct: gets 10

The following function cannot get the size of the array:

void func(int arr[]) {
    // Error! arr has degenerated to a pointer
    int size = sizeof(arr) / sizeof(arr[0]);
}

// Correct approach
void func(int arr[], int size) {
    // Use the passed size parameter
}

Summary

• The array name usually automatically degenerates into a pointer type
• It only maintains the array type during <span>sizeof(arr)</span> and <span>&arr</span>

Have you understood? Although the syntax of C language is simple, there are still many issues that need to be accumulated and mastered gradually!

Some students contacted me, wanting to have a study exchange group. I hesitated to create one before due to concerns about advertisements, but I think having a group would indeed be convenient, so I will try to create one this time.

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When Does the Array Name in C Language Degenerate into a Pointer Type? Here Are Two Common Pitfalls

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