Size

Tags: #◇CCPROG2 #◇C Programming #◇Programming #◇Computer Science #◇Data Structures #◇Data Types #◇Structured Programming #◇Memory Management

◇Overview

The size of an array is the number of elements it can hold, determined at declaration and fixed for the array’s lifetime.

◇Key Concepts

Array size must be a positive integer constant (before C99) C99+ allows Variable-Length Arrays (VLAs) with runtime size sizeof(arr) / sizeof(arr[0]) gives the element count There is no built-in bounds checking

◇Details / Theory

Compile-Time Size

『』

#define N 100
int arr[N];       // Size = 100 (constant expression)
int arr2[5];      // Size = 5 (literal)

Computing Size at Runtime

『』

int arr[10];
int count = sizeof(arr) / sizeof(arr[0]);  // count = 10

This only works when arr is a true array, not a pointer.

Variable-Length Arrays (VLAs — C99)

『』

int n;
scanf("%d", &n);
int arr[n];  // Size determined at runtime

VLAs are allocated on the stack and cannot be initialized with {...}.

Size and Memory

Total memory = size × sizeof(element_type):

Declaration	Elements	Total Bytes (typical)
`int arr[10]`	10	40
`char str[20]`	20	20
`double vals[5]`	5	40

Size in Functions

When an array is passed to a function, it decays to a pointer, so sizeof no longer works. Always pass the size as a separate parameter:

『』

void printArray(int arr[], int size) {
    for (int i = 0; i < size; i++)
        printf("%d ", arr[i]);
}

◇Common Pitfalls

Using sizeof(arr) on a function parameter — gives pointer size, not array size. Off-by-one: accessing arr[size] when valid indices are 0 to size - 1. Forgetting that VLAs cannot be initialized with {...}.

◇Summary

TL;DR — Array size is the number of elements, fixed at declaration. Use sizeof(arr)/sizeof(arr[0]) for element count. Always pass size separately to functions since arrays decay to pointers.

◇Links

Related: ◇Declaration — ◇Element Type — ◇Accessing Elements — ◇Name — ◇Arrays and Functions Source / Reference:

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Related Notes: ◇Characteristics & Attributes, ◇Memory Allocation

SIZE