Arrays (1D and Multidimensional)
A data structure that stores elements of the same type in contiguous memory. Each element is accessed by its index (subscript). In addition to one-dimensional arrays, multidimensional arrays are available for working with tabular data.
Syntax
// Declares a one-dimensional array.
type arrayName[size];
// Declares and initializes an array at the same time.
type arrayName[size] = {value1, value2, value3};
// Omits the size and lets the compiler determine it from the initializer.
type arrayName[] = {value1, value2, value3};
// Accesses an element (indexes are zero-based).
arrayName[index];
// Declares and initializes a two-dimensional array.
type arrayName[rows][cols] = {{value, value}, {value, value}};
Array Characteristics
| Characteristic | Description |
|---|---|
| Zero-based indexing | The first element has index 0; the last element has index (size - 1). |
| Contiguous memory | Array elements are laid out sequentially in memory, so they can be accessed via pointer arithmetic. |
| Fixed size | The size is fixed at declaration and cannot change at runtime. Use dynamic memory allocation when a variable size is needed. |
| Array name is an address | The array name represents the address of the first element. When passed to a function, it decays to a pointer. |
| Strings are char arrays | A string is represented as an array of char, with a null character ('\0') placed at the end. |
Sample Code
#include <stdio.h>
int main(void) {
// Declares and initializes a one-dimensional array.
int scores[5] = {80, 75, 90, 65, 88};
// Accesses individual elements by index.
printf("First element: %d\n", scores[0]); // Outputs '80'.
printf("Last element: %d\n", scores[4]); // Outputs '88'.
// Iterates over all elements with a for loop.
int sum = 0;
for (int i = 0; i < 5; i++) {
sum += scores[i];
}
printf("Sum: %d\n", sum); // Outputs 'Sum: 398'.
printf("Average: %.1f\n", (double)sum / 5); // Outputs 'Average: 79.6'.
// Uses sizeof to get the number of elements.
int count = sizeof(scores) / sizeof(scores[0]);
printf("Count: %d\n", count); // Outputs 'Count: 5'.
// Represents a string as a char array.
char name[] = "Taro";
printf("Name: %s\n", name); // Outputs 'Name: Taro'.
printf("Length: %zu\n", sizeof(name) - 1); // Length excluding the null character.
// Declares and initializes a two-dimensional array.
int matrix[2][3] = {
{1, 2, 3},
{4, 5, 6}
};
// Displays all elements using nested loops.
for (int i = 0; i < 2; i++) {
for (int j = 0; j < 3; j++) {
printf("%d ", matrix[i][j]);
}
printf("\n");
}
// Output: 1 2 3 / 4 5 6
return 0;
}
Notes
C arrays do not perform bounds checking. Accessing an array out of bounds causes undefined behavior — the program may crash or corrupt data in another memory region. Always ensure indexes stay within the valid range.
Arrays and pointers are closely related. scores[i] is equivalent to *(scores + i). For details on how arrays are handled when passed to functions, see Pointers and Arrays.
To determine the size at runtime, use dynamic memory allocation (malloc()). Variable-length arrays (VLAs) are also available since C99, but avoid them for large sizes due to the risk of stack overflow.
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