Created: 2023-01-15 13:09
Status: #concept
Subject: Programming
Tags: C C Array One-Dimensional Array Pointer Arithmetic

Multidimensional Array

Description

They are Arrays that are stored in Row-Major Order, with rows denoting each array and columns for the respective array indices.

Syntax

We have to specify the number of [rows] then [columns].

we can access them like single-dimension arrays, but we need to specify the array[row][col].

int m[5][9]; initializes a 2d multidimensional array with 5 rows and 9 columns.

Multidimensional Array Row & Columns.png

Row Major Order

Multidimensional arrays are stored internally as normal 1d arrays positioned beside each other.

Multidimensional Array Illustration.png

How do we replace nested for loops with Pointers in Multidimensional Arrays?

for(int *p = *a; p < *a + ROWS * COLS; p++)

This works since adding a number i to a multidimensional array pointer a will add i * NUM_ROWS worth of elements to the array's Memory Address.

Example

int arr[2][3], *p = a;

*p points to an an object of size int (*)[3], “an integer array with 3 elements each”. this makes the pointer shift sizeof(int) * 3 elements everytime we do p++.

Multidimensional String Arrays

Remember, when declaring 2d arrays, we must always specify the COLS (the length of each string), but not necessarily ROWS.

char planets[][8] = { "Mercury", "Venus", "Earth", "Mars", "Juputer", "Saturn", "Uranus", "Neptune", "Pluto"};

Multidimensional String Array.png

there is a lot of wasted space due to declaring the length of each string as [8].

Ragged Arrays

A ragged array is an array of Pointer Variables char *str[] whose elements are pointers to Constant strings *str.

they are the data type of the int main(int argc, char *argv[]) to accept CLI arguments.

int main(void)
{
  char *planets[] = {"Mercury", "Venus", "Earth"};

  printf("%s", planets[1]); // outputs Mercury
  printf("%s", *(planets+2)); // outputs Earth 

  printf("%c", planets[0][0]); // outputs M
  printf("%c", *(planets+2)[0]); // outputs E
}

Ragged Arrays Example.png

References

C Programming, A Modern Approach, 2nd Edition, Chapter 12.4