2D Array and Structures in C: Memory Layout, Pointers, and Data Alignment, Schemes and Mind Maps of Computer Programming

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2D Array, Struct, Malloc

Shuai Mu

based on slides from Tiger Wang and

Jinyang Li

2D Arrray 2D arrays are stored contiguously in memory in row-major format

Multi-dimensional arrays int arr[n 1 ][n 2 ][n 3 ]…[n k- 1 ][n k ] Declare a k dimensional array

n

i is the length of the ith dimension Example: 2D array int matrix[2][3]

Multi-dimensional arrays int arr[n 1 ][n 2 ][n 3 ]…[n k- 1 ][n k ] Declare a k dimensional array

n

i is the length of the ith dimension Example: 2D array int matrix[2][3] Row 0 Row 1 Col (^0) Col 1 Col 2

Multi-dimensional arrays int arr[n 1 ][n 2 ][n 3 ]…[n k- 1 ][n k ] Declare a k dimensional array

n

i is the length of the ith dimension Example: 2D array Access an element at second row and third column matrix[1][2] = 10 int matrix[2][3] = {{ 1 , 2 , 3}, {4, 5, 6 }};

Memory layout int matrix[2][3] = {{1, 2 , 3}, { 4 , 5, 6}}; for (int i = 0; i < 2; i++) { for (int j = 0; j < 3; j++) { printf(“%p\n”,&matrix[i][j]); } }

Memory layout 6 (^1) 0x 0x 0x 0x10c 0x 0x 5 4 3 2 1 st^ row

0x matrix[0][0] matrix[0][1] matrix[0][2] matrix[1][0] matrix[1][1] matrix[1][2]

Memory layout 6 (^1) 0x 0x 0x 0x10c 0x 0x 5 4 3 2 1 st^ row 2 nd row … (^) ... 0x matrix[0][0] matrix[0][1] matrix[0][2] matrix[1][0] matrix[1][1] matrix[1][2]

Pointers 6 (^1) 0x 0x 0x 0x10c 0x 0x 5 4 3 2 1 st^ row 2 nd row … (^) ... 0x400 matrix: 0x matrix[0]: 0x matrix[1]: 0x10c matrix[0][0] matrix, matrix[0] matrix[0][1] matrix[0][2] matrix[1][0] matrix[1][1] matrix[1][2] matrix[1]

Pointers 6 (^1) 0x 0x 0x 0x10c 0x 0x 5 4 3 2 1 st^ row 2 nd row … (^) ... 0x matrix[1] matrix[0][0] matrix[0][1] matrix[0][2] matrix[1][0] matrix[1][1] matrix[1][2] matrix, matrix[0] How many ways to define a pointer which points to the head of the array?

Pointers 6 (^1) 0x 0x 0x 0x10c 0x 0x 5 4 3 2 1 st^ row 2 nd row … (^) ... 0x matrix[1] matrix[0][0] matrix[0][1] matrix[0][2] matrix[1][0] matrix[1][1] matrix[1][2] matrix, matrix[0] int *p = &matrix[0][0]; int *p = matrix[0]; int *p = (int *)matrix; How to access matrix[1][0] with p?

Pointers 6 (^1) 0x 0x 0x 0x10c 0x 0x 5 4 3 2 1 st^ row 2 nd row … (^) ... 0x matrix[1] matrix[0][0] matrix[0][1] matrix[0][2] matrix[1][0] matrix[1][1] matrix[1][2] matrix[0] int *p = &matrix[0][0]; int *p = matrix[0]; int *p = (int *)matrix; matrix[1][0]: *(p + 3) p[3]

A general question Given a 2D array matrix[m][n] and a pointer p which points to matrix[0][0], how to use p to access matrix[i][j]? address of matrix[i][j]: p + i * n + j

Accessing 2D array using pointer int *p = matrix[0]; // or int *p = (int *)matrix; for (int i = 0; i < 2 *3; i++) { printf(“%d\n”, p[i]); } int matrix[2][3] = {{1, 2 , 3}, { 4 , 5, 6}}; OR for (int i = 0; i < 2; i++) { for (int j = 0; j < 3; j++) { printf(“%d\n”, matrix[ 2 ][3]); } }