17_multi_array.md

name: main

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template: main

Two Dimensional Arrays

• int bigray[2][4]; Declares a 2-d array of ints.
  • 32 bytes (2 * 4 * sizeof(int)) are allocated sequentially.

--

• bigray[r] refers to a single row, in dereference notation:
  • *(bigray + r)
  • In pointer arithmetic, r is multiplied by the appropriate size.
  • The size in this case is 4 * sizeof(int) becuase that is the size of each row.

--

• bigray[r][c] refers to an element, in dereference notation:
  • *(bigray + r)[c]
    • Here, r is still multiplied by 4 * sizeof(int)
    • Because [] takes presidence over *, this then becomes
    • **(bigray + r) + c).
    • But c is multiplied by sizeof(int)

Two Dimensional Arrays and Pointers

• Because indexing notation requires knowing the size of a row, pointers and 2-d arrays do not work as seamlessly as pointers and 1-d arrays.

• int **bp = bigray will compile, but will not work as desired. Since the type of bp is int **, the pointer arithmetic will only be based on sizeof(int *), instead of the size of a row.

--

• bp[r][c] becomes **(bp + r + c)
  • but r is scaled by sizeof(int *), instead of 4 * sizeof(int)