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c++arraysmultidimensional-array
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Reputation: 992

c++ multidimensional array initialization

In book i found example

static int categoryTable[ 2 ][ 2 ][ 2 ] = {
    //!b!c  !bc  b!c  bc
      0,    3,   2,   2, //!a
      1,    2,   1,   1  // a
};

category = categoryTable[ a ][ b ][ c ]

There is mistake, right?

Right variant is

static int categoryTable[ 2 ][ 2 ][ 2 ] = {
    //!b!c   !bc    b!c  bc
     {{0,    3},   {2,   2}}, //!a
     {{1,    2},   {1,   1}}  // a
};

Or original is right and I don't understand something?

Upvotes: 3

Views: 405

Answers (2)

Piotr Styczyński
Piotr Styczyński

Reputation: 1426

As Beefster said both ways are correct and will compile.

Multidimensional arrays are just plain single-dimension arrays for the compiler but for programmers they are a nice sugar syntax for complex pointer arithmetics.

Because the multidimensional array is in the reality a single dimension array with syntax improvements then there's no point to disallow initialization with single initializer list.

Expression

a[0][2][3] = x;

is equivalent of *(a+(0*DIM_1+2)*DIM_2+3) = x;

What's not a part of your question but also interesting that because it's just pointer arithmetics you could write:

3[a]

That is equivalent of array subscription:

a[3]

So as a fun fact - you can do similar stuff with multidimensional arrays:

#include <stdio.h>

static int categoryTable[ 2 ][ 2 ][ 2 ] = {
    //!b!c  !bc  b!c  bc
      0,    3,   2,   2, //!a
      1,    2,   1,   1  // a
};

int main() {
  // This two printf's are equivalent
  printf("%d\n", 0[0[categoryTable][1]]);
  printf("%d\n", categoryTable[0][1][0]);
  return 0;
}

This is rather an ugly never-do-it thing, but funny anyway.

So you can think about subscription as some kind of mathematical expression to access single plain array - nothing special really.

Upvotes: 3

Anselmo GPP
Anselmo GPP

Reputation: 451

Both are correct. You can use any one of them.

Upvotes: 3

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