Newbie question: Arrays

  • Thread starter Thread starter Blue Streak
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Blue Streak

What exactly is the difference (besides syntax) when you declare multi-
dimentional arrays this way?

int [] [] a = new int[3][3];

int [,] b = new int[3,3];

// What's the difference between a and b?

TIA...
 
Blue Streak said:
What exactly is the difference (besides syntax) when you declare multi-
dimentional arrays this way?

int [] [] a = new int[3][3];

int [,] b = new int[3,3];

// What's the difference between a and b?
Click to expand...

a is a single dimension array of single dimension arrays of ints.

b is a dual dimension array of ints.
 
Blue Streak,
int [] [] a = new int[3][3];
Click to expand...

Once initialized you will have four arrays. That consumes more
memory, and a[1][2] requires two array lookups.
int [,] b = new int[3,3];
Click to expand...

Creates one multidimensional array (a feature of the CLR). Consumes
less memory and a[1,2] requires one lookup.

Oddly enough int[][] is ~10% faster even with the extra lookup. This
is because the Microsoft has optimized the shit out of single
dimension, 0 indexed arrays. If you are planning to iterate over your
arrays I would use int[][], but if you need many dimensions with
infrequent access int[,] will save on memory.

-James
 
Blue said:
What exactly is the difference (besides syntax) when you declare multi-
dimentional arrays this way?

int [] [] a = new int[3][3];

int [,] b = new int[3,3];

// What's the difference between a and b?
Click to expand...

To expand on what Anthony wrote...

In the former case, the primary index returns individual arrays, and so
allows for "jagged" arrays. That is, each sub-array can be of different
size (if you allocate it that way...your example doesn't).

In the latter case, you have a traditional multi-dimensional array,
where each row of the array has the same number of columns.

In both cases, you aren't actually limited to 2 dimensions, so the
behaviors extend as you add dimensions.

Pete
 
Blue said:
What exactly is the difference (besides syntax) when you declare multi-
dimentional arrays this way?

int [] [] a = new int[3][3];

int [,] b = new int[3,3];

// What's the difference between a and b?

TIA...
Click to expand...

The main difference is that a is an array of arrays (a jagged array),
while b is a two dimensional array.

Another difference is the the code for b works, but the code for a doesn't.

To create an array of arrays, you have to create the arrays in the array
separately:

int[][] a = new int[3][];
a[0] = new int[3];
a[1] = new int[3];
a[2] = new int[3];
 
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