问题
What are the differences between multidimensional arrays double[,] and array-of-arrays double[][] in C#?
If there is a difference, what is the best use for each one?
回答1:
Array of arrays (jagged arrays) are faster than multi-dimensional arrays and can be used more effectively. Multidimensional arrays have nicer syntax.
If you write some simple code using jagged and multidimensional arrays and then inspect the compiled assembly with an IL disassembler you will see that the storage and retrieval from jagged (or single dimensional) arrays are simple IL instructions while the same operations for multidimensional arrays are method invocations which are always slower.
Consider the following methods:
static void SetElementAt(int[][] array, int i, int j, int value)
{
array[i][j] = value;
}
static void SetElementAt(int[,] array, int i, int j, int value)
{
array[i, j] = value;
}
Their IL will be the following:
.method private hidebysig static void SetElementAt(int32[][] 'array',
int32 i,
int32 j,
int32 'value') cil managed
{
// Code size 7 (0x7)
.maxstack 8
IL_0000: ldarg.0
IL_0001: ldarg.1
IL_0002: ldelem.ref
IL_0003: ldarg.2
IL_0004: ldarg.3
IL_0005: stelem.i4
IL_0006: ret
} // end of method Program::SetElementAt
.method private hidebysig static void SetElementAt(int32[0...,0...] 'array',
int32 i,
int32 j,
int32 'value') cil managed
{
// Code size 10 (0xa)
.maxstack 8
IL_0000: ldarg.0
IL_0001: ldarg.1
IL_0002: ldarg.2
IL_0003: ldarg.3
IL_0004: call instance void int32[0...,0...]::Set(int32,
int32,
int32)
IL_0009: ret
} // end of method Program::SetElementAt
When using jagged arrays you can easily perform such operations as row swap and row resize. Maybe in some cases usage of multidimensional arrays will be more safe, but even Microsoft FxCop tells that jagged arrays should be used instead of multidimensional when you use it to analyse your projects.
回答2:
A multidimensional array creates a nice linear memory layout while a jagged array implies several extra levels of indirection.
Looking up the value jagged[3][6] in a jagged array var jagged = new int[10][5] works like this: Look up the element at index 3 (which is an array) and look up the element at index 6 in that array (which is a value). For each dimension in this case, there's an additional look up (this is an expensive memory access pattern).
A multidimensional array is laid out linearly in memory, the actual value is found by multiplying together the indexes. However, given the array var mult = new int[10,30], the Length property of that multidimensional array returns the total number of elements i.e. 10 * 30 = 300.
The Rank property of a jagged array is always 1, but a multidimensional array can have any rank. The GetLength method of any array can be used to get the length of each dimension. For the multidimensional array in this example mult.GetLength(1) returns 30.
Indexing the multidimensional array is faster. e.g. given the multidimensional array in this example mult[1,7] = 30 * 1 + 7 = 37, get the element at that index 37. This is a better memory access pattern because only one memory location is involved, which is the base address of the array.
A multidimensional array therefore allocates a continuous memory block, while a jagged array does not have to be square, e.g. jagged[1].Length does not have to equal jagged[2].Length, which would be true for any multidimensional array.
Performance
Performance wise, multidimensional arrays should be faster. A lot faster, but due to a really bad CLR implementation they are not.
23.084 16.634 15.215 15.489 14.407 13.691 14.695 14.398 14.551 14.252
25.782 27.484 25.711 20.844 19.607 20.349 25.861 26.214 19.677 20.171
5.050 5.085 6.412 5.225 5.100 5.751 6.650 5.222 6.770 5.305
The first row are timings of jagged arrays, the second shows multidimensional arrays and the third, well that's how it should be. The program is shown below, FYI this was tested running mono. (The windows timings are vastly different, mostly due to the CLR implementation variations).
On windows, the timings of the jagged arrays are greatly superior, about the same as my own interpretation of what multidimensional array look up should be like, see 'Single()'. Sadly the windows JIT-compiler is really stupid, and this unfortunately makes these performance discussions difficult, there are too many inconsistencies.
These are the timings I got on windows, same deal here, the first row are jagged arrays, second multidimensional and third my own implementation of multidimensional, note how much slower this is on windows compared to mono.
8.438 2.004 8.439 4.362 4.936 4.533 4.751 4.776 4.635 5.864
7.414 13.196 11.940 11.832 11.675 11.811 11.812 12.964 11.885 11.751
11.355 10.788 10.527 10.541 10.745 10.723 10.651 10.930 10.639 10.595
Source code:
using System;
using System.Diagnostics;
static class ArrayPref
{
const string Format = "{0,7:0.000} ";
static void Main()
{
Jagged();
Multi();
Single();
}
static void Jagged()
{
const int dim = 100;
for(var passes = 0; passes < 10; passes++)
{
var timer = new Stopwatch();
timer.Start();
var jagged = new int[dim][][];
for(var i = 0; i < dim; i++)
{
jagged[i] = new int[dim][];
for(var j = 0; j < dim; j++)
{
jagged[i][j] = new int[dim];
for(var k = 0; k < dim; k++)
{
jagged[i][j][k] = i * j * k;
}
}
}
timer.Stop();
Console.Write(Format,
(double)timer.ElapsedTicks/TimeSpan.TicksPerMillisecond);
}
Console.WriteLine();
}
static void Multi()
{
const int dim = 100;
for(var passes = 0; passes < 10; passes++)
{
var timer = new Stopwatch();
timer.Start();
var multi = new int[dim,dim,dim];
for(var i = 0; i < dim; i++)
{
for(var j = 0; j < dim; j++)
{
for(var k = 0; k < dim; k++)
{
multi[i,j,k] = i * j * k;
}
}
}
timer.Stop();
Console.Write(Format,
(double)timer.ElapsedTicks/TimeSpan.TicksPerMillisecond);
}
Console.WriteLine();
}
static void Single()
{
const int dim = 100;
for(var passes = 0; passes < 10; passes++)
{
var timer = new Stopwatch();
timer.Start();
var single = new int[dim*dim*dim];
for(var i = 0; i < dim; i++)
{
for(var j = 0; j < dim; j++)
{
for(var k = 0; k < dim; k++)
{
single[i*dim*dim+j*dim+k] = i * j * k;
}
}
}
timer.Stop();
Console.Write(Format,
(double)timer.ElapsedTicks/TimeSpan.TicksPerMillisecond);
}
Console.WriteLine();
}
}
回答3:
Simply put multidimensional arrays are similar to a table in DBMS.
Array of Array (jagged array) lets you have each element hold another array of the same type of variable length.
So, if you are sure that the structure of data looks like a table (fixed rows/columns), you can use a multi-dimensional array. Jagged array are fixed elements & each element can hold an array of variable length
E.g. Psuedocode:
int[,] data = new int[2,2];
data[0,0] = 1;
data[0,1] = 2;
data[1,0] = 3;
data[1,1] = 4;
Think of the above as a 2x2 table:
1 | 2 3 | 4
int[][] jagged = new int[3][];
jagged[0] = new int[4] { 1, 2, 3, 4 };
jagged[1] = new int[2] { 11, 12 };
jagged[2] = new int[3] { 21, 22, 23 };
Think of the above as each row having variable number of columns:
1 | 2 | 3 | 4 11 | 12 21 | 22 | 23
回答4:
Preface: This comment is intended to address the answer provided by okutane, but because of SO's silly reputation system, I can not post it where it belongs.
Your assertion that one is slower than the other because of the method calls isn't correct. One is slower than the other because of more complicated bounds-checking algorithms. You can easily verify this by looking, not at the IL, but at the compiled assembly. For example, on my 4.5 install, accessing an element (via pointer in edx) stored in a two-dimensional array pointed to by ecx with indexes stored in eax and edx looks like so:
sub eax,[ecx+10]
cmp eax,[ecx+08]
jae oops //jump to throw out of bounds exception
sub edx,[ecx+14]
cmp edx,[ecx+0C]
jae oops //jump to throw out of bounds exception
imul eax,[ecx+0C]
add eax,edx
lea edx,[ecx+eax*4+18]
Here, you can see that there's no overhead from method calls. The bounds checking is just very convoluted thanks to the possibility of non-zero indexes, which is a functionality not on offer with jagged arrays. If we remove the sub,cmp,and jmps for the non-zero cases, the code pretty much resolves to (x*y_max+y)*sizeof(ptr)+sizeof(array_header). This calculation is about as fast (one multiply could be replaced by a shift, since that's the whole reason we choose bytes to be sized as powers of two bits) as anything else for random access to an element.
Another complication is that there are plenty of cases where a modern compiler will optimize away the nested bounds-checking for element access while iterating over a single-dimension array. The result is code that basically just advances an index pointer over the contiguous memory of the array. Naive iteration over multi-dimensional arrays generally involves an extra layer of nested logic, so a compiler is less likely to optimize the operation. So, even though the bounds-checking overhead of accessing a single element amortizes out to constant runtime with respect to array dimensions and sizes, a simple test-case to measure the difference may take many times longer to execute.
回答5:
I would like to update on this, because in .NET Core multi-dimensional arrays are faster than jagged arrays. I ran the tests from John Leidegren and these are the results on .NET Core 2.0 preview 2. I increased the dimension value to make any possible influences from background apps less visible.
Debug (code optimalization disabled)
Running jagged
187.232 200.585 219.927 227.765 225.334 222.745 224.036 222.396 219.912 222.737
Running multi-dimensional
130.732 151.398 131.763 129.740 129.572 159.948 145.464 131.930 133.117 129.342
Running single-dimensional
91.153 145.657 111.974 96.436 100.015 97.640 94.581 139.658 108.326 92.931
Release (code optimalization enabled)
Running jagged
108.503 95.409 128.187 121.877 119.295 118.201 102.321 116.393 125.499 116.459
Running multi-dimensional
62.292 60.627 60.611 60.883 61.167 60.923 62.083 60.932 61.444 62.974
Running single-dimensional
34.974 33.901 34.088 34.659 34.064 34.735 34.919 34.694 35.006 34.796
I looked into disassemblies and this is what I found
jagged[i][j][k] = i * j * k; needed 34 instructions to execute
multi[i, j, k] = i * j * k; needed 11 instructions to execute
single[i * dim * dim + j * dim + k] = i * j * k; needed 23 instructions to execute
I wasn't able to identify why single-dimensional arrays were still faster than multi-dimensional but my guess is that it has to do with some optimalization made on the CPU
回答6:
Multi-dimension arrays are (n-1)-dimension matrices.
So int[,] square = new int[2,2] is square matrix 2x2, int[,,] cube = new int [3,3,3] is a cube - square matrix 3x3. Proportionality is not required.
Jagged arrays are just array of arrays - an array where each cell contains an array.
So MDA are proportional, JD may be not! Each cell can contains an array of arbitrary length!
回答7:
This might have been mentioned in the above answers but not explicitly: with jagged array you can use array[row] to refer a whole row of data, but this is not allowed for multi-d arrays.
回答8:
I am parsing .il files generated by ildasm to build a database of assemnblies, classes, methods, and stored procedures for use doing a conversion. I came across the following, which broke my parsing.
.method private hidebysig instance uint32[0...,0...]
GenerateWorkingKey(uint8[] key,
bool forEncryption) cil managed
The book Expert .NET 2.0 IL Assembler, by Serge Lidin, Apress, published 2006, Chapter 8, Primitive Types and Signatures, pp. 149-150 explains.
<type>[] is termed a Vector of <type>,
<type>[<bounds> [<bounds>**] ] is termed an array of <type>
** means may be repeated, [ ] means optional.
Examples: Let <type> = int32.
1) int32[...,...] is a two-dimensional array of undefined lower bounds and sizes
2) int32[2...5] is a one-dimensional array of lower bound 2 and size 4.
3) int32[0...,0...] is a two-dimensional array of lower bounds 0 and undefined size.
Tom
回答9:
In addition to the other answers, note that a multidimensional array is allocated as one big chunky object on the heap. This has some implications:
- Some multidimensional arrays will get allocated on the Large Object Heap (LOH) where their equivalent jagged array counterparts would otherwise not have.
- The GC will need to find a single contiguous free block of memory to allocate a multidimensional array, whereas a jagged array might be able to fill in gaps caused by heap fragmentation... this isn't usually an issue in .NET because of compaction, but the LOH doesn't get compacted by default (you have to ask for it, and you have to ask every time you want it).
- You'll want to look into <gcAllowVeryLargeObjects> for multidimensional arrays way before the issue will ever come up if you only ever use jagged arrays.
来源：https://stackoverflow.com/questions/597720/what-are-the-differences-between-a-multidimensional-array-and-an-array-of-arrays