问题描述
Matlab 中的单元格(即带有 { })和矩阵(即带有 [ ])有什么区别?
What is the difference between cell (i.e. with { }) and matrix (i.e. with [ ]) in Matlab?
推荐答案
MATLAB 中的元胞数组和矩阵有几个不同点:
There are several differences between a cell array and a matrix in MATLAB:
一个元胞数组可以在每个元胞中包含任意类型的元素；而矩阵要求其元素的类型是齐次的，即类型相同.
A cell array may contain any arbitrary type of element in each cell; while a matrix requires the types of its elements to be homogeneous i.e. of the same type.
就内存布局而言，矩阵的所有元素都在内存中连续布局，而元胞数组包含指向数组每个元素的指针.在考虑高性能代码的缓存位置等事项时，这一点很重要.
As far as memory layout goes, all elements of a matrix are laid out contiguously in memory, while a cell array contains pointers to each element of the array. This can be important when considering things like cache locality for high performance code.
第 2 点的另一面是，当您调整矩阵的大小时，矩阵中的每个元素都必须复制到新分配的内存区域，但在元胞数组的情况下，只需要复制一个指针列表超过.根据您存储的元素的大小和类型，这可能意味着元胞数组调整大小的速度要快得多.
The flip side of point 2 is that when you resize a matrix every element in the matrix must be copied over to the newly allocated memory area, but in case of a cell array only a list of pointers needs to copied over. Depending on the size and type of elements you're storing, this might mean cell arrays are much faster to resize.
为了说明内存布局的差异，让我们考虑一个简单的例子:
To illustrate the differences in memory layout, let's consider a simple example:
A = [10 20 30 40];
这里 MATLAB 创建一个名为 A 的新矩阵变量，分配足够的内存来保存 4 个双精度(32 个字节，假设 8 个双精度)并将此内存分配给一个指针，该指针指向实数A.(如果你创建一个复数矩阵，内存也会分配给虚部，并有一个单独的指针指向这个内存区域).
Here MATLAB creates a new matrix variable named A, allocates enough memory to hold 4 doubles (32 bytes, assuming 8 byte doubles) and assigns this memory to a pointer that points to the real part of A. (If you create a matrix of complex numbers, memory is allocated for the imaginary part also, and a separate pointer points to this memory area).
现在让我们创建一个包含这些元素的元胞数组:
Now let's create a cell array that holds these elements:
B = cell(1, 4);
B{1,1} = 10;
B{1,2} = 20;
B{1,3} = 30;
B{1,4} = 40;
当 MATLAB 执行第一条语句时，它会创建一个包含 4 个指针的元胞数组，每个指针都可以指向任意类型.所以 B 已经使用了 16 个字节(假设是 32 位指针).下一行创建一个包含值 10 的 1x1 矩阵，并将其分配给第一个元胞数组元素.这里的过程类似于我上面描述的创建 1x4 矩阵的过程，不同之处在于分配的内存仅足以容纳一个双精度(8 个字节).对其余 3 个语句中的每一个都重复此操作.因此，至少，第二个示例使用 16 + 8 x 4 = 48 字节.
When MATLAB executes the first statement, it creates a cell array that contains 4 pointers, each of which can point to an arbitrary type. So B is already using 16 bytes (assuming 32-bit pointers).
The next line creates a 1x1 matrix containing the value 10 and assigns it to the first cell array element. The process here is similar to the one I described above for creation of a 1x4 matrix, except that the memory allocated is only large enough to hold one double (8 bytes). This is repeated for each of the remaining 3 statements. So, at the bare minimum, the second example uses 16 + 8 x 4 = 48 bytes.
请注意，MATLAB 中的每个变量还包括一个名为 mxArray 的结构的内存开销，该结构存储诸如维度、数据类型以及有关该变量的更多信息.为简单起见，我忽略了这个开销.
Note that each variable in MATLAB also includes memory overhead for a structure called an mxArray that stores information such as dimension, data type and a lot more about that variable. I've ignored this overhead for the sake of simplicity.
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