Matlab中的单元格和矩阵有什么区别? [英] What is the difference between cell and matrix in Matlab?

问题描述

在Matlab中单元格(即使用{ })和矩阵(即使用[ ])之间有什么区别?

What is the difference between cell (i.e. with { }) and matrix (i.e. with [ ]) in Matlab?

推荐答案

在MATLAB中，单元格数组和矩阵之间存在一些区别:

There are several differences between a cell array and a matrix in MATLAB:

• 一个单元数组可以在每个单元中包含任意类型的元素；而矩阵要求其元素的类型是均质的，即相同类型.

• 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，矩阵的所有元素都连续排列在内存中，而单元格数组包含指向数组中每个元素的指针.当考虑将缓存局部性等用于高性能代码时，这可能很重要.

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，调整矩阵​​大小时，必须将矩阵中的每个元素复制到新分配的存储区中，但是在单元格数组的情况下，只需要复制指针列表即可超过.根据要存储的元素的大小和类型，这可能意味着单元阵列的调整大小要快得多.

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.

为说明内存布局的差异，让我们考虑一个简单的示例:

To illustrate the differences in memory layout, let's consider a simple example:

A = [10 20 30 40];

此处，MATLAB创建了一个名为A的新矩阵变量，分配了足够的内存以容纳4个双精度(32个字节，假设8个字节双精度)，并将此内存分配给一个指向 real 部分的指针.答:(如果创建复数矩阵，则还会为虚构部分分配内存，并且一个单独的指针指向该内存区域).

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以下元素的单元格数组:

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;

MATLAB执行第一条语句时，它将创建一个包含4个指针的单元格数组，每个指针都可以指向任意类型.因此B已经在使用16个字节(假设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单元格数组元素.除了分配的内存仅足以容纳一个双精度(8字节)外，此处的过程与我上面描述的用于创建1x4矩阵的过程类似.对其余3条语句中的每条重复执行此操作.因此，第二个示例至少使用了16 + 8 x 4 = 48个字节.

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名为 mxArray 的结构的内存开销，该结构存储诸如尺寸，数据类型以及有关该变量的更多信息.为了简单起见，我忽略了这些开销.

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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