C ++中的多维向量 [英] Multidimensional Vectors in C++

问题描述

我刚刚开始学习C ++.当我开始感到相当困惑时，我正试图掌握多维数组和向量的语法.我了解如何初始化多维数组.看起来很简单:行后跟列.但是，向量更具挑战性.我必须以相同的方式初始化它们还是创建向量的向量.有人请帮忙.

I just started learning C++. I was trying to grasp the syntax for multidimensional arrays and vectors when I started to get fairly confused. I get how to initialize multidimensional arrays. It seems straightforward: Rows followed by columns. However, vectors are a little more challenging. Do I have to initialize them in the same way or do I create a vector of vectors. Somebody please help.

推荐答案

如果您能够使用C ++ 11，则可以类似的方式初始化向量的多维数组和向量.

If you are able to use C++11, multidimensional arrays and vectors of vectors can be initialized in a similar manner.

int a1[3][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} };
std::vector<std::vector<int>> a2 = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} };

但是，在访问元素而不遇到未定义的行为时，必须理解一些差异.

However, there are differences that must be understood to access the elements without running into undefined behavior.

对于多维数组，需要连续分配数组元素的内存.对于向量的向量，元素的存储空间很可能是不相交的.

For a multidimensional array, memory for the elements of the array is required to be allocated contiguously. For a vector of vector, the memory for the elements is most likely going to be disjoint.

a1的内存:

a1[0][0]    a1[1][0]    a1[2][0]
|           |           |
v           v           v
+---+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+---+

a2的内存(最有可能):

Memory for a2 (most likely):

a2[0][0]
|
v
+---+---+---+
|   |   |   |
+---+---+---+

a2[1][0]
|
v
+---+---+---+
|   |   |   |
+---+---+---+

a2[2][0]
|
v
+---+---+---+
|   |   |   |
+---+---+---+

还可以定义一个向量的向量，其中每行的列数都不相同.

Also, it is possible to defined a vector of vectors in which the number of columns is not same for each row.

std::vector<std::vector<int>> a2 = { {1, 2, 3}, {4, 5}, {6, 7, 8, 9} };

在多维数组中，保证每一行的列数相同.

In a multidimensional array, the number of columns is guaranteed to be same for each row.

鉴于上述多维数组a1，a1[1][2]将是有效元素，而a1[2][3]将是无效元素.对于向量的向量，使用上面的行，a2[1][2]不是有效元素，a2[2][3]是有效元素.

Given the above multidimensional array a1, a1[1][2] will be a valid element and a1[2][3] will be an invalid element. In the case of a vector of vectors, using the above line, a2[1][2] is not a valid element and a2[2][3] is a valid element.

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