矩阵 Matrics

这节课将教你掌握

May the force be with you!

什么是矩阵:

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创建矩阵

来，照上面的格式，你创建一个5 x 5 的矩阵，包含数字1到25，按行填充。

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撸起袖子继续干，来分析一下星战的票房。 在编辑器里已经给你准备好了向量，每个向量是一部电影。第一个数字是前三周该电影的美国本土票房，后一个数字是美国以外前前三周的票房成绩。

  • 你使用 c(new_hope, empire_strikes, return_jedi) 来把三个向量变成一个向量。命名为 box_office.

  • 使用matrix()创建一个有三行的向量，每行是一部电影。

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给向量命名

为了方便我们识别每一行的数据以及后面提取数据，我们要给向量命名。

是这样操作的, 行和列分别命名：

rownames(my_matrix) <- row_names_vector

colnames(my_matrix) <- col_names_vector

电影名称和地区的两个向量已经提前为你准备好了。 你用它们来给你的矩阵star_wars_matrix的行和列来命名吧。

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计算一下全球范围内的票房总成绩，使用 rowSums()来计算每一行的总和，并命名为worldwide_vector。

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我们下面呢，把新生成的这个总票房的向量合并到原来的矩阵里边，成为第三列数据。 使用到的函数是： big_matrix <- cbind(matrix1, matrix2, vector1 ...)。

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有列的合并就有行的合并，没错，就是rbind()。我们使用它来把两个矩阵按行合并在一起。在后台空间，已经给你预备了要被合并的两个矩阵：

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有 cbind() 就有 rbind(), 同样的，有colSums() 就有 rowSums()。 你在上面创建的all_wars_matrix矩阵已经在后台了，你输入all_wars_matrix，看看合并的结果。然后计算这一系列电影的美国本土票房和美国以外票房。

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选取子集或者元素

跟向量类似，你同样可以使用[]来提取矩阵里的元素。

  • my_matrix[1,2] selects the element at the first row and second column.

8 my_matrix[1:3,2:4] results in a matrix with the data on the rows 1, 2, 3 and columns 2, 3, 4.

若是要提取整行或者整列：

  • my_matrix[,1] selects all elements of the first column.

  • my_matrix[1,] selects all elements of the first row.

下面的练习：

  • 提取所有电影的非美国票房，结果存成 non_us_all.
  • 使用 mean() 计算 non_us_all 的平均值，并打印出结果。
  • 选取前两部电影的非美国票房，将结果存成 non_us_some，并计算其均值。
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再来一点算术运算

跟向量一样，加减乘除在矩阵里面也是元素范围内的。 2*my_matrix 是把 my_matrix里边的元素都乘以2。

作为刚刚拿到电影公司数据分析师offer的你，接到一个任务，要计算每一部电影的观影人次。假设一张票售价5元，结合前面的票房数据，这个任务不难完成。

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分析继续！ 在后台，我们给出了每一部电影在不同地区票房的收入矩阵，还有票价矩阵。你来计算一下观影人次的矩阵吧。

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