R 矩阵
矩阵是 R 对象，其中元素以二维矩形布局排列。它们包含相同原子类型的元素。虽然我们可以创建一个只包含字符或仅包含逻辑值的矩阵，但它们没有多大用处。我们使用包含用于数学计算的数字元素的矩阵。
使用
matrix() 函数创建矩阵。
语法
在 R 中创建矩阵的基本语法是-
matrix(data, nrow, ncol, byrow, dimnames)
以下是所用参数的说明-
data 是输入向量，它成为矩阵的数据元素。
nrow 是要创建的行数。
ncol 是要创建的列数。
byrow 是一个合乎逻辑的线索。如果为 TRUE，则输入向量元素按行排列。
dimname 是分配给行和列的名称。
示例
创建一个以数字向量作为输入的矩阵。
# Elements are arranged sequentially by row.
M <-matrix(c(3:14), nrow = 4, byrow = true)
print(M)
# Elements are arranged sequentially by column.
N <-matrix(c(3:14), nrow = 4, byrow = false)
print(N)
# Define the column and row names.
rownames = c("row1", "row2", "row3", "row4")
colnames = c("col1", "col2", "col3")
P <-matrix(c(3:14), nrow = 4, byrow = true, dimnames = list(rownames, colnames))
print(P)
当我们执行上面的代码时，它会产生以下结果-
[,1] [,2] [,3]
[1,] 3 4 5
[2,] 6 7 8
[3,] 9 10 11
[4,] 12 13 14
[,1] [,2] [,3]
[1,] 3 7 11
[2,] 4 8 12
[3,] 5 9 13
[4,] 6 10 14
col1 col2 col3
row1 3 4 5
row2 6 7 8
row3 9 10 11
row4 12 13 14
访问矩阵的元素
可以通过使用元素的列和行索引来访问矩阵的元素。我们考虑上面的矩阵 P 来找到下面的特定元素。
# Define the column and row names.
rownames = c("row1", "row2", "row3", "row4")
colnames = c("col1", "col2", "col3")
# Create the matrix.
P <-matrix(c(3:14), nrow = 4, byrow = true, dimnames = list(rownames, colnames))
# Access the element at 3rd column and 1st row.
print(P[1,3])
# Access the element at 2nd column and 4th row.
print(P[4,2])
# Access only the 2nd row.
print(P[2,])
# Access only the 3rd column.
print(P[,3])
当我们执行上面的代码时，它会产生以下结果-
[1] 5
[1] 13
col1 col2 col3
6 7 8
row1 row2 row3 row4
5 8 11 14
矩阵计算
使用 R 运算符对矩阵执行各种数学运算。运算的结果也是一个矩阵。
操作中涉及的矩阵的维度（行数和列数）应该相同。
矩阵加减法
# Create two 2x3 matrices.
matrix1 <-matrix(c(3, 9,-1, 4, 2, 6), nrow = 2)
print(matrix1)
matrix2 <-matrix(c(5, 2, 0, 9, 3, 4), nrow = 2)
print(matrix2)
# Add the matrices.
result <-matrix1 + matrix2
cat("Result of addition","\n")
print(result)
# Subtract the matrices
result <-matrix1-matrix2
cat("Result of subtraction","\n")
print(result)
当我们执行上面的代码时，它会产生以下结果-
[,1] [,2] [,3]
[1,] 3 -1 2
[2,] 9 4 6
[,1] [,2] [,3]
[1,] 5 0 3
[2,] 2 9 4
Result of addition
[,1] [,2] [,3]
[1,] 8 -1 5
[2,] 11 13 10
Result of subtraction
[,1] [,2] [,3]
[1,] -2 -1 -1
[2,] 7 -5 2
矩阵乘法和除法
# Create two 2x3 matrices.
matrix1 <-matrix(c(3, 9,-1, 4, 2, 6), nrow = 2)
print(matrix1)
matrix2 <-matrix(c(5, 2, 0, 9, 3, 4), nrow = 2)
print(matrix2)
# Multiply the matrices.
result <-matrix1 * matrix2
cat("Result of multiplication","\n")
print(result)
# Divide the matrices
result <-matrix1 / matrix2
cat("Result of division","\n")
print(result)
当我们执行上面的代码时，它会产生以下结果-
[,1] [,2] [,3]
[1,] 3 -1 2
[2,] 9 4 6
[,1] [,2] [,3]
[1,] 5 0 3
[2,] 2 9 4
Result of multiplication
[,1] [,2] [,3]
[1,] 15 0 6
[2,] 18 36 24
Result of division
[,1] [,2] [,3]
[1,] 0.6 -Inf 0.6666667
[2,] 4.5 0.4444444 1.5000000
