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只包含逻辑值的矩阵，但它们并没有多大用处.我们使用包含数字元素的矩阵用于数学计算.
使用矩阵()函数创建矩阵.
在R中创建矩阵的基本语法是 :
matrix(data, nrow, ncol, byrow, dimnames)
以下是所用参数的描述 :
数据是输入向量，它成为矩阵的数据元素.
nrow 是要创建的行数.
ncol 是的数量要创建的列.
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.
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
可以使用元素的列和行索引访问矩阵的元素.我们考虑上面的矩阵P来找到下面的具体元素.
# 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，它产生以下结果 :
[1] 5 [1] 13 col1 col2 col3 6 7 8 row1 row2 row3 row4 5 8 11 14
使用R运算符对矩阵执行各种数学运算.操作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)应相同.
# 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
# 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
