Mainly introduce the use of matrix functions and rep to generate a matrix
In R language, you can use the matrix() function to create a matrix, and its syntax format is as follows:
- matrix(data=NA, nrow = 1, ncol = 1, byrow = FALSE, dimnames = NULL)
The parameters are as follows:
- data: The element of the matrix is NA by default, that is, if the element value is not given, each item is NA
- nrow: The number of rows of the matrix, default is 1;
- ncol: The number of columns of the matrix, default is 1;
- byrow: Whether the element is filled by row, by column by default;
- dimnames: row names and column names represented by character vectors.
Rep function is a function that repeats operations in R language
- rep(x,times,each,)
- x: represents the object you want to copy, which can be a vector or a factor.
- times: represents the number of copies, and can only be positive. Negative numbers and NA values are both wrong values. Copy refers to copying the entire vector.
- each: represents the number of times each element in the vector is copied.
- : represents the length of the final output vector.
Next, we use these two functions to generate a specific matrix
> matrix(rep(1:4,times = 2),nrow = 4 , ncol = 2 ,byrow =T) [,1] [,2] [1,] 1 2 [2,] 3 4 [3,] 1 2 [4,] 3 4 > matrix(rep(1:4,times = 2),nrow = 4 , ncol = 2 ,byrow =F) [,1] [,2] [1,] 1 1 [2,] 2 2 [3,] 3 3 [4,] 4 4
From the above two execution results, it can be seen that rep generates a row vector, and matrix fills the vector of a row according to the specified fill direction by row nibbling.
Next, we generate a 4x4 matrix, requiring that the elements at each position of the matrix are equal to 1/(i+j-1), and understand the meaning of the two parameters of rep
> I <- matrix(rep(1:4,times = 4),nrow = 4 , byrow = F)
> J <- matrix(rep(1:4,each = 4),nrow = 4 , byrow = T)#J is actually the transposition of I> A <- 1/(I+J-1)
> A
[,1] [,2] [,3] [,4]
[1,] 1.0000000 1.0000000 1.0000000 1.0000000
[2,] 0.3333333 0.3333333 0.3333333 0.3333333
[3,] 0.2000000 0.2000000 0.2000000 0.2000000
[4,] 0.1428571 0.1428571 0.1428571 0.1428571
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