R-按维数生成数组与数组下标

向量变成数组，需要指定维数，下面的dim就指定了

> 1:15->z
> c(3,5)->dim(z)
> z
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    4    7   10   13
[2,]    2    5    8   11   14
[3,]    3    6    9   12   15

arry可以更加方便
> array(1:15,dim=c(3,5),dimnames="mmm")
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    4    7   10   13
[2,]    2    5    8   11   14
[3,]    3    6    9   12   15

 

使用matrix也可以的

> matrix(1:15,nrow=3,ncol=5,byrow=TRUE)->Z
> z
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    4    7   10   13
[2,]    2    5    8   11   14
[3,]    3    6    9   12   15
>

 

byrow表示按行还是按列放置

 

 

下标

 

> z[3,1]
[1] 3
> z[3,]
[1]  3  6  9 12 15
> z[,5]
[1] 13 14 15
>

 > z[,2:5]
     [,1] [,2] [,3] [,4]
[1,]    4    7   10   13
[2,]    5    8   11   14
[3,]    6    9   12   15
>

取(1,2)、(3,4)、(1,3）的元素,将位置定义在数组中

> matrix(c(1,2,3,4,1,3),ncol=2,byrow=TRUE)->xx
> z[xx]
[1]  4 12  7
> z
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    4    7   10   13
[2,]    2    5    8   11   14
[3,]    3    6    9   12   15
>

 

 

 

 

 

 

matrix {base}

R Documentation

Matrices

Description

matrix creates a matrix from the given set of values.

as.matrix attempts to turn its argument into a matrix.

is.matrix tests if its argument is a (strict) matrix.

Usage

matrix(data = NA, nrow = 1, ncol = 1, byrow = FALSE,
       dimnames = NULL)

as.matrix(x, ...)
## S3 method for class 'data.frame'
as.matrix(x, rownames.force = NA, ...)

is.matrix(x)

Arguments

 

data	an optional data vector (including a list or expression vector). Other R objects are coerced by as.vector.
nrow	the desired number of rows.
ncol	the desired number of columns.
byrow	logical. If FALSE (the default) the matrix is filled by columns, otherwise the matrix is filled by rows.
dimnames	A dimnames attribute for the matrix: NULL or a list of length 2 giving the row and column names respectively. An empty list is treated as NULL, and a list of length one as row names. The list can be named, and the list names will be used as names for the dimensions.
x	an R object.
...	additional arguments to be passed to or from methods.
rownames.force	logical indicating if the resulting matrix should have character (rather than NULL) rownames. The default, NA, uses NULL rownames if the data frame has ‘automatic’ row.names or for a zero-row data frame.

Details

If one of nrow or ncol is not given, an attempt is made to infer it from the length of data and the other parameter. If neither is given, a one-column matrix is returned.

If there are too few elements in data to fill the matrix, then the elements in data are recycled. If data has length zero, NA of an appropriate type is used for atomic vectors (0 for raw vectors) and NULL for lists.

is.matrix returns TRUE if x is a vector and has a "dim" attribute of length 2) and FALSE otherwise. Note that a data.frame is not a matrix by this test. The function is generic: you can write methods to handle specific classes of objects, see InternalMethods.

as.matrix is a generic function. The method for data frames will return a character matrix if there is any non-(numeric/logical/complex) column, applying format to non-character columns. Otherwise, the usual coercion hierarchy (logical < integer < double < complex) will be used, e.g., all-logical data frames will be coerced to a logical matrix, mixed logical-integer will give a integer matrix, etc.

When coercing a vector, it produces a one-column matrix, and promotes the names (if any) of the vector to the rownames of the matrix.

is.matrix is a primitive function.

Note

If you just want to convert a vector to a matrix, something like

  dim(x) <- c(nx, ny)
  dimnames(x) <- list(row_names, col_names)

will avoid duplicating x.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

See Also

data.matrix, which attempts to convert to a numeric matrix.

A matrix is the special case of a two-dimensional array.

Examples

is.matrix(as.matrix(1:10))
!is.matrix(warpbreaks)# data.frame, NOT matrix!
warpbreaks[1:10,]
as.matrix(warpbreaks[1:10,]) #using as.matrix.data.frame(.) method

# Example of setting row and column names
mdat <- matrix(c(1,2,3, 11,12,13), nrow = 2, ncol=3, byrow=TRUE,
               dimnames = list(c("row1", "row2"),
                               c("C.1", "C.2", "C.3")))
mdat

---

Multi-way Arrays

Description

Creates or tests for arrays.

Usage

array(data = NA, dim = length(data), dimnames = NULL)
as.array(x, ...)
is.array(x)

Arguments

 

data	a vector (including a list or expression vector) giving data to fill the array. Other objects are coerced by as.vector.
dim	the dim attribute for the array to be created, that is a vector of length one or more giving the maximal indices in each dimension.
dimnames	either NULL or the names for the dimensions. This is a list with one component for each dimension, either NULL or a character vector of the length given by dim for that dimension. The list can be named, and the list names will be used as names for the dimensions. If the list is shorter than the number of dimensions, it is extended by NULLs to the length required
x	an R object.
...	additional arguments to be passed to or from methods.

Details

An array in R can have one, two or more dimensions. It is simply a vector which is stored with additional attributes giving the dimensions (attribute "dim") and optionally names for those dimensions (attribute "dimnames").

A two-dimensional array is the same thing as a matrix.

One-dimensional arrays often look like vectors, but may be handled differently by some functions: str does distinguish them in recent versions of R.

The "dim" attribute is an integer vector of length one or more containing non-negative values: the product of the values must match the length of the array.

The "dimnames" attribute is optional: if present it is a list with one component for each dimension, either NULL or a character vector of the length given by the element of the "dim" attribute for that dimension.

is.array is a primitive function.

Value

array returns an array with the extents specified in dim and naming information in dimnames. The values in data are taken to be those in the array with the leftmost subscript moving fastest. If there are too few elements in data to fill the array, then the elements in data are recycled. If data has length zero, NA of an appropriate type is used for atomic vectors (0 for raw vectors) and NULL for lists.

as.array is a generic function for coercing to arrays. The default method does so by attaching a dim attribute to it. It also attaches dimnames if x has names. The sole purpose of this is to make it possible to access the dim[names] attribute at a later time.

is.array returns TRUE or FALSE depending on whether its argument is an array (i.e., has a dim attribute of positive length) or not. It is generic: you can write methods to handle specific classes of objects, see InternalMethods.

Note

is.array is a primitive function.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

See Also

aperm, matrix, dim, dimnames.

Examples

dim(as.array(letters))
array(1:3, c(2,4)) # recycle 1:3 "2 2/3 times"
#     [,1] [,2] [,3] [,4]
#[1,]    1    3    2    1
#[2,]    2    1    3    2

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