An array is a multidimensional data structure. We can determine and manipulate its features with the length, ncol, nrow and dim (dimension) functions.
The length function works on one-dimensional objects such as vectors and lists. In the example below we first create a vector V and then use length() to find out how long V is.
Input:
V<-c(1:41)
length(V)
Output:
41
Matrices are two-dimensional objects so the length function will not work on them. With a two-dimensional object we need to specify the axis along which we want to find the size. This is done using the nrow() and ncol() functions to find out the number of rows and columns respectively. Let's first create a 3x3 matrix.
Input:
M<-matrix(1:9,nrow=3)
M
Output:
1 4 7
2 5 8
3 6 9
Now we can use the nrow() and ncol() functions. The only arguments that they require is the name of the matrix.
Input:
nrow(M)
ncol(M)
Output:
3
3
Knowing the number of rows and columns is useful for iterating over a matrix, for example using a for loop. The dim() function is an alternative to finding the rows and columns separately, it returns both the number of rows and the number of columns.
Input:
dim(M)
Output:
3 3
Arrays can also be three-dimensional. As shown in the example below, two-dimensional arrays can be stacked on top of each other using the concatenation operator c() creating the third dimension. The dim() function can then be used to determine all three dimensions of the array.
Input:
M2<-2*M
M3<-3*M
N<-array(c(M,M2,M3),c(3,3,3))
dim(N)
Output:
3 3 3
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