Well, I have searched a lot of questions but nothing works. Here is my question, I was asked to create a matrix like this
# [,1] [,2] [,3] [,4] [,5]
# [1,] 1 2 3 4 5
# [2,] 2 3 4 5 6
# [3,] 3 4 5 6 7
# [4,] 4 5 6 7 8
# [5,] 5 6 7 8 9
using the rep(), matrix() and seq() function.
I want to add 1 to each repeating cycle in seq(1:5) for 5 times, but I don't know how to. Would any one help me with this question?
Thank you #user20650 ! I finally figured it out.(Excuse me for not being familiar with Stack-overflow's functions)
I use the code like this:
a = matrix(rep((1:5),each = 5),5,5,byrow = TRUE)
b = matrix(rep((0:4),each = 5),5,5)
a+b
And it works
n = 5
r = seq(1,n)
matrix(rep(r,n),n,n,byrow = T) + r - 1
or alternatively, if you don't need to use the stipulated functions
n = 5
r = 1:n
t(matrix(-1,n,n) + r) + r
Check col() and row():
a <- matrix(0, 5, 5)
col(a) + row(a) - 1
[,1] [,2] [,3] [,4] [,5]
[1,] 1 2 3 4 5
[2,] 2 3 4 5 6
[3,] 3 4 5 6 7
[4,] 4 5 6 7 8
[5,] 5 6 7 8 9
Related
This question already has an answer here:
Create all subvectors of a certain length (moving window)
(1 answer)
Closed 1 year ago.
I would like to do something like this question but in R.
Given:
> my_vector
[1] 1 2 3 4 5 6
And window_size = 3, I would like to get the following matrix:
> my_matrix
[,1] [,2] [,3]
[1,] 1 2 3
[2,] 2 3 4
[3,] 3 4 5
[4,] 4 5 6
As you can see, every row of the matrix is a window of the vector.
Is there any efficient way to do it in R without for loop?
We can use embed
apply(embed(my_vector, 4), 1, rev)
# [,1] [,2] [,3]
#[1,] 1 2 3
#[2,] 2 3 4
#[3,] 3 4 5
#[4,] 4 5 6
Or it can be modified to
t(embed(rev(my_vector), 4))[, 3:1]
Or as #lmo suggested
embed(my_vector, 3)[, 3:1]
Or with matrix
matrix(my_vector, 7, 3)[1:4,]
sapply(1:3, function(i) my_vector[i:(i+3)])
# [,1] [,2] [,3]
#[1,] 1 2 3
#[2,] 2 3 4
#[3,] 3 4 5
#[4,] 4 5 6
This can be done using rollapply in zoo:
library(zoo)
rollapply(my_vector, 3, c)
giving:
[,1] [,2] [,3]
[1,] 1 2 3
[2,] 2 3 4
[3,] 3 4 5
[4,] 4 5 6
Note: The input used above, in reproducible form, is:
my_vector <- 1:6
Is it possible in R to say - I want all indices from position i to the end of vector/matrix?
Say I want a submatrix from 3rd column onwards. I currently only know this way:
A = matrix(rep(1:8, each = 5), nrow = 5) # just generate some example matrix...
A[,3:ncol(A)] # get submatrix from 3rd column onwards
But do I really need to write ncol(A)? Isn't there any elegant way how to say "from the 3rd column onwards"? Something like A[,3:]? (or A[,3:...])?
Sometimes it's easier to tell R what you don't want. In other words, exclude columns from the matrix using negative indexing:
Here are two alternative ways that both produce the same results:
A[, -(1:2)]
A[, -seq_len(2)]
Results:
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 3 4 5 6 7 8
[2,] 3 4 5 6 7 8
[3,] 3 4 5 6 7 8
[4,] 3 4 5 6 7 8
[5,] 3 4 5 6 7 8
But to answer your question as asked: Use ncol to find the number of columns. (Similarly there is nrow to find the number of rows.)
A[, 3:ncol(A)]
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 3 4 5 6 7 8
[2,] 3 4 5 6 7 8
[3,] 3 4 5 6 7 8
[4,] 3 4 5 6 7 8
[5,] 3 4 5 6 7 8
For rows (not columns as per your example) then head() and tail() could be utilised.
A <- matrix(rep(1:8, each = 5), nrow = 5)
tail(A, 3)
is almost the same as
A[3:dim(A)[1],]
(the rownames/indices printed are different is all).
Those work for vectors and data frames too:
> tail(1:10, 4)
[1] 7 8 9 10
> tail(data.frame(A = 1:5, B = 1:5), 3)
A B
3 3 3
4 4 4
5 5 5
For the column versions, you could adapt tail(), but it is a bit trickier. I wonder if NROW() and NCOL() might be useful here, rather than dim()?:
> A[, 3:NCOL(A)]
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 3 4 5 6 7 8
[2,] 3 4 5 6 7 8
[3,] 3 4 5 6 7 8
[4,] 3 4 5 6 7 8
[5,] 3 4 5 6 7 8
Or flip this on its head and instead of asking R for things, ask it to drop things instead. Here is a function that encapsulates this:
give <- function(x, i, dimen = 1L) {
ind <- seq_len(i-1)
if(isTRUE(all.equal(dimen, 1L))) { ## rows
out <- x[-ind, ]
} else if(isTRUE(all.equal(dimen, 2L))) { ## cols
out <- x[, -ind]
} else {
stop("Only for 2d objects")
}
out
}
> give(A, 3)
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,] 1 2 3 4 5 6 7 8
[2,] 1 2 3 4 5 6 7 8
[3,] 1 2 3 4 5 6 7 8
> give(A, 3, dimen = 2)
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 3 4 5 6 7 8
[2,] 3 4 5 6 7 8
[3,] 3 4 5 6 7 8
[4,] 3 4 5 6 7 8
[5,] 3 4 5 6 7 8
You can use the following instruction:
A[, 3:length(A[, 1])]
A dplyr readable renewed approach for the same thing:
A %>% as_tibble() %>%
select(-c(V1,V2))
A %>% as_tibble() %>%
select(V3:ncol(A))
Let's say I have a simple vector
v <- 1:5
I can add the vector to each element within the vector with the following code to generate the resulting matrix.
matrix(rep(v, 5), nrow=5, byrow=T) + matrix(rep(v, 5), nrow=5)
[,1] [,2] [,3] [,4] [,5]
[1,] 2 3 4 5 6
[2,] 3 4 5 6 7
[3,] 4 5 6 7 8
[4,] 5 6 7 8 9
[5,] 6 7 8 9 10
But this seems verbose and inefficient. Is there a more concise way to accomplish this? Perhaps some linear algebra concept that is evading me?
outer should do what you want
outer(v, v, `+`)
# [,1] [,2] [,3] [,4] [,5]
# [1,] 2 3 4 5 6
# [2,] 3 4 5 6 7
# [3,] 4 5 6 7 8
# [4,] 5 6 7 8 9
# [5,] 6 7 8 9 10
Posting this answer not for up votes but to highlight Franks comment. You can use
sapply(v,"+",v)
I want to draw a Hankel matrix with R use only matrix(), seq() and rep() function of R. Until now, I draw this in some way:
#Do this exercise with other packages, need to rework
install.packages("matrixcalc")
library(matrixcalc)
E1 <- hankel.matrix( 5, seq( 1, 9 ) )
print(E1)
#Use matrix() only, not efficient
E2 <- matrix(c(1,2,3,4,5,2,3,4,5,6,3,4,5,6,7,4,5,6,7,8,5,6,7,8,9), ncol=5)
print(E2)
#Use seq() but not worked
E3 <- matrix(c(seq(1:5),seq(2:6),seq(3:7),seq(4:8),seq(5:9)), ncol=5)
print(E3)
E1 used a library to draw a Hankel matrix and in E2, I tried to put the number manually to draw one but it will take a lot of time if I want a new big matrix. I tried to use seq() but it not worked. It will draw like this:
[,1] [,2] [,3] [,4] [,5]
[1,] 1 1 1 1 1
[2,] 2 2 2 2 2
[3,] 3 3 3 3 3
[4,] 4 4 4 4 4
[5,] 5 5 5 5 5
I am still very new with R so every idea is welcome.
You can do this :
matrix(rep(1:5,5)+rep(0:4,each=5),ncol=5)
# [,1] [,2] [,3] [,4] [,5]
# [1,] 1 2 3 4 5
# [2,] 2 3 4 5 6
# [3,] 3 4 5 6 7
# [4,] 4 5 6 7 8
# [5,] 5 6 7 8 9
Or more elegant but using outer:
outer(0:4,1:5,'+')
EDIT :
the rep solution works like this:
12345 12345 12345 ... (rep times, repeat the vector n times
+ 00000 11111 22222 ... (rep with each , repeat each element n times
= 12345 23456 34567 .....
outer can be tricky at first, maybe this answer here can help you to understand it and to general debug.
I'm reversing the order of the two different solutions so that the qualifying one is at the top:
A general function (meaning on that doesn't depend on the values being sequential) that only uses a couple of extra functions (like c() and "[") to do the work:
N <- c(9L, 7L, 3L, 2L, 1L, 8L, 4L, 5L, 6L, 10L)
hankel2 <- function(N, n){stopifnot(length(N)==2*n);
matrix( rep(N,n)[c(rep(TRUE,n),rep(FALSE,n+1))], n) }
hankel2(N,5)
[,1] [,2] [,3] [,4] [,5]
[1,] 9 7 3 2 1
[2,] 7 3 2 1 8
[3,] 3 2 1 8 4
[4,] 2 1 8 4 5
[5,] 1 8 4 5 6
The trick with that first (of three) efforts was to depend on argument recycling of logical vectors when used inside the "[" function. It creates a gap of n+1 items after choosing n items by indexing with FALSE ( which has the effect of omitting items.)
Embed is a cute little function that has a fairly opaque help file but occasionally delivers very compact code:
> x <- 1:10
> embed (x, 5)[1:5, 5:1]
[,1] [,2] [,3] [,4] [,5]
[1,] 1 2 3 4 5
[2,] 2 3 4 5 6
[3,] 3 4 5 6 7
[4,] 4 5 6 7 8
[5,] 5 6 7 8 9
You could make a function:
> hankel <- function( n ) embed(1:(2*n),5)[1:n, n:1]
> hankel(5)
[,1] [,2] [,3] [,4] [,5]
[1,] 1 2 3 4 5
[2,] 2 3 4 5 6
[3,] 3 4 5 6 7
[4,] 4 5 6 7 8
[5,] 5 6 7 8 9
(Admittedly not playing by the specifications although I wondered if any of the solutions so far would stand up to a vector that wasn't sequential. This one does:)
> hankel5 <- function( n ) embed(sample(1:10,10),5)[1:n, n:1]
> hankel5(5)
[,1] [,2] [,3] [,4] [,5]
[1,] 3 5 7 9 4
[2,] 5 7 9 4 10
[3,] 7 9 4 10 1
[4,] 9 4 10 1 8
[5,] 4 10 1 8 2
So this is the other general function:
hankel <- function( N, n ) {stopifnot(length(N) == 2*n); embed(N,n)[1:n, n:1]
Is it possible in R to say - I want all indices from position i to the end of vector/matrix?
Say I want a submatrix from 3rd column onwards. I currently only know this way:
A = matrix(rep(1:8, each = 5), nrow = 5) # just generate some example matrix...
A[,3:ncol(A)] # get submatrix from 3rd column onwards
But do I really need to write ncol(A)? Isn't there any elegant way how to say "from the 3rd column onwards"? Something like A[,3:]? (or A[,3:...])?
Sometimes it's easier to tell R what you don't want. In other words, exclude columns from the matrix using negative indexing:
Here are two alternative ways that both produce the same results:
A[, -(1:2)]
A[, -seq_len(2)]
Results:
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 3 4 5 6 7 8
[2,] 3 4 5 6 7 8
[3,] 3 4 5 6 7 8
[4,] 3 4 5 6 7 8
[5,] 3 4 5 6 7 8
But to answer your question as asked: Use ncol to find the number of columns. (Similarly there is nrow to find the number of rows.)
A[, 3:ncol(A)]
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 3 4 5 6 7 8
[2,] 3 4 5 6 7 8
[3,] 3 4 5 6 7 8
[4,] 3 4 5 6 7 8
[5,] 3 4 5 6 7 8
For rows (not columns as per your example) then head() and tail() could be utilised.
A <- matrix(rep(1:8, each = 5), nrow = 5)
tail(A, 3)
is almost the same as
A[3:dim(A)[1],]
(the rownames/indices printed are different is all).
Those work for vectors and data frames too:
> tail(1:10, 4)
[1] 7 8 9 10
> tail(data.frame(A = 1:5, B = 1:5), 3)
A B
3 3 3
4 4 4
5 5 5
For the column versions, you could adapt tail(), but it is a bit trickier. I wonder if NROW() and NCOL() might be useful here, rather than dim()?:
> A[, 3:NCOL(A)]
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 3 4 5 6 7 8
[2,] 3 4 5 6 7 8
[3,] 3 4 5 6 7 8
[4,] 3 4 5 6 7 8
[5,] 3 4 5 6 7 8
Or flip this on its head and instead of asking R for things, ask it to drop things instead. Here is a function that encapsulates this:
give <- function(x, i, dimen = 1L) {
ind <- seq_len(i-1)
if(isTRUE(all.equal(dimen, 1L))) { ## rows
out <- x[-ind, ]
} else if(isTRUE(all.equal(dimen, 2L))) { ## cols
out <- x[, -ind]
} else {
stop("Only for 2d objects")
}
out
}
> give(A, 3)
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,] 1 2 3 4 5 6 7 8
[2,] 1 2 3 4 5 6 7 8
[3,] 1 2 3 4 5 6 7 8
> give(A, 3, dimen = 2)
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 3 4 5 6 7 8
[2,] 3 4 5 6 7 8
[3,] 3 4 5 6 7 8
[4,] 3 4 5 6 7 8
[5,] 3 4 5 6 7 8
You can use the following instruction:
A[, 3:length(A[, 1])]
A dplyr readable renewed approach for the same thing:
A %>% as_tibble() %>%
select(-c(V1,V2))
A %>% as_tibble() %>%
select(V3:ncol(A))