How to make a matrix from a given vector by using for loop

Question

I am trying to make a $n\times 4$ matrix by retrieving the n-th four elements in a given vector. Since I am new to R, don't know how to use loop functions properly.

My code is like

x<-runif(150,-2,2)
x1<-c(0,0,0,0,x)
for (i in 0:150)
 {ai<-x1[1+i,4+i]
 }

However, I got: Error in x1[1 + i, 4 + i] : incorrect number of dimensions.

I also want to combine these ai into a matrix, and each ai will be the i+1-th row of the matrix. Guess I should use the cbind function?

Any help will be appreciated. Thanks in advance.

I guess you got the error because your input data is vector and the indexing ie. [1+i, 4+i] is for matrices/data.frames etc. Could you show the expected result? — akrun
– akrun, Commented Oct 10, 2014 at 10:49

Carl Witthoft · Accepted Answer · 2014-10-10 11:27:50Z

1

You can do this directly with the matrix command:

x <- 1:36

xmat<-matrix(x,nr=9,byrow=TRUE)

answered Oct 10, 2014 at 11:27

Carl Witthoft

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akrun · Accepted Answer · 2014-10-10 12:52:25Z

0

May be this helps:

n <- length(x1)-1
res <- sapply((4:n)-3, function(i) x1[(i+3):i])
dim(res)
#[1]   4 150

edited Oct 10, 2014 at 12:52

answered Oct 10, 2014 at 11:13

akrun

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3 Comments

Xst67 Over a year ago

@ Roman Luštrik @akrun Let $X1=(0,0,0,0,a1,....,a150)$, then my matrix is like row1 = (0,0,0,0), row2=(a1,0,0,0),row3=(a2,a1,0,0),row4=(a3,a2,a1,0),row5=(a4,a3,a2,a1),....,row150=(a149,a148,a147,a146). So n-th row of the matrix consists of the n-th four elements of the vector X1, and it is in reverse order. Also a150 is left out. The dimension of the matrix is $x\times 150$.

Xst67 Over a year ago

I tried n<-length(x1)-1 res<-sapply((4:n)-3,function(i) x1[(i+3):i]), and it worked somehow. Thanks!

akrun Over a year ago

@Xst67 Yes, I guess that should do it. I was away from the computer.

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