# [R] matrix values linked to vector index

Dennis Murphy djmuser at gmail.com
Fri Oct 11 22:41:18 CEST 2013

```Attempting to follow the OP's conditions and assuming I understood
them correctly, here is one way to wrap this up into a function:

makeMat <- function(x)
{
stopifnot(is.integer(x))
nr <- length(x)
nc <- max(x)

# Initialize a matrix of zeros
m <- matrix(0, nr, nc)
# Conditionally replace with ones
for(i in seq_len(nr)) if(x[i] != 0)  m[i, 1:x[i]] <- 1
m
}

## Examples:
x1 <- 1:3
x2 <- as.integer(c(2, 0, 4, 3, 1))
x3 <- c(2, 1, 2.2)

makeMat(x1)
makeMat(x2)
makeMat(x3)
makeMat(4:6)

On Fri, Oct 11, 2013 at 9:49 AM, arun <smartpink111 at yahoo.com> wrote:
> Hi,
>
> In the example you showed:
>
> m1<- matrix(0,length(vec),max(vec))
> 1*!upper.tri(m1)
>
> #or
>  m1[!upper.tri(m1)] <-  rep(rep(1,length(vec)),vec)
>
> #But, in a case like below, perhaps:
> vec1<- c(3,4,5)
>
>  m2<- matrix(0,length(vec1),max(vec1))
>  indx <- cbind(rep(seq_along(vec1),vec1),unlist(tapply(vec1,list(vec1),FUN=seq),use.names=FALSE))
> m2[indx]<- 1
>  m2
> #     [,1] [,2] [,3] [,4] [,5]
> #[1,]    1    1    1    0    0
> #[2,]    1    1    1    1    0
> #[3,]    1    1    1    1    1
>
>
>
>
> A.K.
>
>
> Hi-
>
> I'd like to create a matrix of 0's and 1's where the number of
> 1's in each row defined by the value indexed in another vector, and
> where the (value-1) is back-filled by 0's.
>
> For example, given the following vector:
> vec= c(1,2,3)
>
> I'd like to produce a matrix with dimensions (length(vec), max(vec)):
>
> 1,0,0
> 1,1,0
> 1,1,1
>
> Thank you!
>
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