CODEWITHSUNDEEP
r
user3270948
user3270948

Reputation: 35

Set some values in a matrix equal to zero

I have a matrix with ID values at grid points and I have a vector containing some of those ID's. I would like to write a function so that for all the ID's contained in the vector, the corresponding ID in the matrix is set to 0.

For example:

ID <- matrix(c(2,3), nrow=2)

A <- matrix(c(0, 0, 2, 2, 0, 3,
        1, 0, 0, 0, 0, 0,
        0, 1, 0, 0, 0, 4,
        1, 0, 1, 0, 0, 4), nrow=4, byrow=TRUE)

I would like to transform my matrix A so that the 2's and 3 become 0.

I found a way to do this manually with the "if" function, setting conditions:

a <- nrow(A)
b <- ncol(A)
vide <- array(NaN, dim=c(a,b))

for (i in c(1:a)) {for (j in c(1:b)) {
  if ( (A[i,j]==ID[1,1]) | (A[i,j]==ID[2,1]) ) {
    vide[i,j]==0} else {vide[i,j]=A[i,j]}
  }
}

vide[is.na(vide)] <- 0

But I would like to create an automatic function that will do this for all the ID's contained in the ID vector, regardless of the number of rows. Because I will then run this over many years and the ID matrix will not always have the same number of rows. Is there a way to do that ?

I also tried another way, to make R write the conditions in the "if" function by itself:

d <- nrow(ID)
Cond <- array(NaN, dim=c(d,1))

for (k in c(1:d)) {
  Cond[k,1]=paste("(A[i,j]==ID[",k,",1]) ", sep="")
}

Cond <- t(Cond)
Cond <- paste(Cond,collapse="| ")
Cond <- noquote(Cond)

for (i in c(1:a)) {for (j in c(1:b)) {
  if (Cond) {
    vide[i,j]==0
  } else {
    vide[i,j]=A[i,j]
  }
}

but the problem is that Cond is a character matrix and I get the error message "argument is not interpretable as logical".

I am a beginner in R so maybe there is an easier way to do what I would like to do ? I tried searching on Internet but never seemed to find something appropriate to my case. Thanks for your help!

Upvotes: 0

Views: 3678

Answers (1)

Roman Luštrik
Roman Luštrik

Reputation: 70653

How's this? The trick is to handle matrix is a vector. R will take care of the second dimension on the fly. What I do below is find where in A elements from ID appear and overwrite them with a zero.

> A
     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    0    0    2    2    0    3
[2,]    1    0    0    0    0    0
[3,]    0    1    0    0    0    4
[4,]    1    0    1    0    0    4

> A[A %in% ID] <- 0
> A
     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    0    0    0    0    0    0
[2,]    1    0    0    0    0    0
[3,]    0    1    0    0    0    4
[4,]    1    0    1    0    0    4

Upvotes: 3

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