Vectorizing which operation across the rows of a matrix

问题

I would like to vectorize (apply) a which operation on matrix X as illustrated by the following for loop having as result the vector ind:

X   = matrix( 1:20, 4, 5 )
V   = sample( 1:20, 4 )
ind = numeric()
for( i in 1:nrow(X) ) ind[i] = max( c(0, which(X[i,] < V[i]) ))

The operation returns in ind for each row in X the index of the element with the highest value smaller than the value indicated by the X-row-corresponding element of V.

The operation max maps the vector of all eligible indices to a scalar. Alternatively I would by happy with an operation returning e.g. a list of all indices (to which I can lapply max).

回答1:

If your matrix has increasing values like the example you shared (which of course I doubt), but If it does you can simply do,

rowSums(X < V)
#[1] 4 3 4 0

However, If this is not the case, then a simple apply will suffice, i.e.

apply(X < V, 1, function(i)max(which(i)))
#[1]    4    3    4 -Inf

Remember that all mathematical operations are vectorized, so < is vectorized

You can replace -Inf as per usual

回答2:

apply(
    (X < V) * X
    , 1
    , which.max
)

回答3:

Here's a simple lapply example

X   = matrix( 1:20, 4, 5 )
V   = sample( 1:20, 4 )
ind = numeric()
for( i in 1:nrow(X) ) ind[i] = max( c(0, which(X[i,] < V[i]) ))

mymax = function(matrix, sample) {
    whichlist = which(matrix < sample)
    max(0, whichlist)
}
ind2 = unlist(lapply(1:nrow(X), function(r) mymax(X[r,], V[r])))

identical(ind, ind2)
# [1] TRUE

回答4:

Here is another answer where you first sweep the vector V across each row of X and then you use apply to determine the maximum TRUE element of each row.

set.seed(14)
X   = matrix( 1:20, 4, 5 )
V   = sample( 1:20, 4 )
ind1 = numeric()
for( i in 1:nrow(X) ) ind1[i] =  max(c(0, which(X[i,] < V[i]) ))



ind2 <- apply(
  sweep(X, 1, V, "<"),
  1,
  function(x){
    max(
    which(
      x,
      arr.ind = TRUE)
    )
  }
)


> ind1
[1] 2 5 2 4
> ind2
[1] 2 5 2 4

来源：https://stackoverflow.com/questions/60097628/vectorizing-which-operation-across-the-rows-of-a-matrix

标签

vectorization