Question

我有一个大的稀疏矩阵，并希望保留所有列的前N个和后N个记录（对于此示例N = 2）并用零替换所有其他非零元素。我所遵循的当前方法是，通过生成原始矩阵的秩矩阵，然后使用它来保留前N个和后N个元素。

此示例的问题是，所有列中非零元素的总数不唯一。

以下是可重现的代码。

TestMatrix = Matrix(c(0,100,12,0,11,1,2,3,4,5,0,31,29,0,28,69,61,0,72,27,0,0,0,0,5,19,0,0,17,33),6,sparse = TRUE)
n <- diff(TestMatrix@p)  ## number of non-zeros per column
lst <- split(TestMatrix@x, rep.int(1:ncol(TestMatrix), n))  ## columns to list
r <- unlist(lapply(lapply(lst,function(x) x * -1), rank))   ## column-wise ranking and result collapsing
RankMatrix <- TestMatrix  ## copy sparse matrix
RankMatrix@x <- r  ## replace non-zero elements with rank
#Once the rank matrix is obtained I use something like

TestMatrix[(RankMatrix <= 1) | (RankMatrix > 3)] = 0 #This particular snippet is just trying to show what I want to achieve.

Answer 1

以下是我的解决方案。我只是想知道它是否可以在处理大型稀疏矩阵时进行优化。

TestMatrix = Matrix(c(0.80,0.9,0.6,0,0,0.3,0.5,
         0,0,0.3,0,0,0,0,
         0.4,0.5,0.6,0,0,0.1,0,
         0,0,0,0,0,0,0,
         0.3,0.4,0.5,0.2,0.1,0.7,0.8,
         0.6,0.7,0.5,0.8,0,0,0),7,sparse = TRUE)


n <- diff(TestMatrix@p)  ## number of non-zeros per column
lst <- split(TestMatrix@x, rep.int(1:ncol(TestMatrix), n))  ## columns to list
r <- unlist(lapply(lapply(lst,function(x) x * -1), rank))  ## column-wise ranking and result collapsing
RankMatrix <- TestMatrix  ## copy sparse matrix
RankMatrix@x <- r  ## replace non-zero elements with rank

#TestMatrix
#7 x 6 sparse Matrix of class "dgCMatrix"

# [1,] 0.8 .   0.4 . 0.3 0.6
# [2,] 0.9 .   0.5 . 0.4 0.7
# [3,] 0.6 0.3 0.6 . 0.5 0.5
# [4,] .   .   .   . 0.2 0.8
# [5,] .   .   .   . 0.1 .  
# [6,] 0.3 .   0.1 . 0.7 .  
# [7,] 0.5 .   .   . 0.8 .  
## Function to retain Top N and Bottom N records
GetTopNBottomN <- function(aMatrix,rMatrix){
  #aMatrix = original SparseMatrix, rMatrix = RankMatrix
  n = 2 ## Top 2 and Bottom 2 Elements across all the columns
  for(j in 1:ncol(aMatrix)){ 
    MaxValue = max(rMatrix[,j])
    if(MaxValue <= 2*n) next  ##Ignore the column if there are less than or equal to 2*n nonzero values
aMatrix[(rMatrix[,j] > n & rMatrix[,j] <= MaxValue-n),j] = 0
  }   
return(aMatrix)
}

GetTopNBottomN(TestMatrix,RankMatrix)
## Obtained Output
# 7 x 6 sparse Matrix of class "dgCMatrix"
# 
# [1,] 0.8 .   0.4 . .   0.6
# [2,] 0.9 .   0.5 . .   0.7
# [3,] .   0.3 0.6 . .   0.5
# [4,] .   .   .   . 0.2 0.8
# [5,] .   .   .   . 0.1 .  
# [6,] 0.3 .   0.1 . 0.7 .  
# [7,] 0.5 .   .   . 0.8 .

保留稀疏矩阵中所有列的前N个和后N个记录，并将所有其他非零元素设置为零

1 个答案: