r - 基于另一个稀疏矩阵将稀疏矩阵中的非零值替换为零的优化方法

Question

我有两个大的稀疏矩阵(比如 A 和 B)。我想根据 B 矩阵将 A 中的非零元素替换为零，该矩阵包含在每一列中排名的所有单个元素的等级。我的输出矩阵应包含来自 A 矩阵的前 n 个和后 n 个排名元素，所有其他非零值应设为零。

下面是我的方法。我在函数 GetTopNBottomN 中使用循环，我想知道它是否可以优化，因为当矩阵变大时它需要很长时间。

#input matrix
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) 

#function to genrate ranks across all the columns for the input matrix
GenerateRankMatrix <- function(aMatrix){ ## Function Begins
  n <- diff(aMatrix@p)  ## number of non-zeros per column
  lst <- split(aMatrix@x, rep.int(1:ncol(aMatrix), n))  ## columns to list
  r <- unlist(lapply(lapply(lst,function(x) x * -1), rank))  ## column-wise   ranking and result collapsing
  RankMatrix <- aMatrix  ## copy sparse matrix
  RankMatrix@x <- r  ## replace non-zero elements with rank
  return(RankMatrix)
} # Function Ends

## 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 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
    for (i in 1: nrow(aMatrix)){
      if(rMatrix[i,j] >n & rMatrix[i,j] <= MaxValue-n){ #IF Cond
        aMatrix[i,j] = 0
      } #IF ends
    }

  }   
  return(aMatrix)
}

#Output
RankMatrix = GenerateRankMatrix(TestMatrix) #Genrate Rank Matrix
#Output Matrix
GetTopNBottomN(TestMatrix,RankMatrix)

Answer 1

我提取了非零元素的索引，并使用 ave() 计算了分组排名

idx <- which(TestMatrix != 0, arr.ind=TRUE)
ranks = ave(-TestMatrix[idx], idx[,2], FUN=rank)

或者实际上是您想要的结果，即要保留的值

keep = ave(-TestMatrix[idx], idx[,2], FUN=function(elt) {
    elt = rank(elt)
    (elt > 2) & (elt <= length(elt) - 2)
}) == 0
idx = idx[keep,]

然后创建一个新的稀疏矩阵

sparseMatrix(idx[,1], idx[,2], x=TestMatrix[idx])