基于另一个锯齿状data.frame的data.frame的有效子集

Question

我正在一个项目中，需要根据属性的不同组合重复子集data.frame。 现在，我正在使用合并功能对data.frame进行子集设置，因为我不知道运行时输入的属性是什么，并且可以正常工作。 但是，我想知道是否有更快的方法来创建子集。

require(data.table)
df <- structure(list(att1 = c("e", "a", "c", "a", "d", "e", "a", "d", "b", "a", "c", "a", "b", "e", "e", "c", "d", "d", "a", "e", "b"), 
                     att2 = c("b", "d", "c", "a", "e", "c", "e", "d", "e", "b", "e", "e", "c", "e", "a", "a", "e", "c", "b", "b", "d"), 
                     att3 = c("c", "b", "e", "b", "d", "d", "d", "c", "c", "d", "e", "a", "d", "c", "e", "a", "d", "e", "d", "a", "e"), 
                     att4 = c("c", "a", "b", "a", "e", "c", "a", "a", "b", "a", "a", "e", "c", "d", "b", "e", "b", "d", "d", "b", "e")), 
                .Names = c("att1", "att2", "att3", "att4"), class = "data.frame", row.names = c(NA, -21L))

#create combinations of attributes
#attributes to search through
cnames <- colnames(df)
att_combos <- data.table()
for(i in 2:length(cnames)){
  combos <- combn(cnames, i)
  for(x in 1:ncol(combos)){
    df_sub <- unique(df[,combos[1:nrow(combos), x]])
    att_combos <- rbind(att_combos, df_sub, fill = T)
  }
}
rm(df_sub, i, x, combos, cnames)
for(i in 1:nrow(att_combos)){
  att_sub <- att_combos[i, ]
  att_sub <- att_sub[, is.na(att_sub)==F, with = F]

  #need to subset data.frame here - very slow on large data.frames
  #anyway to speed this up?
  df_subset_for_analysis <- merge(df, att_sub)
}

Answer 1

我将在要子集化的列上使用data.table keys ，然后（在运行时）使用您感兴趣的组合生成一个data.table ，然后merge两者merge 。

下面是与属性（的单一组合的示例simple_combinations ）和一个具有属性（多个组合multiple_combinations ）：

require(data.table)
df <- structure(list(att1 = c("e", "a", "c", "a", "d", "e", "a", "d", "b", "a", "c", "a", "b", "e", "e", "c", "d", "d", "a", "e", "b"), 
                 att2 = c("b", "d", "c", "a", "e", "c", "e", "d", "e", "b", "e", "e", "c", "e", "a", "a", "e", "c", "b", "b", "d"), 
                 att3 = c("c", "b", "e", "b", "d", "d", "d", "c", "c", "d", "e", "a", "d", "c", "e", "a", "d", "e", "d", "a", "e"), 
                 att4 = c("c", "a", "b", "a", "e", "c", "a", "a", "b", "a", "a", "e", "c", "d", "b", "e", "b", "d", "d", "b", "e")), 
            .Names = c("att1", "att2", "att3", "att4"), class = "data.frame", row.names = c(NA, -21L))

# Convert to data.table
dt <- data.table(df)
# Set key on the columns used for "subsetting"
setkey(dt, att1, att2, att3, att4)

# Simple subset on a single set of attributes
simple_combinations <- data.table(att1 = "d", att2 = "e", att3 = "d", att4 = "e")
setkey(simple_combinations, att1, att2, att3, att4)
# Merge to generate simple output subset (simple_combinations of att present in dt)
simple_subset <- merge(dt, simple_combinations)

# Complex (multiple) sets of attributes
multiple_combinations <- data.table(expand.grid(att1=c("d"), att2=c("c", "d", "e"),
  att3 = c("d"), att4 = c("b", "e")))
setkey(multiple_combinations, att1, att2, att3, att4)
# Merge to generate  output subset (multiple_combinations of att present in dt)
multiple_subset <- merge(dt, multiple_combinations)

输出在simple_subset和multiple_subset 。