当使用 data.table 的 DT[ i , j, by] 时，是否可以预先设置列类型？

我正在尝试计算多个不同组的两个变量之间的相关性（例如DT[, cor.test(var1, var2), group]）。每当我使用时这都非常有效cor.test(var1, var2, method = 'pearson')但当我使用时抛出错误cor.test(var1, var2, method = 'spearman').

library(data.table)
DT <- as.data.table(iris)

# works perfectly 
DT[,cor.test(Sepal.Length,Sepal.Width, method = 'pearson'), Species]
#       Species statistic parameter      p.value  estimate null.value
# 1:     setosa  7.680738        48 6.709843e-10 0.7425467          0
# 2:     setosa  7.680738        48 6.709843e-10 0.7425467          0
# 3: versicolor  4.283887        48 8.771860e-05 0.5259107          0
# 4: versicolor  4.283887        48 8.771860e-05 0.5259107          0
# 5:  virginica  3.561892        48 8.434625e-04 0.4572278          0
# 6:  virginica  3.561892        48 8.434625e-04 0.4572278          0
#    alternative                               method
# 1:   two.sided Pearson's product-moment correlation
# 2:   two.sided Pearson's product-moment correlation
# 3:   two.sided Pearson's product-moment correlation
# 4:   two.sided Pearson's product-moment correlation
# 5:   two.sided Pearson's product-moment correlation
# 6:   two.sided Pearson's product-moment correlation
#                       data.name  conf.int
# 1: Sepal.Length and Sepal.Width 0.5851391
# 2: Sepal.Length and Sepal.Width 0.8460314
# 3: Sepal.Length and Sepal.Width 0.2900175
# 4: Sepal.Length and Sepal.Width 0.7015599
# 5: Sepal.Length and Sepal.Width 0.2049657
#> 6: Sepal.Length and Sepal.Width 0.6525292

# error
DT[,cor.test(Sepal.Length,Sepal.Width, method = 'spearman'), Species]
# Error in `[.data.table`(DT, , cor.test(Sepal.Length, Sepal.Width, method = "spearman"), : 
# Column 2 of j's result for the first group is NULL. We rely on the column types of the first 
# result to decide the type expected for the remaining groups (and require consistency). NULL 
# columns are acceptable for later groups (and those are replaced with NA of appropriate type 
# and recycled) but not for the first. Please use a typed empty vector instead, such as 
# integer() or numeric().

问题：

我知道这个特定示例有解决方法，但可以告诉data.table事先确定在任何情况下使用的列类型是什么DT[i,j,by = 'something']?

如果您想保留所有列，而不是删除带有 NULL 的列，您可以手动设置“问题”列的类（在这种情况下，给出问题的列是“参数”）。如果列确实包含某些组的值但不包含其他组的值，则这比删除 NULL 更可取。

DT[, {
  res <- cor.test(Sepal.Length, Sepal.Width, method = 'spearman')
  class(res$parameter) <- 'integer'
  res
  }, Species]

#      Species statistic parameter      p.value  estimate null.value alternative                          method                    data.name
#1:     setosa  5095.097        NA 2.316710e-10 0.7553375          0   two.sided Spearman's rank correlation rho Sepal.Length and Sepal.Width
#2: versicolor 10045.855        NA 1.183863e-04 0.5176060          0   two.sided Spearman's rank correlation rho Sepal.Length and Sepal.Width
#3:  virginica 11942.793        NA 2.010675e-03 0.4265165          0   two.sided Spearman's rank correlation rho Sepal.Length and Sepal.Width