R Data.table创建新列时报错，寻求解决方法

作为R语言初学者，运行以下代码时，data.table在创建新列环节抛出错误，推测是语法问题，查阅资料得知添加by参数可能解决，但不知如何添加。

运行代码 #

NF = MFDETAILED[,c(1,2)]
feature_names = colnames(compare)
NF = cbind(NF, data.table( 
  NF = apply(compare,1,function(x) {paste0(feature_names[(which(x[]==1))],collapse = ", ")}), 
  NMF = apply(compare,1,function(x) {paste0(feature_names[(which(x[]==0))],collapse = ", ")})
))
# 报错行
**NF[, c("LOOKUP", "MAT") := tstrsplit(COMBINATION, "-", fixed=TRUE)]**
NF[, ("COMBINATION"):=NULL]
setcolorder(NF, c( "LOOKUP","MAT","MS", "NF","NMF"))
setnames(NF,c("LOOKUP MATERIAL", "MATERIAL", "MATCH SCORE","MATCH FEATURES","NON MATCH FEATURES"))

错误信息 #

Error in [.data.table(NF, , :=(c("LOOKUP", "MAT"), tstrsplit(COMBINATION, : Supplied 2 columns to be assigned 4 items. Please see NEWS for v1.12.2.
中文翻译：在[.data.table操作中，提供了2列，但需要分配4个条目。请查看v1.12.2版本的NEWS文档。

这个错误的核心原因是：tstrsplit返回的结果总长度和data.table的行数不匹配，导致无法将分割结果正确映射到要创建的2个新列。当部分COMBINATION字段分割后得到的元素数量不一致时，批量处理模式下就会出现条目数不匹配的情况。

给data.table的操作添加by = .I参数，强制按行索引逐行处理分割逻辑，确保每行的分割结果都能正确对应到该行的LOOKUP和MAT列。修改后的报错行代码如下：

NF[, c("LOOKUP", "MAT") := tstrsplit(COMBINATION, "-", fixed=TRUE), by = .I]

完整修改后代码 #

NF = MFDETAILED[,c(1,2)]
feature_names = colnames(compare)
NF = cbind(NF, data.table( 
  NF = apply(compare,1,function(x) {paste0(feature_names[(which(x[]==1))],collapse = ", ")}), 
  NMF = apply(compare,1,function(x) {paste0(feature_names[(which(x[]==0))],collapse = ", ")})
))
# 修改后的行
NF[, c("LOOKUP", "MAT") := tstrsplit(COMBINATION, "-", fixed=TRUE), by = .I]
NF[, ("COMBINATION"):=NULL]
setcolorder(NF, c( "LOOKUP","MAT","MS", "NF","NMF"))
setnames(NF,c("LOOKUP MATERIAL", "MATERIAL", "MATCH SCORE","MATCH FEATURES","NON MATCH FEATURES"))

原理说明 #

by = .I会让data.table以每行作为一个分组来执行操作，tstrsplit会单独处理每行的COMBINATION值，分割后的结果直接分配到当前行的两个新列中，避免了批量处理时因条目数不匹配导致的报错。

内容的提问来源于stack exchange，提问作者Arvind Menon