R多条件列赋值：按subject分组新增列，取CS+对应Per_size值

你可以直接使用data.table内置的nafill函数完成需求，代码简洁且执行效率高，适合处理大样本数据集：

# 加载data.table包（如果未加载）
library(data.table)

# 你的示例数据集
dt <- structure(list(subject = c("subject1", "subject1", "subject1", 
"subject1", "subject1", "subject1", "subject1", "subject1", "subject1", 
"subject2", "subject2", "subject2", "subject2", "subject2", "subject2", 
"subject2", "subject2", "subject2"), stimulus = c("CS-", "CS-", 
"CS+", "CS-", "S8", "S7", "S2", "CS+", "CS+", "CS-", "CS-", "CS+", 
"S4", "S9", "S7", "CS+", "CS+", "S8"), Per_size = c(60L, 70L, 
30L, 85L, 62L, 56L, 42L, 36L, 34L, 24L, 21L, 70L, 65L, 31L, 35L, 
79L, 79L, 34L)), row.names = c(NA, -18L), class = c("data.table", 
"data.frame"))

# 新增列逻辑
dt[, new_col := nafill(nafill(fifelse(stimulus == "CS+", Per_size, NA), type = "nocb"), type = "locf"), by = subject]

逻辑说明 #

• 按subject分组独立计算，符合分组要求
• fifelse(stimulus == "CS+", Per_size, NA)：仅保留CS+对应的Per_size值，其余行设为缺失值
• 内层nafill(..., type = "nocb")：后向填充，将首次CS+出现前的所有缺失值替换为该分组第一个CS+的Per_size，满足规则2
• 外层nafill(..., type = "locf")：前向填充，将两次CS+之间的缺失值替换为最近一次出现的CS+的Per_size，仅当新CS+出现时更新取值，满足规则1

运行结果示例 #

内容的提问来源于stack exchange，提问作者Kenny