从dplyr转用data.table：自定义函数调用报错及修改咨询

我此前使用dplyr，刚接触data.table，编写了如下自定义函数tabs：

tabs <- function(dt, x) {
tab2 <- dt[!is.na(x), ][, .(Freq = sum(nwgt0)), by = .(inc_cat, year, x)][, Prop := Freq / sum(Freq), by= .(inc_cat, year)][order(inc_cat, year)][x == 1 & !is.na(inc_cat), ] %>%
   ggplot(., aes(x= year, y = Prop, color = factor(inc_cat, levels = c(1,2,3,4),labels = c("0% to 100% FPL", "101-138% FPL", "139-200% FPL", ">200% FPL")))) +
    labs(color = "Income Categories") +
    geom_line() +
    theme_minimal() +
  ylab("Weighted proportion") +
   theme(
  panel.border = element_blank(),
  panel.grid.major = element_blank(),
  panel.grid.minor = element_blank(),
  )
return(tab2)
}

尝试通过result <- hints_dt[ , tabs(.SD, x='internet_use')]调用时出现报错：

Error in `[.data.table`(dt[!is.na(x), ], , .(Freq = sum(nwgt0)), by = .(inc_cat,  : 
  The items in the 'by' or 'keyby' list are length(s) (22344,22344,1). Each must be length 22344; the same length as there are rows in x (after subsetting if i is provided).

附测试用reprex（基于NHANES数据集）：

tabs <- function(dt, x) {
tab2 <- dt[!is.na(x), ][, .(Freq = sum(WTMEC2YR)), by = .(race, agecat, x)][, Prop := Freq / sum(Freq), by= .(race, agecat)][order(race, agecat)][x == 1 & !is.na(race), ] %>%
   ggplot(., aes(x= year, y = Prop, color = factor(race, levels = c(1,2,3,4),labels = c("hispanic", "white", "black", "other")))) +
    labs(color = "Race") +
    geom_line() +
    theme_minimal() +
  ylab("Weighted proportion") +
   theme(
  panel.border = element_blank(),
  panel.grid.major = element_blank(),
  panel.grid.minor = element_blank(),
  )
return(tab2)
}

调用result <- nhanes[ , tabs(.SD, x="RIAGENDR")]时可复现报错：

Error in `[.data.table`(dt[!is.na(get(x)), ], , .(Freq = sum(WTMEC2YR)),  : 
  The items in the 'by' or 'keyby' list are length(s) (8591,8591,1). Each must be length 8591; the same length as there are rows in x (after subsetting if i is provided).

错误原因 #

报错核心是直接用字符串变量x引用列名不符合data.table语法规则：

• dt[!is.na(x)]中，x是传入的字符串（如"internet_use"），并非列对象，!is.na(x)会返回长度为1的逻辑值，导致子集筛选逻辑错误；
• by = .(inc_cat, year, x)里的x同样是字符串，长度为1，和inc_cat、year的行长度不匹配，触发分组长度不一致的报错。

修改方案 #

用get(x)动态引用列名，确保data.table能识别到目标列；同时可结合.SDcols优化列传递（可选，减少不必要的列加载）。

修改后的原始函数 #

tabs <- function(dt, x) {
  tab2 <- dt[!is.na(get(x)), ] %>%
    .[, .(Freq = sum(nwgt0)), by = .(inc_cat, year, get(x))] %>%
    .[, Prop := Freq / sum(Freq), by = .(inc_cat, year)] %>%
    .[order(inc_cat, year)] %>%
    .[get(x) == 1 & !is.na(inc_cat), ] %>%
    ggplot(aes(x = year, y = Prop, 
               color = factor(inc_cat, levels = c(1,2,3,4),
                              labels = c("0% to 100% FPL", "101-138% FPL", "139-200% FPL", ">200% FPL")))) +
    labs(color = "Income Categories") +
    geom_line() +
    theme_minimal() +
    ylab("Weighted proportion") +
    theme(
      panel.border = element_blank(),
      panel.grid.major = element_blank(),
      panel.grid.minor = element_blank()
    )
  return(tab2)
}

修改后的测试用函数（NHANES数据集） #

tabs <- function(dt, x) {
  tab2 <- dt[!is.na(get(x)), ] %>%
    .[, .(Freq = sum(WTMEC2YR)), by = .(race, agecat, get(x))] %>%
    .[, Prop := Freq / sum(Freq), by = .(race, agecat)] %>%
    .[order(race, agecat)] %>%
    .[get(x) == 1 & !is.na(race), ] %>%
    ggplot(aes(x = year, y = Prop, 
               color = factor(race, levels = c(1,2,3,4),
                              labels = c("hispanic", "white", "black", "other")))) +
    labs(color = "Race") +
    geom_line() +
    theme_minimal() +
    ylab("Weighted proportion") +
    theme(
      panel.border = element_blank(),
      panel.grid.major = element_blank(),
      panel.grid.minor = element_blank()
    )
  return(tab2)
}

调用方式优化（可选） #

指定.SDcols只传递需要的列，提升效率：

# 原始数据调用
result <- hints_dt[, tabs(.SD, x='internet_use'), .SDcols = c('inc_cat', 'year', 'nwgt0', 'internet_use')]

# NHANES测试数据调用
result <- nhanes[, tabs(.SD, x="RIAGENDR"), .SDcols = c('race', 'agecat', 'year', 'WTMEC2YR', 'RIAGENDR')]

内容的提问来源于stack exchange，提问作者Felippe Marcondes