R 中使用 Data.Table 滚动文本串联

Question

我有一个如下所示的数据集:

rownum<-c(1,2,3,4,5,6,7,8,9,10)
name<-c("jeff","jeff","mary","jeff","jeff","jeff","mary","mary","mary","mary")
text<-c("a","b","c","d","e","f","g","h","i","j")
a<-data.table(rownum,name,text)

我想添加一个新的文本列，该文本列按 rownum 和名称从前一列添加。新列的向量为:

rolltext<-c("a","ab","c","abd","abde","abdef","cg","cgh","cghi","cghij"

我现在不知道该怎么办。对于数字，我只会使用 cumsum 函数，但对于文本，我想我需要一个 for 循环或使用 apply 函数之一？

Answer 1

您可以将 Reduce 与 accumulate 选项结合使用:

a[, rolltext := Reduce(paste0, text, accumulate = TRUE), by = name]

    rownum name text rolltext
 1:      1 jeff    a        a
 2:      2 jeff    b       ab
 3:      3 mary    c        c
 4:      4 jeff    d      abd
 5:      5 jeff    e     abde
 6:      6 jeff    f    abdef
 7:      7 mary    g       cg
 8:      8 mary    h      cgh
 9:      9 mary    i     cghi
10:     10 mary    j    cghij

或者，正如 @DavidArenburg 建议的那样，使用 sapply 构造每一行:

a[, rolltext := sapply(1:.N, function(x) paste(text[1:x], collapse = '')), by = name]

<小时/>

这是一个运行总和，而滚动总和(在OP的标题中)是不同的，至少在R语言中是这样。