父进程何时获取子进程的退出状态，如果子进程调用exec（）(when will the parent process get exit status of the child, if child calls exec())

 在分组data.frame上使用combn的tidyverse选项： 
 
library(tidyverse)

df %>% group_by(index) %>% 
        summarise(gene = list(as_data_frame(t(combn(gene, 2))))) %>% 
        unnest(.sep = '_')

## # A tibble: 4 × 3
##   index gene_V1 gene_V2
##   <int>   <chr>   <chr>
## 1     1       a       b
## 2     1       a       c
## 3     1       b       c
## 4     2       d       e
 
 可以在基R中复制相同的逻辑： 
 
df2 <- aggregate(gene ~ index, df, function(x){t(combn(x, 2))})

do.call(rbind, apply(df2, 1, data.frame))

##   index gene.1 gene.2
## 1     1      a      b
## 2     1      a      c
## 3     1      b      c
## 4     2      d      e
 
 
 数据 
 
df <- structure(list(gene = c("a", "b", "c", "d", "e"), index = c(1L, 
    1L, 1L, 2L, 2L)), .Names = c("gene", "index"), row.names = c(NA, 
    -5L), class = "data.frame")

A tidyverse option using combn on a grouped data.frame:
 
library(tidyverse)

df %>% group_by(index) %>% 
        summarise(gene = list(as_data_frame(t(combn(gene, 2))))) %>% 
        unnest(.sep = '_')

## # A tibble: 4 × 3
##   index gene_V1 gene_V2
##   <int>   <chr>   <chr>
## 1     1       a       b
## 2     1       a       c
## 3     1       b       c
## 4     2       d       e
 
The same logic can be replicated in base R:
 
df2 <- aggregate(gene ~ index, df, function(x){t(combn(x, 2))})

do.call(rbind, apply(df2, 1, data.frame))

##   index gene.1 gene.2
## 1     1      a      b
## 2     1      a      c
## 3     1      b      c
## 4     2      d      e
 
 
Data
 
df <- structure(list(gene = c("a", "b", "c", "d", "e"), index = c(1L, 
    1L, 1L, 2L, 2L)), .Names = c("gene", "index"), row.names = c(NA, 
    -5L), class = "data.frame")