CODEWITHSUNDEEP
r
Christian
Christian

Reputation: 243

R add column based on condition of two rows

i have the following data frame, containing three groups and two values within a group:

df <- data.frame(var1 = c("A","A","B","B","C","C"),
                 var2 = c(0,1,1,1,2,1)
                )

I want to compare them for each line based on the second within the group and add a column with characters. There are some possibilities (i think) for each line:

df$var2[1] <  df$var2[2] # if TRUE write "N"
df$var2[1] >  df$var2[2] # if TRUE write "S"
df$var2[1] == df$var2[2] # if TRUE write "U"

df$var2[2] <  df$var2[1] # if TRUE write "N"
df$var2[2] >  df$var2[1] # if TRUE write "S"
df$var2[2] == df$var2[1] # if TRUE write "U"

I want to test that for each group in and add a column that mark the result:

df <- data.frame(var1 = c("A","A","B","B","C","C"),
                 var2 = c(0,1,1,1,2,1),
                 var3 = c("N","S","U","U","S","N")
                )

Hope somebody can help!

Upvotes: 0

Views: 61

Answers (1)

phiver
phiver

Reputation: 23608

Answer based on adjusted rules:

with dplyr:

library(dplyr)

df %>% 
  group_by( var1 ) %>%
  mutate( var3 = case_when(
    var2 < lead(var2) | var2 < lag(var2)  ~ "N",
    var2 > lead(var2) | var2 > lag(var2) ~ "S",
    var2 == lead(var2) | var2 == lag(var2) ~ "U" 
  ))
# A tibble: 6 x 3
# Groups:   var1 [3]
  var1   var2 var3 
  <fct> <dbl> <chr>
1 A         0 N    
2 A         1 S    
3 B         1 U    
4 B         1 U    
5 C         2 S    
6 C         1 N

with data.table:

library(data.table)
dt <- setDT(df)
dt[, var3 := ifelse(var2 < shift(var2, n=1L, fill=0, type="lead") | var2 < shift(var2, n=1L, fill=0, type="lag"), 
                     "N", 
                     ifelse(var2 == shift(var2, n=1L, fill=0, type="lead") | var2 == shift(var2, n=1L, fill=0, type="lag"), 
                             "U", 
                             "S" )),
   by = var1]

dt
   var1 var2 var3
1:    A    0    N
2:    A    1    S
3:    B    1    U
4:    B    1    U
5:    C    2    S
6:    C    1    N

Upvotes: 1

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