Delete rows after a certain sequence of values in a certain column - r

Question

a <- c("A","A","A","B","B","B","C","C","C","C","D","D","D","D","D")
b <- c("x","y","z","x","x","z","y","z","z","z","y","z","z","z","x")
df = data.frame(a,b)
a b
1 A x
2 A y
3 A z
4 B x
5 B x
6 B z
7 C y
8 C z
9 C z
10 C z
11 D y
12 D z
13 D z
14 D z
15 D x
For every group A, B, C, D, I'd like to delete the value z in column b every time the combination y,z appears at the end of the group.
If we have the case of a=="C", where the b-values are y,z,z,z, I'd like to delete all z's. However, in a=="D", nothing has to change as x is the last value.
The results looks like this:
a b
1 A x
2 A y
4 B x
5 B x
6 B z
7 C y
11 D y
12 D z
13 D z
14 D z
15 D x
By grouping in dplyr, I can identify the last occurence of each value in A, so the basic case depictured in a=="A"is not a problem. I have trouble finding a solution for the case of a=="C", where I could have one occurence of y followed by 20 occurences of z.

Answer 1

You can use by and cummin in base R:
df[unlist(by(df$b, interaction(df$a), FUN = function(x) {
tmp <- rev(cummin(rev(x == "z")))
if (tail(x[!tmp], 1) == "y") !tmp else rep(TRUE, length(x))
})), ]
The result:
a b
1 A x
2 A y
4 B x
5 B x
6 B z
7 C y
11 D y
12 D z
13 D z
14 D z
15 D x

Answer 2

Here's a possible data.table solution. Basically, I'm creating an logical index that satisfies 3 conditions at once: being a z, that the first z comes after y and that the last value is z and then I'm just evaluating it.
library(data.table)
setDT(df)[, indx := b == "z" &
max(which(b == "z")) == .N &
ifelse(min(which(b == "z")) == 1L,
TRUE,
b[min(which(b == "z")) - 1L] == "y"),
by = a][!(indx)]
# a b indx
# 1: A x FALSE
# 2: A y FALSE
# 3: B x FALSE
# 4: B x FALSE
# 5: B z FALSE
# 6: C y FALSE
# 7: D y FALSE
# 8: D z FALSE
# 9: D z FALSE
# 10: D z FALSE
# 11: D x FALSE

Answer 3

Here's a base solution:
do.call("rbind", by(df, df$a, FUN = function(x) {
if(x$b[length(x$b)] == "z") {
y <- which(x$b == "y")
if(!length(y)) {
return(x)
}
z <- which(x$b == "z")
if(!length(z)) {
return(x)
}
# check if y isn't immediately before z
if(max(y) - min(z) > 1) {
return(x)
} else {
return(x[-z,])
}
} else {
return(x)
}
}))
And the result:
a b
A.1 A x
A.2 A y
B.4 B x
B.5 B x
B.6 B z
C C y
D.11 D y
D.12 D z
D.13 D z
D.14 D z
D.15 D x

Answer 4

Not so efficient, but works fine:
require(stringr)
df2 <- data.frame(row.names = c("a", "b"))
for(i in levels(factor(df$a))) {
temp <- paste(df$b[df$a == i], collapse = "")
if(str_detect(temp, "yz") & str_detect(temp, "z$")) {
temp <- gsub("z", "", temp)
df2 <- rbind(df2, data.frame(a = rep(i, nchar(temp)), b = substring(temp, seq(1,nchar(temp),1), seq(1,nchar(temp),1))))
} else df2 <- rbind(df2, data.frame(a = rep(i, nchar(temp)), b = substring(temp, seq(1,nchar(temp),1), seq(1,nchar(temp),1))))
}
# a b
# 1 A x
# 2 A y
# 3 B x
# 4 B x
# 5 B z
# 6 C y
# 7 D y
# 8 D z
# 9 D z
# 10 D z
# 11 D x