在R中某个值之后删除行

Question

I have a data frame in R,

df <- data.frame(a=c(1,1,1,2,2,5,5,5,5,5,6,6), b=c(0,1,0,0,0,0,0,1,0,0,0,1))

I want to remove the rows which has values for the variable b equal to 0 which occurs after the value equals to 1 for the duplicated variable a values.

So the output I am looking for is,

df.out <- data.frame(a=c(1,1,2,2,5,5,5,6,6), b=c(0,1,0,0,0,0,1,0,1))

Is there a way to do this in R?

Answer 1

This should do the trick?

ind = intersect(which(df$b==0), which(df$b==1)+1)
df.out = df[-ind,]

The which(df$b==1) returns the index of the df where b==1. add one to this and intersect with the indexes where b==0.

Answer 2

How about

df[ ave(df$b, df$a, FUN=function(x) x>=cummax(x))==1, ]

#        a b
#     1  1 0
#     2  1 1
#     4  2 0
#     5  2 0
#     6  5 0
#     7  5 0
#     8  5 1
#     11 6 0
#     12 6 1

Here we use ave to look within each level of a and we test to see if we've seen a 1 yet with cummax .