Split data.frame by value

You could find which values of the indexing vector equal "a", then create a grouping variable based on that and then use split.

df[,1] == "a"
# [1]  TRUE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE
#[13] FALSE FALSE
cumsum(df[,1] == "a")
# [1] 1 1 1 1 2 2 2 2 2 2 2 3 3 3
split(df, cumsum(df[,1] == "a"))
#$`1`
#  var1 var2
#1    a    1
#2    1    2
#3    2    3
#4    3    4
#
#$`2`
#   var1 var2
#5     a    5
#6     1    6
#7     2    7
#8     3    8
#9     4    9
#10    5   10
#11    6   11
#
#$`3`
#   var1 var2
#12    a   12
#13    1   13
#14    2   14