Purpose
I came across 350 commonly used functions by Hadley Wickham. I thought atleast I should know the basic vocabulory of R. Going over the list, I figured out that there are quite a number of functions that I have no clue on

Ordering and tabulating
I will code atleast a few lines of code duplicated, unique, merge,order, rank, quantile, sort,table, ftable

duplicated function

> x <- c(9:20, 1:5, 3:7, 0:8)
> cbind(x, duplicated(x))
       x
 [1,]  9 0
 [2,] 10 0
 [3,] 11 0
 [4,] 12 0
 [5,] 13 0
 [6,] 14 0
 [7,] 15 0
 [8,] 16 0
 [9,] 17 0
[10,] 18 0
[11,] 19 0
[12,] 20 0
[13,]  1 0
[14,]  2 0
[15,]  3 0
[16,]  4 0
[17,]  5 0
[18,]  3 1
[19,]  4 1
[20,]  5 1
[21,]  6 0
[22,]  7 0
[23,]  0 0
[24,]  1 1
[25,]  2 1
[26,]  3 1
[27,]  4 1
[28,]  5 1
[29,]  6 1
[30,]  7 1
[31,]  8 0
> cbind(rev(x), duplicated(x, fromLast = T))
      [,1] [,2]
 [1,]    8    0
 [2,]    7    0
 [3,]    6    0
 [4,]    5    0
 [5,]    4    0
 [6,]    3    0
 [7,]    2    0
 [8,]    1    0
 [9,]    0    0
[10,]    7    0
[11,]    6    0
[12,]    5    0
[13,]    4    1
[14,]    3    1
[15,]    5    1
[16,]    4    1
[17,]    3    1
[18,]    2    1
[19,]    1    1
[20,]   20    1
[21,]   19    1
[22,]   18    1
[23,]   17    0
[24,]   16    0
[25,]   15    0
[26,]   14    0
[27,]   13    0
[28,]   12    0
[29,]   11    0
[30,]   10    0
[31,]    9    0

unique

> unique(x)
 [1]  9 10 11 12 13 14 15 16 17 18 19 20  1  2  3  4  5  6  7  0  8
> unique(x, fromLast = T)
 [1]  9 10 11 12 13 14 15 16 17 18 19 20  0  1  2  3  4  5  6  7  8

merge

> authors <- data.frame(surname = I(c("Tukey", "Venables", "Tierney",
+     "Ripley", "McNeil")), nationality = c("US", "Australia",
+     "US", "UK", "Australia"), deceased = c("yes", rep("no", 4)))
> books <- data.frame(name = I(c("Tukey", "Venables", "Tierney",
+     "Ripley", "Ripley", "McNeil", "R Core")), title = c("Exploratory Data Analysis",
+     "Modern Applied Statistics ...", "LISP-STAT", "Spatial Statistics",
+     "Stochastic Simulation", "Interactive Data Analysis", "An Introduction to R"),
+     other.author = c(NA, "Ripley", NA, NA, NA, NA, "Venables & Smith"))
> merge(authors, books, by.x = "surname", by.y = "name")
   surname nationality deceased                         title other.author
1   McNeil   Australia       no     Interactive Data Analysis         <NA>
2   Ripley          UK       no            Spatial Statistics         <NA>
3   Ripley          UK       no         Stochastic Simulation         <NA>
4  Tierney          US       no                     LISP-STAT         <NA>
5    Tukey          US      yes     Exploratory Data Analysis         <NA>
6 Venables   Australia       no Modern Applied Statistics ...       Ripley
> merge(authors, books, by.x = "surname", by.y = "name", all = T)
   surname nationality deceased                         title     other.author
1   McNeil   Australia       no     Interactive Data Analysis             <NA>
2   R Core        <NA>     <NA>          An Introduction to R Venables & Smith
3   Ripley          UK       no            Spatial Statistics             <NA>
4   Ripley          UK       no         Stochastic Simulation             <NA>
5  Tierney          US       no                     LISP-STAT             <NA>
6    Tukey          US      yes     Exploratory Data Analysis             <NA>
7 Venables   Australia       no Modern Applied Statistics ...           Ripley
> x <- data.frame(k1 = c(NA, NA, 3, 4, 5), k2 = c(1, NA, NA, 4,
+     5), data = 1:5)
> y <- data.frame(k1 = c(NA, 2, NA, 4, 5), k2 = c(NA, NA, 3, 4,
+     5), data = 1:5)
> x
  k1 k2 data
1 NA  1    1
2 NA NA    2
3  3 NA    3
4  4  4    4
5  5  5    5
> y
  k1 k2 data
1 NA NA    1
2  2 NA    2
3 NA  3    3
4  4  4    4
5  5  5    5
> merge(x, y, by = c("k1", "k2"))
  k1 k2 data.x data.y
1  4  4      4      4
2  5  5      5      5
3 NA NA      2      1
> merge(x, y, by = c("k1", "k2"), incomparables = NA)
  k1 k2 data.x data.y
1  4  4      4      4
2  5  5      5      5
3 NA NA      2      1
> merge(x, y, by = "k1")
  k1 k2.x data.x k2.y data.y
1  4    4      4    4      4
2  5    5      5    5      5
3 NA    1      1   NA      1
4 NA    1      1    3      3
5 NA   NA      2   NA      1
6 NA   NA      2    3      3
> merge(x, y, by = "k2", incomparables = NA)
  k2 k1.x data.x k1.y data.y
1  4    4      4    4      4
2  5    5      5    5      5

order , sort ,quantile, table
Have used it numerous times

ftable

> y <- mtcars[c("cyl", "vs", "am", "gear")]
> x <- ftable(y)
> x
          gear  3  4  5
cyl vs am
4   0  0        0  0  0
       1        0  0  1
    1  0        1  2  0
       1        0  6  1
6   0  0        0  0  0
       1        0  2  1
    1  0        2  2  0
       1        0  0  0
8   0  0       12  0  0
       1        0  0  2
    1  0        0  0  0
       1        0  0  0