# [R] Efficient means to link two data frames

Dennis Fisher fisher at plessthan.com
Thu Sep 29 15:38:39 CEST 2016

```R 3.3.1
OS X

Colleagues,

I have two large data frames that I am trying to link efficiently.   A small example is as follows:

structure(list(Day = c(1L, 2L, 3L, 5L, 6L, 7L, 8L, 9L, 10L, 11L,
13L, 14L, 16L, 17L, 18L, 19L, 20L, 21L, 22L, 23L, 24L, 25L, 26L,
27L, 28L, 29L, 30L), Value = c(76, 116, 111, 103, 114, 99, 128,
96, 81, 84, 81, 108, 109, 106, 125, 128, 92, 90, 83, 89, 76,
89, 101, 93, 98, 77, 92)), .Names = c("Day", "Value"), row.names = c(NA,
-27L), class = "data.frame")

which becomes:
Day Value
1    1    76
2    2   116
3    3   111
4    5   103
5    6   114
6    7    99
7    8   128
8    9    96
9   10    81
10  11    84
11  13    81
12  14   108
13  16   109
14  17   106
15  18   125
16  19   128
17  20    92
18  21    90
19  22    83
20  23    89
21  24    76
22  25    89
23  26   101
24  27    93
25  28    98
26  29    77
27  30    92

The second dataframe is merely:
TESTDAYS			<- data.frame(TestDay = c(4, 11, 15))

For each row in the second dataframe, I would like to identify the first row in the first dataframe in which Day is >= TestDay.
For example, for TestDay == 4, Day would equal 5.  I would then append the corresponding “Value” in the TestValue column
The result would be:
TestDay TestValue
1       4       103
2      11        84
3      15       109

I can accomplish this with brute force but I suspect that there is some clever day to vectorize this.  Any help would be appreciated.

Dennis

Dennis Fisher MD
P < (The "P Less Than" Company)
Phone / Fax: 1-866-PLessThan (1-866-753-7784)
www.PLessThan.com <http://www.plessthan.com/>

[[alternative HTML version deleted]]

```