How do I install RODBC on Mac with unixodbc and fr

After a fairly extensive search, I noticed that a number of people are having a hard time finding a start-to-finish guide that answers this question. (At least one question notes that a solution exists, but the proposed solution does not get around the fact that, by default, RODBC attempts to compile against iODBC, which is not included with Yosemite.) I just went through this process, so I thought I would document it here in the hope that it will benefit others. I am connecting to a SQL Server database.

Using Homebrew as my OS X package manager, I can install RODBC with the following steps (assuming I have already installed R).

Install unixodbc:
```
$ brew install unixodbc
```

Install freetds (replacing /usr/local/Cellar/unixodbc/2.3.2_1 with your unixodbc directory, if necessary):

$ brew install --with-tdsver=8.0 --with-msdblib --with-unixodbc=/usr/local/Cellar/unixodbc/2.3.2_1 freetds

Configure your freetds installation (the following is a minimal configuration file):

`freetds.conf`

# server specific section
[global]
;       tds version = 8.0
;       dump file = /tmp/freetds.log
;       debug flags = 0xffff
;       timeout = 10
;       connect timeout = 10
        text size = 64512

[TESTSQL]
        # insert the actual host below
        host = <xxx.xx.x.xx>
        port = 1433
        tds version = 8.0

Test the freetds config:

$ tsql -H `<xxx.xx.x.xx>` -p 1433 -U `<username>` -P `<password>`

locale is "en_US.UTF-8"
locale charset is "UTF-8"
using default charset "UTF-8"
1> exit

Configure your unixodbc installation (the following is a minimal configuration file):

$ sudo vim /usr/local/Cellar/unixodbc/2.3.2_1/etc/odbcinst.ini

`odbcinst.ini`

[MSSQL]
Description   = Microsoft SQL Server driver
Driver        = /usr/local/Cellar/freetds/0.95.18/lib/libtdsodbc.so

(and another minimal installation file):

$ sudo vim /usr/local/Cellar/unixodbc/2.3.2_1/etc/odbc.ini

`odbc.ini`

[ODBC Data Sources]
TESTSQL     = Test database

[TESTSQL]
Driver      = MSSQL
Servername  = TESTSQL
Port        = 1433
Database    = TMSEPRD
TDS_Version = 8.0

Test the new configuration with isql:

$ isql TESTSQL `<username>` `<password>`

+---------------------------------------+
| Connected!                            |
|                                       |
| sql-statement                         |
| help [tablename]                      |
| quit                                  |
|                                       |
+---------------------------------------+
SQL> quit

Create a symbolic link to the files in your home directory:

$ ln -vs /usr/local/Cellar/freetds/0.95.18/etc/freetds.conf ~/.freetds.conf
$ ln -vs /usr/local/Cellar/unixodbc/2.3.2_1/etc/odbc.ini ~/.odbc.ini
$ ln -vs /usr/local/Cellar/unixodbc/2.3.2_1/etc/odbcinst.ini ~/.odbcinst.ini

Find and modify your RProfile file by appending the following line(s) of code to the file (replacing /usr/local/include with the include directory that contains your sql.h and sqlext.h files; the second line may be unnecessary if the directory does not exist):
```
$ vim /Library/Frameworks/R.framework/Versions/3.2/Resources/library/base/R/Rprofile
Sys.setenv(ODBC_INCLUDE="/usr/local/include")
Sys.setenv(ODBC_LIBS="/usr/local/lib")
```
Now download the RODBC package source (which you an download here) to your Downloads folder.
Open a new R console session and install the package (replacing RODBC_1.3-12.tar.gz with the name of your package source):

install.packages("~/Downloads/RODBC_1.3-12.tar.gz", repos=NULL, type="source")

The package should now work:

> library(RODBC)
> myconn <- odbcConnect("TESTSQL", uid="<userid>", pwd="<password>")

Thanks to Jared Folkins and Gabi Huiber for help with figuring out what directories R looks in by default for the requisite files for RODBC.