filehash (version 2.3)

filehash-class: Class "filehash"

Description

These functions form the interface for a simple file-based key-value database (i.e. hash table).

Objects from the Class

Objects can be created by calls of the form new("filehash", ...).

Slots

name:

Object of class "character", name of the database.

Additional slots for "filehashDB1"

datafile:

full path to the database file.

meta:

list containing an environment for database metadata.

Additional slots for "filehashRDS"

dir:

Directory where files are stored.

Methods

dbDelete

The dbDelete function is for deleting elements, but for the "DB1" format all it does is remove the key from the lookup table. The actual data are still in the database (but inaccessible). If you reinsert data for the same key, the new data are simply appended on to the end of the file. Therefore, it's possible to have multiple copies of data lying around after a while, potentially making the database file big. The "RDS" format does not have this problem.

dbExists

check to see if a key exists.

dbFetch

retrieve the value associated with a given key.

dbMultiFetch

retrieve values associated with multiple keys (a list of those values is returned).

dbInsert

insert a key-value pair into the database. If that key already exists, its associated value is overwritten. For "RDS" type databases, there is a safe option (defaults to TRUE) which allows the user to insert objects somewhat more safely (objects should not be lost in the event of an interrupt).

dbList

list all keys in the database.

dbReorganize

The dbReorganize function is there for the purpose of rewriting the database to remove all of the stale entries. Basically, this function creates a new copy of the database and then overwrites the old copy. This function has not been tested extensively and so should be considered experimental. dbReorganize is not needed when using the "RDS" format.

dbUnlink

delete an entire database from the disk

show

print method

with

allows with to be used with "filehash" objects much like it can be used with lists or data frames

[[,[[<-

elements of a database can be accessed using the [[ operator much like a list or environment, but only character indices are allowed

$,$<-

elements of a database can be accessed using the $ operator much like with a list or environment

lapply

works much like lapply with lists; a list is returned.

names

returns all of the keys in the database

length

returns the number of elements in the database

Examples

dbCreate("myDB")  ## Create database 'myDB'
db <- dbInit("myDB")
dbInsert(db, "a", 1:10)
dbInsert(db, "b", rnorm(1000))
dbExists(db, "b")  ## 'TRUE'

dbList(db)  ## c("a", "b")
dbDelete(db, "a")
dbList(db) ## "b"

with(db, mean(b))