README.md
In deflist: Deferred List - A Read-Only List-Like Object with Deferred Access

deflist

The goal of deflist is to provide a list-like object for which element access is routed through a user-supplied function. This can be used, for example, when list-like syntax is required but one cannot afford to hold all elements in memory at once.

You can install the released version of deflist from CRAN with:

install.packages("deflist")

And the development version from GitHub with:

# install.packages("devtools")
devtools::install_github("bbuchsbaum/deflist")


library(deflist)

dl <- deflist(function(i) i, len=10)
print(dl[[1]])
#> [1] 1

dl2 <- deflist(function(i) { Sys.sleep(5); i*2  }, len=10)

for (i in 1:10) {
  print(dl2[[i]])
}
#> [1] 2
#> [1] 4
#> [1] 6
#> [1] 8
#> [1] 10
#> [1] 12
#> [1] 14
#> [1] 16
#> [1] 18
#> [1] 20

The value at an index may change across calls, for example:

dl3 <- deflist(function(i) { rnorm(1) }, len=10)
print(dl3[[1]])
#> [1] 0.3200255
print(dl3[[1]])
#> [1] 0.3937322

Memoisation can be enabled so that values at a given index are cached:

dl4 <- deflist(function(i) { rnorm(1)  }, len=10, memoise=TRUE)
print(dl4[[1]])
#> [1] 0.1447534
print(dl4[[1]])
#> [1] 0.1447534

In addition, memoisation can be set to store cached values to the file system:

dl5 <- deflist(function(i) { rnorm(1000)  }, len=10, memoise=TRUE, cache="file", cachedir = tempdir())
print(dl5[[1]][1:10])
#>  [1]  0.23399448  0.93722807 -0.34288042  0.36736236  0.87024915 -0.86566821
#>  [7]  1.57174277 -0.38036486 -0.04497923 -0.34889371
print(dl5[[1]][1:10])
#>  [1]  0.23399448  0.93722807 -0.34288042  0.36736236  0.87024915 -0.86566821
#>  [7]  1.57174277 -0.38036486 -0.04497923 -0.34889371