how to append results of a function in a for loop that are variable in length-CodePudding

Applying a function in a for loop gives a vector of results that are variable in length every time (for example the first iteration might give 4 numbers the 2nd might give 10 numbers etc) How can you apply a function in a for loop and save every iteration without using external to R packages?

CodePudding user response：

You can create a list (which can take elements of undefined lengths and class) beforehand, and update the list at each iteration with [[.

To do so, you can use vector with mode = "list" and define the length of the list beforehand.

In this example, I use numeric to create a numeric vector of 0s and of length i, but you can use any function.

l <- vector(mode = "list", length = 10)

for (i in 1:10){
  l[[i]] <- numeric(i)
}

output

# [[1]]
# [1] 0
# 
# [[2]]
# [1] 0 0
# 
# [[3]]
# [1] 0 0 0
# 
# [[4]]
# [1] 0 0 0 0
# 
# [[5]]
# [1] 0 0 0 0 0
# 
# [[6]]
# [1] 0 0 0 0 0 0
# 
# [[7]]
# [1] 0 0 0 0 0 0 0
# 
# [[8]]
# [1] 0 0 0 0 0 0 0 0
# 
# [[9]]
# [1] 0 0 0 0 0 0 0 0 0
# 
# [[10]]
# [1] 0 0 0 0 0 0 0 0 0 0

CodePudding user response：

This is exactly what lapply is designed for. Same effect as Mael's answer but in a single line.

lapply(seq(10), numeric)
#> [[1]]
#> [1] 0
#> 
#> [[2]]
#> [1] 0 0
#> 
#> [[3]]
#> [1] 0 0 0
#> 
#> [[4]]
#> [1] 0 0 0 0
#> 
#> [[5]]
#> [1] 0 0 0 0 0
#> 
#> [[6]]
#> [1] 0 0 0 0 0 0
#> 
#> [[7]]
#> [1] 0 0 0 0 0 0 0
#> 
#> [[8]]
#> [1] 0 0 0 0 0 0 0 0
#> 
#> [[9]]
#> [1] 0 0 0 0 0 0 0 0 0
#> 
#> [[10]]
#>  [1] 0 0 0 0 0 0 0 0 0 0

I know you were looking for only base packages, but purrr::map also does this.

purrr::map(seq(10), numeric)
#> [[1]]
#> [1] 0
#> 
#> [[2]]
#> [1] 0 0
#> 
#> [[3]]
#> [1] 0 0 0
#> 
#> [[4]]
#> [1] 0 0 0 0
#> 
#> [[5]]
#> [1] 0 0 0 0 0
#> 
#> [[6]]
#> [1] 0 0 0 0 0 0
#> 
#> [[7]]
#> [1] 0 0 0 0 0 0 0
#> 
#> [[8]]
#> [1] 0 0 0 0 0 0 0 0
#> 
#> [[9]]
#> [1] 0 0 0 0 0 0 0 0 0
#> 
#> [[10]]
#>  [1] 0 0 0 0 0 0 0 0 0 0

There is Map in base R, but I personally don't like it because the function comes first making it less "pipeable"

Map(numeric, seq(10))