There are too many ways of looping in R data structures. Here is an unfinished summary of them.

Table of Contents

Looping Basic

mylist = list(10, 20)
for (e in mylist) print(e)
  #> [1] 10
  #> [1] 20

myvec = c(10, 20)
for (e in myvec) print (e)
  #> [1] 10
  #> [1] 20

myvec = c(10, 20)
for (i in seq_along(myvec)) print (myvec[i])
  #> [1] 10
  #> [1] 20

dfa = data.frame( a_id = 1:3, a_title = letters[1:3], b_id = c(10,10,11) )
for (i in 1:nrow(dfa))
  print(dfa[i, ])
  #>   a_id a_title b_id
  #> 1    1       a   10
  #>   a_id a_title b_id
  #> 2    2       b   10
  #>   a_id a_title b_id
  #> 3    3       c   11

dfa = data.frame( a_id = 1:3, a_title = letters[1:3], b_id = c(10,10,11) )
rutils::applyr(dfa, print)
  #>    a_id a_title    b_id 
  #>     "1"     "a"    "10" 
  #>    a_id a_title    b_id 
  #>     "2"     "b"    "10" 
  #>    a_id a_title    b_id 
  #>     "3"     "c"    "11"

Looping over Data Frame Rows

Setup

library(dplyr, warn.conflicts = F)
dfa = data_frame( a_id = 1:3, a_title = letters[1:3], b_id = c(10,10,11) )
dfb = data_frame( b_id = 10:12, b_title = letters[10:12] )

When Can We Not Use Loop Functions?

There are cases when we cannot use loop functions like lapply, apply, Map

  1. When we loop over each distinct value of a field of a table. See: ex_sql_generation.Rmd