在R中的循环内创建向量

Question

I have the following code:

    generator <- function(n){
  nodes <- c()
  distances <- c()
  for (main in 1:n){
    for (i in 1:n) {
      for (j in 1:n){
        if (main != i & i != j & main != j & i < j){
          nodes <- c(nodes, paste(main, i, j, sep=",", collapse=""))
          distances <- c(distances, main+i+j)}}}}
  data <- data.frame(nodes, distances)
  return(data)}

Running generator(4), I get the following output:

      nodes distances
1  1,2,3         6
2  1,2,4         7
3  1,3,4         8
4  2,1,3         6
5  2,1,4         7
6  2,3,4         9
7  3,1,2         6
8  3,1,4         8
9  3,2,4         9
10 4,1,2         7
11 4,1,3         8
12 4,2,3         9

What I would like is to have the items in the "nodes" column be actual vectors of values, with the goal of comparing different triplets of nodes and finding the common members. It is currently just a string. So, for instance, I would like a$nodes[1][1] , to yield 1 , or something along those lines, so I can extract every individual value from the nodes triplets.

I currently have paste(main, i, j, sep=",") and I tried replacing this with c(main, i, j) but what happened is that I got a data frame of 36 rows instead of twelve: each individual node was its own row.

Thank you.

Answer 1

You need to work with list columns to do what you want to do.

I suggest working with tibbles instead of regular data frames, they're much more convenient for this type of work.

If you'd rather stick with base R you can replace data <- tibble::tibble(nodes, distances) by:

data <- data.frame(distances)
data$nodes <- nodes  

new function

generator <- function(n){
  nodes <- list()      # changed this from `c()` to `list()`
  distances <- c()
  for (main in 1:n){
    for (i in 1:n) {
      for (j in 1:n){
        if (main != i & i != j & main != j & i < j){
          nodes <- append(nodes, list(c(main, i, j))) # changed this to append a list instead of a vector
          distances <- c(distances, main+i+j)}}}}
  data <- tibble::tibble(nodes, distances) # changed `data.frame` to `tibble`
  return(data)}

output

df <- generator(4)

df
# A tibble: 12 x 2
#        nodes distances
#      <list>     <int>
#  1 <int [3]>         6
#  2 <int [3]>         7
#  3 <int [3]>         8
#  4 <int [3]>         6
#  5 <int [3]>         7
#  6 <int [3]>         9
#  7 <int [3]>         6
#  8 <int [3]>         8
#  9 <int [3]>         9
# 10 <int [3]>         7
# 11 <int [3]>         8
# 12 <int [3]>         9

df$nodes[[1]]
# [1] 1 2 3

side note

You're growing a list, which can be slow, if you run into performance issues try defining nodes with nodes <- vector("list", length = max_length) and trim it in the end with nodes <- nodes[lengths(nodes)!=0] .