根据条件将列的子集复制到 R 中的另一个 dataframe

Question

I have very limited R skills, and after hours searching for a solution I could not see an option that would work. I have several large data tables. From each one, I would like to copy part of a column into an dataframe, to populate a column there. My data tables (tabn1, tabn2, tabn3) all have the same format, but with different lengths. Each subset will have a different number of rows. I would want empty spaces to be filled with NA. I can't even copy the first column, so the subsequent are the next problem!

Ro  Co  Red Green   Yellow
1   3   123 999 265
1   3   223 875 5877
1   4   21488   555 478
1   4   558 23698   5558
2   3   558 559 148
2   3   4579    557 59
2   4   1489    545 2369
2   4   123 999 265
3   3   558 559 148
3   3   558 23698   5558
3   4   4579    557 59
3   4   1478 4579   557
4   3   1488    555 478
4   3   1478    2945    5889
4   4   448 259 4548
4   4   26576   158 15

My new data frame col names:

cls <- c("n1","n2","n3")

I created a dataframe with the column names:

df <- setNames(data.frame(matrix(ncol=3)),cls)

For each of my tables, I want to subset Ro > = 3, Co = 3, column "Red" only I have tried:

sub1 <- (filter(tabn1, tabn1$Ro >=3 | tabn$Co == 3)
df$n1 <- sub1$Red

> Error in `$<-.data.frame`(`*tmp*`, n1, value = c(183.94, 180.884,  : 
  replacement has 32292 rows, data has 1

Also:

df$n1 <- cut(sub1$Red)

> Error in cut.default(sub1$Red) : 
  argument "breaks" is missing, with no default

I tried using df as a datatable instead of dataframe, but also got the following errors:

df <- setNames(data.table(matrix(ncol=3)),cls)
df$n1 <- sub1$Red
> Error in set(x, j = name, value = value) : 
  Supplied 32292 items to be assigned to 1 items of column 'nn1'. If you wish to 'recycle' the RHS please use rep() to make this intent clear to readers of your code.

I would subsequently tried to subset and copy from tabn2 to df$n2, and so forth. As indicated above, the original tables have different lengths. Thanks in advance!

Answer 1

The issue is that the number of rows in 'df' and 'sub1' are different. 'df' is created with 1 row. Instead, we can create the 'df' directly from the 'sub1' itself

df <- sub1['Red']
names(df) <- cls[1]

---

Also, another way to create the data.frame, would be to specify the nrow as well

df <- as.data.frame(matrix(nrow = nrow(sub1), ncol = length(cls)),
       dimnames = list(NULL, cls))

Regarding the second error with cut , it needs breaks . Either we specify the number of breaks

cut(sub1$Red, breaks = 3)

Or a vector of break points

cut(sub1$Red, breaks = c(-Inf, 100, 500, 1000, Inf))

---

If there are many 'tabn' objects, get them into a list , loop over the list with lapply

lst1 <- mget(ls(pattern = '^tabn\\d+$'))
out_lst <- lapply(lst1, function(x) subset(x, Ro >=3 | Co == 3)$Red)

It is possible that after subset ting and selecting the 'Red' column, the number of elements may be different. If the lengths are different, a option is to pad NA at the end for those having lesser number of elements before cbind ing it

mx <- max(lengths(out_lst))
df <- do.call(cbind, lapply(out_lst, `length<-`, mx))