Get Data Frame from Character Variable

How to use a character variable to refer to a data.frame in R?

You can use get() to fetch the object, i.e.,

p <-lapply(t,function(x){x <- get(x);x[x$X == "a",]})

such that

> p
[[1]]
  X        Y
1 a 5.550481

[[2]]
  X        Y
1 a 5.365116

[[3]]
  X        Y
1 a 5.783017

[[4]]
  X        Y
1 a 2.782952

[[5]]
  X        Y
1 a 2.123357

Get data frame from character variable

I think you'll need a get in there if the input is a string. Also, depending on your usage of the function, the explicit print might not be necessary:

printdf <- function(dataframe) {
  get(dataframe)
  # print(get(dataframe))
}
head(printdf("mtcars"))
#                    mpg cyl disp  hp drat    wt  qsec vs am gear carb
# Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
# Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
# Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
# Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
# Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
# Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1

Dataframe from a character vector where variable name and its data were stored jointly

We may use read.dcf from base R

out <- type.convert(as.data.frame(read.dcf(
    textConnection(paste(gsub("\\s+\\|\\s+", "\n", foo$vars), 
    collapse="\n\n")))), as.is = TRUE)

-output

> out
  animal wks site PI  GI
1  mouse  12 cage 78  NA
2    dog  32 <NA> NA 0.2
3    cat   8 wild 13  NA
> str(out)
'data.frame':   3 obs. of  5 variables:
 $ animal: chr  "mouse" "dog" "cat"
 $ wks   : int  12 32 8
 $ site  : chr  "cage" NA "wild"
 $ PI    : int  78 NA 13
 $ GI    : num  NA 0.2 NA

Create a character variable with data.frame function

stringsAsFactors is your friend. Namely:

df = data.frame(var1 = c("1","2","3","4"),var2 = c(1,2,3,4),stringsAsFactors = F) 

yielding:

> str(df)
'data.frame':   4 obs. of  2 variables:
 $ var1: chr  "1" "2" "3" "4"
 $ var2: num  1 2 3 4

Using a character vector to refer to a data frame

It would be get to return the value of the object name as string

C <- get(B)

If there are more than objects, use mget to return the values in a list

collapse a dataframe in R that contains both numeric and character variables

Is this what you are looking for?

library(dplyr)

data %>%
  dplyr::group_by(ag, date) %>%
  summarise(across(everything(), ~
                     if(is.numeric(.x)) mean(.x) else first(.x)))

#> `summarise()` has grouped output by 'ag'. You can override using the `.groups` argument.
#> # A tibble: 12 x 6
#> # Groups:   ag [4]
#>    ag     date num_var1 num_var2 alpha_var1 alpha_var2
#>    <chr> <int>    <dbl>    <dbl> <chr>      <chr>     
#>  1 A         1        3       22 A          Y         
#>  2 A         2       11       14 I          Q         
#>  3 A         3       19        6 Q          I         
#>  4 B         1        4       21 B          X         
#>  5 B         2       12       13 J          P         
#>  6 B         3       20        5 R          H         
#>  7 C         1        5       20 C          W         
#>  8 C         2       13       12 K          O         
#>  9 C         3       21        4 S          G         
#> 10 D         1        6       19 D          V         
#> 11 D         2       14       11 L          N         
#> 12 D         3       22        3 T          F

^{Created on 2022-03-03 by the reprex package (v2.0.1)}

Dynamically select data frame columns using $ and a character value

You can't do that kind of subsetting with $. In the source code (R/src/main/subset.c) it states:

/*The $ subset operator.

We need to be sure to only evaluate the first argument.

The second will be a symbol that needs to be matched, not evaluated.

*/

Second argument? What?! You have to realise that $, like everything else in R, (including for instance ( , + , ^ etc) is a function, that takes arguments and is evaluated. df$V1 could be rewritten as

`$`(df , V1)

or indeed

`$`(df , "V1")

But...

`$`(df , paste0("V1") )

...for instance will never work, nor will anything else that must first be evaluated in the second argument. You may only pass a string which is never evaluated.

Instead use [ (or [[ if you want to extract only a single column as a vector).

For example,

var <- "mpg"
#Doesn't work
mtcars$var
#These both work, but note that what they return is different
# the first is a vector, the second is a data.frame
mtcars[[var]]
mtcars[var]

You can perform the ordering without loops, using do.call to construct the call to order. Here is a reproducible example below:

#  set seed for reproducibility
set.seed(123)
df <- data.frame( col1 = sample(5,10,repl=T) , col2 = sample(5,10,repl=T) , col3 = sample(5,10,repl=T) )

#  We want to sort by 'col3' then by 'col1'
sort_list <- c("col3","col1")

#  Use 'do.call' to call order. Seccond argument in do.call is a list of arguments
#  to pass to the first argument, in this case 'order'.
#  Since  a data.frame is really a list, we just subset the data.frame
#  according to the columns we want to sort in, in that order
df[ do.call( order , df[ , match( sort_list , names(df) ) ]  ) , ]

   col1 col2 col3
10    3    5    1
9     3    2    2
7     3    2    3
8     5    1    3
6     1    5    4
3     3    4    4
2     4    3    4
5     5    1    4
1     2    5    5
4     5    3    5

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