berryFunctions (version 1.12.3)

getColumn: get column from data.frame

Description

Extract columns if they are given in a data frame. Watch out not to define objects with the same name as x if you are using getColumn in a function!

Usage

getColumn(x, df, trace = TRUE)

Arguments

x

Column name to be subsetted. The safest is to use character strings or substitute(input). If there is an object "x" in a function environment, its value will be used as name! (see upper2 example)

df

dataframe object

trace

Logical: Add function call stack to the message? DEFAULT: TRUE WARNING: in do.call settings with large objects, tracing may take a lot of computing time.

Value

Vector (or array, factor, etc) with values in the specified column

See Also

subset, https://mran.revolutionanalytics.com/web/packages/car/vignettes/embedding.pdf

Examples

getColumn(Air.Flow, stackloss)
getColumn(2, stackloss)
getColumn("2",  stackloss) # works too...
is.error(   getColumn(Acid, stackloss)   , tell=TRUE)
is.error(   getColumn(2:3,  stackloss)   , tell=TRUE)
 
upper <- function(x) getColumn(x, stackloss)
upper(Water.Temp)
upper(2)
# upper(Water) # error with useful message (design choice: partial matching not supported)

upper2 <- function(xx) {xx <- 17; getColumn(xx, stackloss)} # will break!
stopifnot(is.error(      upper2(Water.Temp)       )) # breaks

upper3 <- function(xx, dd) getColumn(substitute(xx), dd)
upper3(Air.Flow, stackloss) # may be safer in many scoping situations

# In packages use "colname" with quotation marks in level 2 functions to avoid 
# the CRAN check NOTE "no visible binding for global variable"

df <- data.frame(x=letters[1:3],y=letters[4:6]) 
is.vector(df$x)
is.vector(getColumn("x", df)) # FALSE
# cannot force output to be a vector, as this will convert:
as.Date("2016-09-14")  ;  as.vector(as.Date("2016-09-14"))
# same problem with dfs from tapply results
# better ideas welcome!! ([e-mail])