Extract a named element from an object with a default

Description

Get element name of object. If object does not have an element name, return default.

If the name element of object is NULL the result depends on warn.NULL: If TRUE, issue a warning and return default. Otherwise, return NULL

Usage

getElement2(object, name=1, default=NA, warn.NULL=TRUE, 
            envir=list(), returnName)

Arguments

Details

1. If is.numeric(name) In <- (1 <= name <= length(object))

2. else In <- if(name %in% names(object))

3. El <- if(In) object[[name]] else default

4. warn.NULL?

5. if(returnName) return(as.character(El)) else return(eval(El, envir=object))

Value

an object of the form of object[[name]]; if object does not have an element or slot name, return default.

Author(s)

Spencer Graves with help from Marc Schwartz and Hadley Wickham

See Also

getElement, which also can return slots from S4 objects.

Examples

##
## 1.  name in object, return 
##
e1 <- getElement2(list(ab=1), 'ab', 2) # 1
# check 

all.equal(e1, 1)


##
## 2.  name not in object, return default
##
eNA <- getElement2(list(), 'ab') # default default = NA
# check 

all.equal(eNA, NA)


e0 <- getElement2(list(), 'ab', 2) # name not in object

all.equal(e0, 2)


e2 <- getElement2(list(ab=1), 'a', 2) # partial matching not used 

all.equal(e2, 2)


##
## 3.  name NULL in object, return default 
##
ed <- getElement2(list(a=NULL), 'a',2) # 2 with a warning

all.equal(ed, 2)


e. <- getElement2(list(a=NULL), 'a', 2, warn.NULL=FALSE) # NULL

all.equal(e., NULL)


eNULL <- getElement2(list(a=NULL), 'a', NULL) # NULL

all.equal(eNULL, NULL)


##
## 4.  Language:  find, eval, return 
##
Qte <- quote(plot(1:4, y=x, col=c2))
if(require(pryr)){ 
  Qt <- pryr::standardise_call(Qte) # add the name 'x' 
  fn <- getElement2(Qt)
  eQuote <- getElement2(Qt, 'y')
  Col2 <- getElement2(Qt, 'col', envir=list(c2=2))
# check

  all.equal(fn, 'plot')


  all.equal(eQuote, 1:4)


  all.equal(Col2, 2)

}

Results

R version 3.3.1 (2016-06-21) -- "Bug in Your Hair"
Copyright (C) 2016 The R Foundation for Statistical Computing
Platform: x86_64-pc-linux-gnu (64-bit)

R is free software and comes with ABSOLUTELY NO WARRANTY.
You are welcome to redistribute it under certain conditions.
Type 'license()' or 'licence()' for distribution details.

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Type 'contributors()' for more information and
'citation()' on how to cite R or R packages in publications.

Type 'demo()' for some demos, 'help()' for on-line help, or
'help.start()' for an HTML browser interface to help.
Type 'q()' to quit R.

> library(Ecfun)

Attaching package: 'Ecfun'

The following object is masked from 'package:base':

    sign

> png(filename="/home/ddbj/snapshot/RGM3/R_CC/result/Ecfun/getElement2.Rd_%03d_medium.png", width=480, height=480)
> ### Name: getElement2
> ### Title: Extract a named element from an object with a default
> ### Aliases: getElement2
> ### Keywords: manip
> 
> ### ** Examples
> 
> ##
> ## 1.  name in object, return 
> ##
> e1 <- getElement2(list(ab=1), 'ab', 2) # 1
> # check 
> ## Don't show: 
> stopifnot(
+ ## End(Don't show)
+ all.equal(e1, 1)
+ ## Don't show: 
+ )
> ## End(Don't show)
> 
> ##
> ## 2.  name not in object, return default
> ##
> eNA <- getElement2(list(), 'ab') # default default = NA
> # check 
> ## Don't show: 
> stopifnot(
+ ## End(Don't show)
+ all.equal(eNA, NA)
+ ## Don't show: 
+ )
> ## End(Don't show)
> 
> e0 <- getElement2(list(), 'ab', 2) # name not in object
> ## Don't show: 
> stopifnot(
+ ## End(Don't show)
+ all.equal(e0, 2)
+ ## Don't show: 
+ )
> ## End(Don't show)
> 
> e2 <- getElement2(list(ab=1), 'a', 2) # partial matching not used 
> ## Don't show: 
> stopifnot(
+ ## End(Don't show)
+ all.equal(e2, 2)
+ ## Don't show: 
+ )
> ## End(Don't show)
> 
> ##
> ## 3.  name NULL in object, return default 
> ##
> ed <- getElement2(list(a=NULL), 'a',2) # 2 with a warning
Warning message:
In getElement2(list(a = NULL), "a", 2) :
  element a is NULL; returning default
> ## Don't show: 
> stopifnot(
+ ## End(Don't show)
+ all.equal(ed, 2)
+ ## Don't show: 
+ )
> ## End(Don't show)
> 
> e. <- getElement2(list(a=NULL), 'a', 2, warn.NULL=FALSE) # NULL
> ## Don't show: 
> stopifnot(
+ ## End(Don't show)
+ all.equal(e., NULL)
+ ## Don't show: 
+ )
> ## End(Don't show)
> 
> eNULL <- getElement2(list(a=NULL), 'a', NULL) # NULL
> ## Don't show: 
> stopifnot(
+ ## End(Don't show)
+ all.equal(eNULL, NULL)
+ ## Don't show: 
+ )
> ## End(Don't show)
> 
> ##
> ## 4.  Language:  find, eval, return 
> ##
> Qte <- quote(plot(1:4, y=x, col=c2))
> if(require(pryr)){ 
+   Qt <- pryr::standardise_call(Qte) # add the name 'x' 
+   fn <- getElement2(Qt)
+   eQuote <- getElement2(Qt, 'y')
+   Col2 <- getElement2(Qt, 'col', envir=list(c2=2))
+ # check
+ ## Don't show: 
+ stopifnot(
+ ## End(Don't show)
+   all.equal(fn, 'plot')
+ ## Don't show: 
+ )
+ ## End(Don't show)
+ ## Don't show: 
+ stopifnot(
+ ## End(Don't show)
+   all.equal(eQuote, 1:4)
+ ## Don't show: 
+ )
+ ## End(Don't show)
+ ## Don't show: 
+ stopifnot(
+ ## End(Don't show)
+   all.equal(Col2, 2)
+ ## Don't show: 
+ )
+ ## End(Don't show)
+ }
Loading required package: pryr
> 
> 
> 
> 
> 
> dev.off()
null device 
          1 
>