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POIs it possible to modify an object on a list in a parent frame in R?
 

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  1. POIs it possible to modify an object on a list in a parent frame in R?
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    <p>I'm working on an R package that has a number of functions that follow a non-R-standard practice of modifying in place the object passed in as an argument. This normally works OK, but fails when the object to be modified is on a list. </p> <p>An function to give an example of the form of the assignments:</p> <pre><code>myFun&lt;-function(x){ xn &lt;- deparse(substitute(x)) ev &lt;- parent.frame() # would do real stuff here .. # instead set simple value to modify local copy x[[1]]&lt;-"b" # assign in parent frame if (exists(xn, envir = ev)) on.exit(assign(xn, x, pos = ev)) # return invisibly invisible(x) } </code></pre> <p>This works:</p> <pre><code>&gt; myObj &lt;-list("a") &gt; myFun(myObj) &gt; myObj [[1]] [1] "b" </code></pre> <p>But it does not work if the object is a member of a list:</p> <pre><code>&gt; myObj &lt;-list("a") &gt; myList&lt;-list(myObj,myObj) &gt; myFun(myList[[1]]) &gt; myList [[1]] [[1]][[1]] [1] "a" [[2]] [[2]][[1]] [1] "a" </code></pre> <p>After reading answers to other questions here, I see the docs for <code>assign</code> clearly state:</p> <p><code>assign does not dispatch assignment methods, so it cannot be used to set elements of vectors, names, attributes, etc.</code></p> <p>Since there is an existing codebase using these functions, we cannot abandon the modify-in-place syntax. Does anyone have suggestions for workarounds or alternative approaches for modifying objects which are members of a list in a parent frame?</p> <p>UPDATE:</p> <p>I've considered trying to roll my own assignment function, something like:</p> <pre><code>assignToListInEnv&lt;-function(name,env,value){ # assume name is something like "myList[[1]]" #check for brackets index&lt;-regexpr('[[',name,fixed=TRUE)[1] if(index&gt;0){ lname&lt;-substr(name,0,index-1) #check that it exists if (exists(lname,where=env)){ target&lt;-get(lname,pos=env) # make sure it is a list if (is.list(target)){ eval(parse(text=paste('target',substr(name,index,999),'&lt;-value',sep=''))) assign(lname, target, pos = env) } else { stop('object ',lname,' is not a list in environment ',env) } } else { stop('unable to locate object ',lname,' in frame ',env) } } } </code></pre> <p>But it seems horrible brittle, would need to handle many more cases (<code>$</code> and <code>[</code> as well as <code>[[</code>) and would probably still fail for <code>[[x]]</code> because <code>x</code> would be evaluated in the wrong frame... </p>
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    1. COThe first task will be to locate them by name and you have not provided any guidance about how the R interpreter is supposed to find it/them. It is supposed to page through all the items in the workspace looking for ones that have names-attributes that contain the targets? I suspect you need to learn more about how to use environments or Reference Classes.
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    2. COI guess I don't understand the first part of you comment. I'm assuming that `myFun` doesn't know the 'real' name of the `myObj` argument passed in, just its class. Thanks for the suggestion to learn about Reference Classes, I'm not familiar with them. I like that they seem closer to Java classes, which seems appropriate for this use case. Is there a source beyond the man page to learn about them? Are you suggesting that `myObj` should be re-implemented as a Reference Class?
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    3. COYou seem to want to change component-"a" to be "b" (if it exists?) in all leaves of `myList`, but are not wanting to give the function the names of both of these items?
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