[R] Accessing Data Frame

Duncan Murdoch murdoch@dunc@n @ending from gm@il@com
Thu Jan 3 19:24:07 CET 2019


On 03/01/2019 12:39 p.m., Benoit Galarneau wrote:
> Thanks for the feedback.
> Point taken about the data.table package, I will take a look for sure.
> As I am new to the R programming, I'm exploring with the default
> libraries as a start.
> 
> I have various options that works like this one:
> 
> topCard <- deck[1,]
> #Remove card from deck using row name
> deck <- deck[!rownames(deck) %in% row.names(topCard),]
> 
> Is it recommended/safe/good practice to work on items using the item names?

I think the answer to "recommended or good practice" depends on your 
priorities.  If I was running a big simulation where speed really 
matters, I wouldn't do it that way:  dataframes are slow and looking for 
a name in a vector of names can be slow.  You'd get faster results using 
matrices and row indices (which can be negated to remove items, as you 
know.)  But then you have to deal with the issue that matrices can't mix 
types as your dataframe does, so your code is likely to be less clear.

If speed isn't so much of an issue but clarity is, then you really want 
to write your own small functions to do the removal, e.g.

whichCard <- function(deck, card) {
   which(deck$face == card$face & deck$suit == card$suit & deck$value = 
card$value)
}

removeCard <- function(deck, card) {
   deck[-whichCard(deck, card), ]
}

deck <- removeCard(deck, topCard)

Duncan Murdoch

> Benoit
> 
> Jeff Newmiller <jdnewmil using dcn.davis.ca.us> a écrit :
> 
>>> In my programmer's head, something similar to this should "work": ...
>>> deck[aCard]
>>
>> There are some people who agree with you... see the data.table
>> package, which can be made to behave like this.
>>
>> Keep in mind that the aCard data frame in general may have a
>> different set of column names or more than one row. (I would be
>> concerned that the logic of your application was inefficiently
>> designed if `deck` actually has the same columns as `aCard` as in
>> your example.) Others have pointed out that data frames are
>> typically combined using the merge function, which allows matching
>> columns to be specified very flexibly.
>>
>>
>> On January 3, 2019 6:50:22 AM PST, Benoit Galarneau
>> <benoit.galarneau using polymtl.ca> wrote:
>>> Hi everyone,
>>> I'm new to the R world.
>>> Probably a newbie question but I am stuck with some concept with data
>>> frame.
>>> I am following some examples in the "Hands-On Programming with R".
>>>
>>> In short, how can I access/filter items in a data frame using a
>>> variable.
>>>
>>> One example consists of manipulating elements from a deck of card:
>>>
>>>> deck
>>>      face     suit value
>>> 1   king   spades    13
>>> 2  queen   spades    12
>>> 3   jack   spades    11
>>> 4    ten   spades    10
>>> etc.
>>>
>>> Let's say I want to remove or filter out the first card. I know I
>>> could do deck[-1].
>>>
>>> But let's say I have: topCard <- deck[1,]
>>>
>>> topCard is then a list of 3 elements
>>>> topCard
>>>    face   suit value
>>> 1 king spades    13
>>>
>>> My question is the following, how can I remove or filter out the deck
>>> using the topCard variable.
>>>
>>> In my programmer's head, something similar to this should "work":
>>>> deck[10,]
>>>     face   suit value
>>> 10 four spades     4
>>>> aCard <- deck[10,]
>>>> aCard
>>>     face   suit value
>>> 10 four spades     4
>>>> deck[aCard]
>>> Error in `[.default`(deck, aCard) : invalid subscript type 'list'
>>>
>>> Wihout having to specify all elements in the logical tests.
>>>
>>> deck[deck$face == aCard$face & deck$suit == aCard$suit & deck$value ==
>>>
>>> aCard$value,]
>>>     face   suit value
>>> 10 four spades     4
>>>
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>>> http://www.R-project.org/posting-guide.html
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>>
>> --
>> Sent from my phone. Please excuse my brevity.
> 
> ______________________________________________
> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>



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