[R] Accessing Data Frame

[Berry, Charles]

See below.

> On Jan 3, 2019, at 6:50 AM, 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

matchRow <- Reduce( "&", mapply("==", deck, aCard, SIMPLIFY=FALSE))

deck[ !matchRow, ]

There is a little `trick' here. A "data.frame" object is a "list" object and both `deck` and `aCard` are "data.frame" objects. So using `mapply` as above operates on the successive columns of each.  


You will find many useful idioms using the *apply functions. 

As you are new to R, I recommend running the example() for each of them as well as browsing the help page.

HTH,

Chuck