What is the R function `c( )` coercing when inputs are numeric and bigq?-CodePudding

Consider these two contrasting examples:

> library(Rmpfr)
  > K = 1:5
    > denoms = c(as.bigz(1), as.bigq(rep(1,times = length(K)), K) )
    #Big Rational ('bigq') object of length 6:
    #[1] 1   1   1/2 1/3 1/4 1/5
    > foo = c(1, as.bigq(rep(1,times = length(K)), K) )
    
    # [1] 1 5 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1
    # [39] 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0    

    > bar <- c(as.bigz(1), as.bigq(rep(1,times = length(K)), K) )
    
    #Big Rational ('bigq') object of length 6:
    # [1] 1   1   1/2 1/3 1/4 1/5

I understand that the method c.bigq may be incomplete, leading to failure of the desired interpretation and coercion of data types. My question is: how did some bigq objects get turned into those vectors of 1s and 0s ? The output foo is numeric.

CodePudding user response：

The dispatch on c() is done based on the class of the first element, so c(1, as.bigq(rep(1,times = length(K)), K) ) calls the default method.

The second parameter as.bigq(rep(1,times = length(K)), K) has class "bigq", but that's irrelevant, because the default method ignores the class. It just looks at the type, and typeof(as.bigq(rep(1,times = length(K)), K)) gives "raw", so it's stored as raw bytes. If you call unclass() on it, you'll see something like

 [1] 05 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 01 00 00
[40] 00 01 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00
attr(,"denominator")
 [1] 05 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 02 00 00 00 01 00 00 00 01 00 00 00 03 00 00
[40] 00 01 00 00 00 01 00 00 00 04 00 00 00 01 00 00 00 01 00 00 00 05 00 00 00

so that's where all those numbers came from.

On the other hand, c(as.bigz(1), as.bigq(rep(1,times = length(K)), K) ) has a bigz object as the first argument, so it will call c.bigz. That method knows about bigz and bigq objects and does the right thing with them.