Variable shows values but str shows NULL in r. How to check structure of such variable?

Question

require(MTS)
rt=rnorm(200)
b<-archTest(rt, lag = 10)

This code results in the values

Q(m) of squared series(LM test):  
Test statistic:  7.694531  p-value:  0.6586466 
Rank-based Test:  
Test statistic:  20.80503  p-value:  0.02249487

But when the structure of b is checked using typeof(b) and str(b), it gives NULL.

How is it possible? How can one knows the structure of b as it is needed to extract values from this variable.

Answer 1

If you look at the source code of archTest, you're question boils down to more or less this one: can I stop cat from returning NULL? To which the answer is no.

You can modify archTest however such that it returns the test statistic and p-value (see below). b is a named vector.

b <- my_archTest(rt, lag = 10)
str(b)
# Named num [1:2] 7.565 0.671
# - attr(*, "names")= chr [1:2] "Test statistic" "p-value"
b
#Test statistic        p-value 
#     7.5653283      0.6712114
b["Test statistic"]
#Test statistic 
#      7.565328

I added three lines at the end of MTS::archTest.

my_archTest <- function (rt, lag = 10) {
  at = rt
  if (is.matrix(at)) 
    at = at[, 1]
  m1 = acf(at^2, lag.max = lag, plot = F)
  acf = m1$acf[2:(lag + 1)]
  nT = length(at)
  c1 = c(1:lag)
  deno = rep(nT, lag) - c1
  Q = sum(acf^2/deno) * nT * (nT + 2)
  pv1 = 1 - pchisq(Q, lag)
  cat("Q(m) of squared series(LM test): ", "\n")
  cat("Test statistic: ", Q, " p-value: ", pv1, "\n")
  rk = rank(at^2)
  m2 = acf(rk, lag.max = lag, plot = F)
  acf = m2$acf[2:(lag + 1)]
  mu = -(rep(nT, lag) - c(1:lag))/(nT * (nT - 1))
  v1 = rep(5 * nT^4, lag) - (5 * c(1:lag) + 9) * nT^3 + 9 * 
    (c(1:lag) - 2) * nT^2 + 2 * c(1:lag) * (5 * c(1:lag) + 
                                              8) * nT + 16 * c(1:lag)^2
  v1 = v1/(5 * (nT - 1)^2 * nT^2 * (nT + 1))
  QR = sum((acf - mu)^2/v1)
  pv2 = 1 - pchisq(QR, lag)
  cat("Rank-based Test: ", "\n")
  cat("Test statistic: ", QR, " p-value: ", pv2, "\n")

  out <- c("Test statistic" = QR, # new lines added here
           "p-value" = pv2)
  return(out)

# or instead simply
# c("Test statistic" = QR,
#   "p-value" = pv2)
}

As pointed out by @42- in the comments, the return call is not necessary.