diff --git a/.ci/.lintr.R b/.ci/.lintr.R
index bfc454768..a62745b15 100644
--- a/.ci/.lintr.R
+++ b/.ci/.lintr.R
@@ -38,8 +38,6 @@ linters = c(dt_linters, all_linters(
   function_argument_linter = NULL,
   indentation_linter = NULL,
   infix_spaces_linter = NULL,
-  # TODO(R>3.2.0): Activate this, extending to recognize vapply_1i(x, length).
-  lengths_linter = NULL,
   line_length_linter = NULL,
   missing_package_linter = NULL,
   namespace_linter = NULL,
@@ -57,26 +55,20 @@ linters = c(dt_linters, all_linters(
   # TODO(michaelchirico): Enforce these and re-activate them one-by-one.
   brace_linter = NULL,
   condition_call_linter = NULL,
-  conjunct_test_linter = NULL,
   fixed_regex_linter = NULL,
-  function_left_parentheses_linter = NULL,
   if_not_else_linter = NULL,
   implicit_assignment_linter = NULL,
   implicit_integer_linter = NULL,
   keyword_quote_linter = NULL,
-  length_levels_linter = NULL,
-  matrix_apply_linter = NULL,
   missing_argument_linter = NULL,
   nzchar_linter = NULL,
   object_overwrite_linter = NULL,
   paren_body_linter = NULL,
   redundant_equals_linter = NULL,
   return_linter = NULL,
-  sample_int_linter = NULL,
   scalar_in_linter = NULL,
   undesirable_function_linter = NULL,
   unnecessary_concatenation_linter = NULL,
-  unnecessary_lambda_linter = NULL,
   unnecessary_nesting_linter = NULL,
   unreachable_code_linter = NULL,
   unused_import_linter = NULL
@@ -98,7 +90,8 @@ exclusions = c(local({
       undesirable_function_linter = Inf
     )),
     exclusion_for_dir("vignettes", list(
-      quotes_linter = Inf
+      quotes_linter = Inf,
+      sample_int_linter = Inf
       # strings_as_factors_linter = Inf
       # system_time_linter = Inf
     )),
@@ -112,7 +105,9 @@ exclusions = c(local({
       equals_na_linter = Inf,
       paste_linter = Inf,
       rep_len_linter = Inf,
-      seq_linter = Inf
+      sample_int_linter = Inf,
+      seq_linter = Inf,
+      unnecessary_lambda_linter = Inf
     ))
   )
 }),
diff --git a/R/IDateTime.R b/R/IDateTime.R
index 185952fe7..712fff1d8 100644
--- a/R/IDateTime.R
+++ b/R/IDateTime.R
@@ -83,7 +83,7 @@ as.list.IDate = function(x, ...) NextMethod()
 ## round.IDate = function (x, digits, units=digits, ...) {
 ##     if (missing(digits)) digits = units # workaround to provide a units argument to match the round generic and round.POSIXt
 ##     units = match.arg(digits, c("weeks", "months", "quarters", "years"))
-round.IDate = function (x, digits=c("weeks", "months", "quarters", "years"), ...) {
+round.IDate = function(x, digits=c("weeks", "months", "quarters", "years"), ...) {
   units = match.arg(digits)
   as.IDate(switch(units,
           weeks  = round(x, "year") + 7L * (yday(x) %/% 7L),
@@ -93,7 +93,7 @@ round.IDate = function (x, digits=c("weeks", "months", "quarters", "years"), ...
 }
 
 #Adapted from `+.Date`
-`+.IDate` = function (e1, e2) {
+`+.IDate` = function(e1, e2) {
   if (nargs() == 1L)
     return(e1)
   # TODO: investigate Ops.IDate method a la Ops.difftime
@@ -108,7 +108,7 @@ round.IDate = function (x, digits=c("weeks", "months", "quarters", "years"), ...
   (setattr(as.integer(unclass(e1) + unclass(e2)), "class", c("IDate", "Date")))  # () wrap to return visibly
 }
 
-`-.IDate` = function (e1, e2) {
+`-.IDate` = function(e1, e2) {
   if (!inherits(e1, "IDate")) {
     if (inherits(e1, 'Date')) return(base::`-.Date`(e1, e2))
     stopf("can only subtract from \"IDate\" objects")
@@ -228,7 +228,7 @@ print.ITime = function(x, ...) {
   print(format(x))
 }
 
-rep.ITime = function (x, ...)
+rep.ITime = function(x, ...)
 {
   y = rep(unclass(x), ...)
   class(y) = "ITime"   # unlass and rep could feasibly not copy, hence use class<- not setattr()
diff --git a/R/data.table.R b/R/data.table.R
index aba3dacd3..ce5da744a 100644
--- a/R/data.table.R
+++ b/R/data.table.R
@@ -129,7 +129,7 @@ replace_dot_alias = function(e) {
   }
 }
 
-"[.data.table" = function (x, i, j, by, keyby, with=TRUE, nomatch=NA, mult="all", roll=FALSE, rollends=if (roll=="nearest") c(TRUE,TRUE) else if (roll>=0) c(FALSE,TRUE) else c(TRUE,FALSE), which=FALSE, .SDcols, verbose=getOption("datatable.verbose"), allow.cartesian=getOption("datatable.allow.cartesian"), drop=NULL, on=NULL, env=NULL)
+"[.data.table" = function(x, i, j, by, keyby, with=TRUE, nomatch=NA, mult="all", roll=FALSE, rollends=if (roll=="nearest") c(TRUE,TRUE) else if (roll>=0) c(FALSE,TRUE) else c(TRUE,FALSE), which=FALSE, .SDcols, verbose=getOption("datatable.verbose"), allow.cartesian=getOption("datatable.allow.cartesian"), drop=NULL, on=NULL, env=NULL)
 {
   # ..selfcount <<- ..selfcount+1  # in dev, we check no self calls, each of which doubles overhead, or could
   # test explicitly if the caller is [.data.table (even stronger test. TO DO.)
@@ -1920,7 +1920,7 @@ replace_dot_alias = function(e) {
     attrs = attr(x, 'index', exact=TRUE)
     skeys = names(attributes(attrs))
     if (!is.null(skeys)) {
-      hits  = unlist(lapply(paste0("__", names_x[cols]), function(x) grep(x, skeys, fixed = TRUE)))
+      hits  = unlist(lapply(paste0("__", names_x[cols]), grep, skeys, fixed=TRUE))
       hits  = skeys[unique(hits)]
       for (i in seq_along(hits)) setattr(attrs, hits[i], NULL) # does by reference
     }
@@ -2074,19 +2074,17 @@ as.matrix.data.table = function(x, rownames=NULL, rownames.value=NULL, ...) {
     if (is.ff(X[[j]])) X[[j]] = X[[j]][]   # nocov to bring the ff into memory, since we need to create a matrix in memory
     xj = X[[j]]
     if (length(dj <- dim(xj)) == 2L && dj[2L] > 1L) {
-      if (inherits(xj, "data.table"))
+      if (is.data.table(xj))
         xj = X[[j]] = as.matrix(X[[j]])
       dnj = dimnames(xj)[[2L]]
-      collabs[[j]] = paste(collabs[[j]], if (length(dnj) >
-        0L)
-        dnj
-      else seq_len(dj[2L]), sep = ".")
+      collabs[[j]] = paste(
+        collabs[[j]],
+        if (length(dnj) > 0L) dnj else seq_len(dj[2L]), sep = ".")
     }
     if (!is.logical(xj))
       all.logical = FALSE
-    if (length(levels(xj)) > 0L || !(is.numeric(xj) || is.complex(xj) || is.logical(xj)) ||
-        (!is.null(cl <- attr(xj, "class", exact=TRUE)) && any(cl %chin%
-        c("Date", "POSIXct", "POSIXlt"))))
+    if (nlevels(xj) > 0L || !(is.numeric(xj) || is.complex(xj) || is.logical(xj)) ||
+        (!is.null(cl <- attr(xj, "class", exact=TRUE)) && any(cl %chin% c("Date", "POSIXct", "POSIXlt"))))
       non.numeric = TRUE
     if (!is.atomic(xj))
       non.atomic = TRUE
@@ -2104,7 +2102,7 @@ as.matrix.data.table = function(x, rownames=NULL, rownames.value=NULL, ...) {
       if (is.character(X[[j]])) next
       xj = X[[j]]
       miss = is.na(xj)
-      xj = if (length(levels(xj))) as.vector(xj) else format(xj)
+      xj = if (nlevels(xj)) as.vector(xj) else format(xj)
       is.na(xj) = miss
       X[[j]] = xj
     }
@@ -2136,7 +2134,7 @@ tail.data.table = function(x, n=6L, ...) {
   x[i]
 }
 
-"[<-.data.table" = function (x, i, j, value) {
+"[<-.data.table" = function(x, i, j, value) {
   # [<- is provided for consistency, but := is preferred as it allows by group and by reference to subsets of columns
   # with no copy of the (very large, say 10GB) columns at all. := is like an UPDATE in SQL and we like and want two symbols to change.
   if (!cedta()) {
@@ -2240,7 +2238,7 @@ dimnames.data.table = function(x) {
   list(NULL, names(x))
 }
 
-"dimnames<-.data.table" = function (x, value)   # so that can do  colnames(dt)=<..>  as well as names(dt)=<..>
+"dimnames<-.data.table" = function(x, value)   # so that can do  colnames(dt)=<..>  as well as names(dt)=<..>
 {
   if (!cedta()) return(`dimnames<-.data.frame`(x,value))  # nocov ; will drop key but names<-.data.table (below) is more common usage and does retain the key
   if (!is.list(value) || length(value) != 2L) stopf("attempting to assign invalid object to dimnames of a data.table")
@@ -2264,7 +2262,7 @@ dimnames.data.table = function(x) {
   x   # this returned value is now shallow copied by R 3.1.0 via *tmp*. A very welcome change.
 }
 
-within.data.table = function (data, expr, ...)
+within.data.table = function(data, expr, ...)
 # basically within.list but retains key (if any)
 # will be slower than using := or a regular query (see ?within for further info).
 {
@@ -2289,7 +2287,7 @@ within.data.table = function (data, expr, ...)
   ans
 }
 
-transform.data.table = function (`_data`, ...)
+transform.data.table = function(`_data`, ...)
 # basically transform.data.frame with data.table instead of data.frame, and retains key
 {
   if (!cedta()) return(NextMethod()) # nocov
@@ -2299,7 +2297,7 @@ transform.data.table = function (`_data`, ...)
   `_data`
 }
 
-subset.data.table = function (x, subset, select, ...)
+subset.data.table = function(x, subset, select, ...)
 {
   key.cols = key(x)
 
@@ -2348,7 +2346,7 @@ subset.data.table = function (x, subset, select, ...)
 is_na = function(x, by=seq_along(x)) .Call(Cdt_na, x, by)
 any_na = function(x, by=seq_along(x)) .Call(CanyNA, x, by)
 
-na.omit.data.table = function (object, cols = seq_along(object), invert = FALSE, ...) {
+na.omit.data.table = function(object, cols = seq_along(object), invert = FALSE, ...) {
   # compare to stats:::na.omit.data.frame
   if (!cedta()) return(NextMethod()) # nocov
   if ( !missing(invert) && is.na(as.logical(invert)) )
@@ -2374,7 +2372,7 @@ which_ = function(x, bool = TRUE) {
   .Call(Cwhichwrapper, x, bool)
 }
 
-is.na.data.table = function (x) {
+is.na.data.table = function(x) {
   if (!cedta()) return(`is.na.data.frame`(x))
   do.call("cbind", lapply(x, "is.na"))
 }
diff --git a/R/fcast.R b/R/fcast.R
index 014bd76d4..d2423cd31 100644
--- a/R/fcast.R
+++ b/R/fcast.R
@@ -251,5 +251,5 @@ dcast.data.table = function(data, formula, fun.aggregate = NULL, sep = "_", ...,
     setattr(ans, 'names', c(lhsnames, allcols))
     setDT(ans); setattr(ans, 'sorted', lhsnames)
   } else stopf("Internal error -- empty rhsnames in dcast; please report") # nocov
-  return (ans)
+  return(ans)
 }
diff --git a/R/fdroplevels.R b/R/fdroplevels.R
index c7025dda0..69f23cb61 100644
--- a/R/fdroplevels.R
+++ b/R/fdroplevels.R
@@ -1,7 +1,7 @@
 # 647 fast droplevels.data.table method
 fdroplevels = function(x, exclude = if (anyNA(levels(x))) NULL else NA, ...) {
   stopifnot(inherits(x, "factor"))
-  lev = which(tabulate(x, length(levels(x))) & (!match(levels(x), exclude, 0L)))
+  lev = which(tabulate(x, nlevels(x)) & (!match(levels(x), exclude, 0L)))
   ans = match(as.integer(x), lev)
   setattr(ans, 'levels', levels(x)[lev])
   setattr(ans, 'class', class(x))
diff --git a/R/foverlaps.R b/R/foverlaps.R
index 54dc61f93..a7d207428 100644
--- a/R/foverlaps.R
+++ b/R/foverlaps.R
@@ -179,10 +179,10 @@ foverlaps = function(x, y, by.x=if (!is.null(key(x))) key(x) else key(y), by.y=k
   # CsubsetDT bug has been fixed by Matt. So back to using it! Should improve subset substantially.
   if (which) {
     if (mult %chin% c("first", "last"))
-      return (olaps$yid)
+      return(olaps$yid)
     else if (!is.na(nomatch))
-      return (.Call(CsubsetDT, olaps, which(olaps$yid > 0L), seq_along(olaps)))
-    else return (olaps)
+      return(.Call(CsubsetDT, olaps, which(olaps$yid > 0L), seq_along(olaps)))
+    else return(olaps)
   } else {
     if (!is.na(nomatch))
       olaps = .Call(CsubsetDT, olaps, which(olaps$yid > 0L), seq_along(olaps))
@@ -195,7 +195,7 @@ foverlaps = function(x, y, by.x=if (!is.null(key(x))) key(x) else key(y), by.y=k
     xcols2 = setdiff(names(ans), xcols1)
     ans[, (ycols) := .Call(CsubsetDT, origy, olaps$yid, chmatch(ycols, names(origy)))]
     setcolorder(ans, c(xcols1, ycols, xcols2))
-    return (ans[])
+    return(ans[])
   }
 }
 
diff --git a/R/fread.R b/R/fread.R
index a7cab35eb..da44d6be0 100644
--- a/R/fread.R
+++ b/R/fread.R
@@ -33,8 +33,8 @@ yaml=FALSE, autostart=NA, tmpdir=tempdir(), tz="UTC")
   if (is.na(nrows) || nrows<0) nrows=Inf   # accept -1 to mean Inf, as read.table does
   if (identical(header,"auto")) header=NA
   stopifnot(
-    is.logical(header) && length(header)==1L,  # TRUE, FALSE or NA
-    is.numeric(nThread) && length(nThread)==1L
+    is.logical(header), length(header)==1L,  # TRUE, FALSE or NA
+    is.numeric(nThread), length(nThread)==1L
   )
   nThread=as.integer(nThread)
   stopifnot(nThread>=1L)
diff --git a/R/print.data.table.R b/R/print.data.table.R
index f80a5833c..f16a39625 100644
--- a/R/print.data.table.R
+++ b/R/print.data.table.R
@@ -136,7 +136,7 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
   invisible(x)
 }
 
-format.data.table = function (x, ..., justify="none") {
+format.data.table = function(x, ..., justify="none") {
   if (is.atomic(x) && !is.null(x)) { ## future R can use  if (is.atomic(x))
 
     stopf("Internal structure doesn't seem to be a list. Possibly corrupt data.table.")
diff --git a/R/tables.R b/R/tables.R
index 6a0209c86..7662598d8 100644
--- a/R/tables.R
+++ b/R/tables.R
@@ -12,7 +12,7 @@ type_size = function(DT) {
     tt = lookup[storage.mode(col)]
     if (is.na(tt)) tt = .Machine$sizeof.pointer
     tt = tt*nrow(DT)
-    if (is.factor(col)) tt = tt + length(levels(col))*.Machine$sizeof.pointer
+    if (is.factor(col)) tt = tt + nlevels(col)*.Machine$sizeof.pointer
     ans = ans + tt
   }
   ans + ncol(DT)*.Machine$sizeof.pointer  # column name pointers
diff --git a/R/test.data.table.R b/R/test.data.table.R
index 1a6b725be..574bcb4f3 100644
--- a/R/test.data.table.R
+++ b/R/test.data.table.R
@@ -311,7 +311,7 @@ INT = function(...) { as.integer(c(...)) }   # utility used in tests.Rraw
 gc_mem = function() {
   # nocov start
   # gc reports memory in MB
-  m = apply(gc()[, c(2L, 4L, 6L)], 2L, sum)
+  m = colSums(gc()[, c(2L, 4L, 6L)])
   names(m) = c("GC_used", "GC_gc_trigger", "GC_max_used")
   m
   # nocov end
diff --git a/R/uniqlist.R b/R/uniqlist.R
index 4f3600f83..9bd64efc6 100644
--- a/R/uniqlist.R
+++ b/R/uniqlist.R
@@ -1,5 +1,4 @@
-
-uniqlist = function (l, order = -1L)
+uniqlist = function(l, order = -1L)
 {
   # Assumes input list is ordered by each list item (or by 'order' if supplied), and that all list elements are the same length
   # Finds the non-duplicate rows. Was called duplist but now grows vector - doesn't over-allocate result vector and
diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index 83d672bea..0a5de42c8 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -3282,7 +3282,7 @@ test(1034, as.data.table(x<-as.character(sample(letters, 5))), data.table(V1=x))
     test(1036.01, dim(DT), INT(1,327))
     test(1036.02, dim(ans<-melt(DT, 1:2)), INT(325,4),
       warning="'measure.vars' [[]Geography, Estimate; SEX AND AGE - Total population, Margin of Error; SEX AND AGE - Total population, Percent; SEX AND AGE - Total population, [.][.][.][]] are not all of the same type.*the molten data value column will be of type 'character'.*not of type 'character' will be coerced too")
-    test(1036.03, length(levels(ans$variable)), 317L)
+    test(1036.03, nlevels(ans$variable), 317L)
     test(1036.04, levels(ans$variable)[c(1,2,316,317)],
       tt <- c("Geography",
         "Estimate; SEX AND AGE - Total population",
@@ -3702,7 +3702,7 @@ test(1100, dt1[dt2,roll=-Inf,rollends=c(FALSE,TRUE)]$ind, INT(NA,NA,1,2,2,2,2,2,
 
   # issues/713 - dcast and fun.aggregate
   DT = data.table(id=rep(1:2, c(3,4)), k=c(rep(letters[1:3], 2), 'c'), v=1:7)
-  foo = function (tbl, fun.aggregate) {
+  foo = function(tbl, fun.aggregate) {
       dcast(tbl, id ~ k, value.var='v', fun.aggregate=fun.aggregate, fill=NA_integer_)
   }
   test(1102.22, foo(DT, last), dcast(DT, id ~ k, value.var='v', fun.aggregate=last, fill=NA_integer_))
@@ -8314,7 +8314,7 @@ test(1581.18, DT[, v:=l[[f1]][f2], by=c("f1","f2")],
 #   need eval to have enclos=parent.frame(), #4612
 DT = data.table(id = c(1, 1, 2), value = c("a", "b", "c"))
 DT0 = copy(DT)
-fun = function (DT, tag = c("A", "B")) DT[, var := tag[[.GRP]], by = "id"]
+fun = function(DT, tag = c("A", "B")) DT[, var := tag[[.GRP]], by = "id"]
 fun(DT)
 test(1581.19, DT, DT0[ , var := c('A', 'A', 'B')])
 
@@ -8708,7 +8708,9 @@ test(1613.35, all.equal(dt1, dt2))
 test(1613.36, !isTRUE(all.equal(dt1, dt2, trim.levels = FALSE)))
 test(1613.37, !isTRUE(all.equal(dt1, dt2, trim.levels = FALSE, check.attributes = FALSE)))
 test(1613.38, all.equal(dt1, dt2, trim.levels = FALSE, ignore.row.order = TRUE))
-test(1613.39, length(levels(dt1$A)) == 10L && length(levels(dt2$A)) == 5L, TRUE) # dt1 and dt2 not updated by reference
+# dt1 and dt2 not updated by reference
+test(1613.391, nlevels(dt1$A), 10L)
+test(1613.392, nlevels(dt2$A), 5L)
 # unsupported column types: list
 dt = data.table(V1 = 1:4, V2 = letters[1:4], V3 = lapply(1:4, function(x) new.env()))
 test(1613.40, all.equal(dt, dt), TRUE)