Closes #3014, new warning for %between%

NEWS.md

@@ -81,6 +81,7 @@ Warning message:
 
 4. `hour`, `minute`, and `second` utility functions use integer arithmetic when the input is already (explicitly) UTC-based `POSIXct` for 4-10x speedup vs. using `as.POSIXlt`.
 
+5. Helpful warning added when potentially incorrect usage of `%between%` is detected, [#3014](https://github.com/Rdatatable/data.table/issues/3014). Thanks @peterlittlejohn for offering his user experience and providing the impetus. 
 
 ### Changes in v1.11.4  (on CRAN 27 May 2018)
 

R/between.R

@@ -12,7 +12,18 @@ between <- function(x,lower,upper,incbounds=TRUE) {
 }
 
 # %between% is vectorised, #534.
-"%between%" <- function(x, y) between(x, y[[1L]], y[[2L]], incbounds=TRUE)
+"%between%" <- function(x, y) {
+  if (length(y) > 2L) {
+    ysub = substitute(y)
+    warning("RHS has length() greater than 2. ",
+            if (is.call(ysub) && ysub[[1L]] == 'c')
+              sprintf("Perhaps you meant %s? ",
+                      capture.output(print(`[[<-`(ysub, 1L, quote(list))))),
+            "The first element should be the lower bound(s); ",
+            "the second element should be the upper bound(s).")
+  }
+  between(x, y[[1L]], y[[2L]], incbounds=TRUE)
+}
 # If we want non inclusive bounds with %between%, just +1 to the left, and -1 to the right (assuming integers)
 
 # issue FR #707

inst/tests/tests.Rraw

@@ -12134,6 +12134,13 @@ test(1925.12, as.ITime(x, ms='ceil'),    structure(c(13L, 68L), class="ITime"))
 test(1936.1, fread("A,B\n1,3\n2,4", autostart=1), data.table(A=1:2, B=3:4), warning="autostart.*deprecated.*Consider skip")
 if (.Platform$OS.type == "unix") test(1936.2, is.data.table(fread("ls .")))
 
+# add helpful warning to %between%
+d <- data.table(A=1:10, B = 3)
+test(1937.1, d[A %between% c(B,B+1)], data.table(A = 3L, B = 3),
+     warning = 'RHS has length().*Perhaps you meant')
+test(1937.2, d[A %between% B], data.table(A = 3L, B = 3),
+     warning = 'greater than 2. The first')
+
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 #  Add new tests above this line  #
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