From Martin Maechler <maechler@stat.math.ethz.ch>  Fri Sep  4 14:26:29 1998
From: Martin Maechler <maechler@stat.math.ethz.ch> (Martin Maechler)
Date: Fri, 4 Sep 1998 15:26:29 +0200
Subject: a nasty error  --> patch to seq.R
In-Reply-To: <199809031726.TAA00634@aryness.ethz.ch> (message from Peter
 Holzer on Thu, 3 Sep 1998 19:26:44 +0200)
Message-ID: <199809041326.PAA04572@sophie.ethz.ch>


>>>>> "Peter" == Peter Holzer <holzer@stat.math.ethz.ch> writes:

    Peter> ... The basic function seq doesn't work properly:

    >> seq(1,6,by=3)
    Peter> [1] 1 4 7

    Peter> Looking at the source code of seq.default I found that strange
    Peter> fuzz thing "+ 0.4". Taking that away the seq works fine.

    Peter> My question is: Why has this fuzz thing been added?

    Peter> If it was for some rounding problems I would suggest to replace
    Peter> "0.4" by something much smaller.

This bug was introduced between 0.62.1 and 0.62.2  --
with the aim to fix another one.

After some thinking and some experimentation, I conclude that there is no
easy architecture-independent ``fudge constant'' that can be used here
instead of 0.4.  One needs to use a subsequent test.

Here is a patch that seems "robust" (and passes my tests):

--- ../R-0.62.3/src/library/base/R/seq.R	Tue Jun 16 14:32:08 1998
+++ src/library/base/R/seq.R	Fri Sep  4 15:23:11 1998
@@ -16,9 +16,11 @@
 		if(missing(by))
 			from:to
 		else {
-			n <- (to - from)/by + 0.4 # fuzz needed
+			n <- (to - from)/by
 			if(n < 0)
 				stop("Wrong sign in by= argument")
+                        n <- as.integer(n + 1e-10)
+                        if(n*by + from > to) n <- n - 1
 			from + (0:n) * by
 		}
 	else if(length.out < 0)


---
Martin Maechler <maechler@stat.math.ethz.ch>	http://stat.ethz.ch/~maechler/
Seminar fuer Statistik, ETH-Zentrum SOL G1;	Sonneggstr.33
ETH (Federal Inst. Technology)	8092 Zurich	SWITZERLAND
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a nasty error --> patch to seq.R
Martin Maechler