[R] "for" loop in R - strange behaviour

[Peter Alspach]

Tena koe

Not really strange:  (7+2):11 is 9:11, 7+2:11 is 7+c(2,3,4,5,6,7,8,9,10,11); i.e., 9:18

Peter Alspach



-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Supriya Jain
Sent: Thursday, 30 January 2014 5:32 a.m.
To: r-help at r-project.org
Subject: [R] "for" loop in R - strange behaviour

Hi, I notice the following from a "for" loop in R, which seems strange to
me:

When I  do this:

-------
first <- 0
nstep <- 10
N <- 14
while(first < N)
{
 print("-------> ")
 last <- first + nstep
 if(last > N)
    last <- N
 #start <- first+2
 for(i in (first+2):last)                            #  line 11
    print(sprintf("i: %d; last: %d", i, last))  first <- first + nstep } # end of while loop
-------

the output is as expected:

[1] "-------> "
[1] "i: 2; last: 10"
[1] "i: 3; last: 10"
[1] "i: 4; last: 10"
[1] "i: 5; last: 10"
[1] "i: 6; last: 10"
[1] "i: 7; last: 10"
[1] "i: 8; last: 10"
[1] "i: 9; last: 10"
[1] "i: 10; last: 10"
[1] "-------> "
[1] "i: 12; last: 14"
[1] "i: 13; last: 14"
[1] "i: 14; last: 14"

but, if, on line 11 in code, I forget to enclose first+2 in parenthesis, that is, do:
                for(i in first+2:last)
the output from the second iteration is very non-intuitive:

[1] "-------> "
[1] "i: 2; last: 10"
[1] "i: 3; last: 10"
[1] "i: 4; last: 10"
[1] "i: 5; last: 10"
[1] "i: 6; last: 10"
[1] "i: 7; last: 10"
[1] "i: 8; last: 10"
[1] "i: 9; last: 10"
[1] "i: 10; last: 10"
[1] "-------> "
[1] "i: 12; last: 14"
[1] "i: 13; last: 14"
[1] "i: 14; last: 14"
[1] "i: 15; last: 14"
[1] "i: 16; last: 14"
[1] "i: 17; last: 14"
[1] "i: 18; last: 14"
[1] "i: 19; last: 14"
[1] "i: 20; last: 14"
[1] "i: 21; last: 14"
[1] "i: 22; last: 14"
[1] "i: 23; last: 14"
[1] "i: 24; last: 14"

Here, even though the "last" is 14, "i" does not stop at 14, but goes on till first (= 10)+14 = 24.

--
Any insights on why this is happening?

Thanks,
SJ

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