[R] "for" loop in R - strange behaviour
Peter Alspach
Peter.Alspach at plantandfood.co.nz
Wed Jan 29 20:22:45 CET 2014
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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