[R] [EXT] Re: Beginner problem - using mod function to print odd numbers

[David K Stevens]

This thread, in addition to being a nice Thursday morning chuckle, 
should be de rigueur for 'R 101' users. It illustrates much of the 
thinking behind R in a very simple way, without taking itself too 
seriously. Well done all!

David Stevens
**/**/

On 6/9/2021 8:35 AM, Bill Dunlap wrote:
> Martin wrote
>    Use
>        num[num %% 2 == 1]
>    instead of much slower and ...@#^$
>        num[ifelse(num %% 2 == 1, TRUE, FALSE)]
>
> Read the '[' as 'such that' when the subscript is logical
> (=="Boolean"==TRUE/FALSE-values).
>
> [The original post had a typo/thinko, num<-num+i instead of num<-num+1,
> which was simply an error, not a matter of style.  R's vectorization makes
> it easy to avoid such errors.]
>
> -Bill
>
> On Wed, Jun 9, 2021 at 2:56 AM Martin Maechler <maechler using stat.math.ethz.ch>
> wrote:
>
>>>>>>> David Carlson    on Sun, 6 Jun 2021 15:21:34 -0400 writes:
>>> There is really no need for a loop:
>>> num <- 1:100
>>> num[ifelse(num %% 2 == 1, TRUE, FALSE)]
>>> [1]  1  3  5  7  9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
>> 47 49
>>> [26] 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93
>> 95 97 99
>>
>> Well, and the above "works" but is really another proof of my
>> year-long claim that people use  ifelse(.)  *MUCH MUCH* too often,
>> and should really learn to use alternatives, in this case,
>> "R 101" (*long* before fooverse):
>>
>> Use
>>
>>      num[num %% 2 == 1]
>>
>> instead of much slower and ...@#^$
>>
>>      num[ifelse(num %% 2 == 1, TRUE, FALSE)]
>>
>> Martin Maechler
>> ETH Zurich
>>
>>> On Sat, Jun 5, 2021 at 2:05 PM William Michels via R-help
>>> <r-help using r-project.org> wrote:
>>      >>
>>      >> > i <- 1L; span <- 1:100; result <- NA;
>>      >> > for (i in span){
>>      >> + ifelse(i %% 2 != 0, result[i] <- TRUE, result[i] <- FALSE)
>>      >> + }
>>      >> > span[result]
>>      >> [1]  1  3  5  7  9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41
>> 43
>>
>>   [............]
>>
>> ______________________________________________
>> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
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>
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-- 
David K Stevens, PhD,PE
Professor
Civil and Environmental Engineering
Utah State University
Logan, UT 84322-8200
david.stevens using usu.edu
014357973229


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