[R] apply block of if statements with menu function

[PIKAL Petr]

Hi

One comment: I never used menu or switch, this is just how I understand its function. So you probably are on a wrong track and do not understand what are objects and functions in R languange.

menu

Description
menu presents the user with a menu of choices labelled from 1 to the number of choices. To exit without choosing an item one can select 0.

Value
The number corresponding to the selected item, or 0 if no choice was made.

It is obvious (for me :) that results of menu command is 0,1,2, ... depending on how many items are in menu

switch

Description
switch evaluates EXPR and accordingly chooses one of the further arguments (in ...).

Details
switch works in two distinct ways depending whether the first argument evaluates to a character string or a number.

If the value of EXPR is not a character string it is coerced to integer. If the integer is between 1 and nargs()-1 then the corresponding element of ... is evaluated and the result returned: thus if the first argument is 3 then the fourth argument is evaluated and returned.

> switch(1,"a","b","c","d")
[1] "a"
> switch(2,"a","b","c","d")
[1] "b"
> switch(3,"a","b","c","d")
[1] "c"
> switch(4,"a","b","c","d")
[1] "d"
> switch(0,"a","b","c","d")
>

So if result of menu is 0 (you did not choose anything) you can either stay with 0, then switch does not return anything or add 1 and let evaluate something meaningful specified in second and following positions of switch command.

Regards
Petr


> -----Original Message-----
> From: rl at openmailbox.org [mailto:rl at openmailbox.org]
> Sent: Tuesday, September 16, 2014 2:17 PM
> To: PIKAL Petr
> Cc: r-help at r-project.org
> Subject: RE: [R] apply block of if statements with menu function
>
> On 2014-09-16 10:50, PIKAL Petr wrote:
> >
> >> switch(menu(c("List letters", "List LETTERS")) + 1,
> >>       cat("Nothing done\n"), letters, LETTERS)
> >>
> >> why is the result changed if ' + 1,' removed?
> >
> > Because +1 belongs to switch not to menu. You can translate above to:
> >
>
> The help pages ?switch, ?menu do not describe (in my eyes! :) ) the
> significance of '+ 1'; where is this described in more detail please?


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