[R] Conditional expressions in commands -- basic

[Sarah Goslee]

Hi Richard,

?if
?ifelse
?subset
?[

depending on the specific operation. See inline.

Given a data frame tdf with x, y, and a (or with vectors creating a
data frame using):
tdf <- data.frame(x=x, y=y, a=a)

On Sat, Mar 22, 2014 at 10:21 PM, Richard Sherman <rss.pdx at gmail.com> wrote:
> Hi all,
>
> A simple question, new-ish to R, coming from Stata, and I yes I've looked at great length and not found ...
>
> In Stata I might write (with * in place of #)
>
> # regress y on x in the set of observations where a==1
> reg y x if a==1

lm(y ~ x, data=subset(tdf, a==1))

>
> # descriptives on x where a==1
> su x if a==1

summary(subset(tdf, a==1)$x)
or
summary(tdf[tdf$a == 1,])
or if they are vectors
summary(x[a==1])

>
> # generate a logical depending on the value of a variable
> gen z = (a == 1)

z <- a==1

>
> # or likewise for a numeric
> gen x = y if a == 1

I'm not sure what this one is supposed to do. What's x if a isn't 1?

But those examples should be enough to get you started, especially if
you also read the Intro to R that comes with your R installation.
Subsetting is a very basic operation, as you say, and pretty much
every intro/tutorial I've read talks about it. I'm not sure where you
looked in your extensive searching.

Sarah

>
> # show the values of a variable based on the value of another variable
> di y if a ~= .
>
> # generate a logical based on missing-ness of a variable
> gen w = (a~=.)
>
> or any of many similar operations conditional on the value of a variable.
>
> Believe it or not this is a big obstacle to learning R.
>
> ---
> Richard Sherman


-- 
Sarah Goslee
http://www.functionaldiversity.org