R help - Nov 2012 - Performing operations only on selected data

I spent some time on this simple question, also searched the forum,
eventually hacked my way to an ugly solution for my particular problem but I
would like to improve my coding:

I have data of the form:
df <- expand.grid(group=c('copper', 'zinc',
'aluminum', 'nickel'),
condition1=c(1:4))

I would like to add a new data column "condition2", with values equal
to the
value of condition1 plus a random number from 0-1 (uniform distribution)  if
the value of condition1 is < 1, or just condition1 if the value of
condition1 is > 1. More generally, my interest is in manipulating the values
of condition1 if they meet one or more criteria, or keeping the values the
same otherwise. Thanks for any thoughts!

 



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Marcel Curlin <cemarcel at u.washington.edu> wrote:
>I spent some time on this simple question, also searched the forum,
>eventually hacked my way to an ugly solution for my particular problem
>but I
>would like to improve my coding:
>
>I have data of the form:
>df <- expand.grid(group=c('copper', 'zinc',
'aluminum', 'nickel'),
>condition1=c(1:4))
>
>I would like to add a new data column "condition2", with values
equal
>to the
>value of condition1 plus a random number from 0-1 (uniform
>distribution)  if
>the value of condition1 is < 1, or just condition1 if the value of
>condition1 is > 1. More generally, my interest is in manipulating the
>values
>of condition1 if they meet one or more criteria, or keeping the values
>the
>same otherwise. Thanks for any thoughts!
>
> 
>
>
>
>--
>View this message in context:
>http://r.789695.n4.nabble.com/Performing-operations-only-on-selected-data-tp4650646.html
>Sent from the R help mailing list archive at Nabble.com.
>
>______________________________________________
>R-help at r-project.org mailing list
>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.

Thank you, this works very well. My only remaining question about this is
about how ifelse is working; I understand the basic syntax (df$condition2
gets assigned the value *runif(nrow(df1[df1$condition1<=1,]),0,1)* or the
value *df$condition1* depending on whether or not df$condition1 meets the
criterion "<=1".

As I understand it, "runif(nrow(df1[df1$condition1<=1,]),0,1)" is a
vector
of random values with vector length equal to the number of rows meeting
"df$condition1<=1" and df$condition1 is just my column of
condition1 values.
So the command seems to be going down row by row and assigning condition2
values from one of two vectors in an "interleaved" way. 

So my question is, how does R keep track of which item in each of the
vectors to assign to condition2? For example, if the first 4 entries of
condition1 are 1, 3, 4, 1,  how does R know to use the *first* entry of
vector runif(nrow(df1[df1$condition1<=1,]),0,1) then the *second* and
*third* values of vector df$condition1, then the *second* value of vector
runif(nrow(df1[df1$condition1<=1,]),0,1)?



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