R help - Sep 2011 - Selecting row indices from a data.frame by a factor and simple calculation (miniumum)

Hello

I wish to extract the row indices from a data.frame in which a column contains
numeric data by calculating the minimum value, but grouped on another column
factor:

An example data.frame:

Code                absdiff
NY14/3070         2
NY14/3070         4
NY14/3070         1
NY14/5459         5
NY14/5459         7

So in this case the factor is Code and the numeric vector is absdiff

The query would return row indices 3 and 4 which have the minimum values for the
factor.
I am sure it would involve some combination of which and tapply but cannot
figure it out?

Many thanks, Barry

Dr Barry Rawlins
British Geological Survey
Keyworth
Nottingham
NG12 5GG
UK
Tel: 0115 9363140
Mob: 07884235473
homepage http://sites.google.com/site/barryrawlins/



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Code <- c(rep("NY14/3070", 3), rep("NY14/5459", 2))
Code <- as.factor(Code)
absdiff <- c(2, 4, 1, 5, 7)
df <- data.frame(Code, absdiff)
which(
? ? paste(df$Code, df$absdiff) ==?
? ? paste(
? ? ? ? aggregate(df$absdiff, by = list(df$Code), min)$Group.1,
? ? ? ? aggregate(df$absdiff, by = list(df$Code), min)$x
? ? )
)



----- Original Message -----
From: "Rawlins, Barry G." <bgr at bgs.ac.uk>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc: 
Sent: Tuesday, September 13, 2011 11:31 AM
Subject: [R] Selecting row indices from a data.frame by a factor and simple
calculation (miniumum)

Hello

I wish to extract the row indices from a data.frame in which a column contains
numeric data by calculating the minimum value, but grouped on another column
factor:

An example data.frame:

Code? ? ? ? ? ? ? ? absdiff
NY14/3070? ? ? ?  2
NY14/3070? ? ? ?  4
NY14/3070? ? ? ?  1
NY14/5459? ? ? ?  5
NY14/5459? ? ? ?  7

So in this case the factor is Code and the numeric vector is absdiff

The query would return row indices 3 and 4 which have the minimum values for the
factor.
I am sure it would involve some combination of which and tapply but cannot
figure it out?

Many thanks, Barry

Dr Barry Rawlins
British Geological Survey
Keyworth
Nottingham
NG12 5GG
UK
Tel: 0115 9363140
Mob: 07884235473
homepage http://sites.google.com/site/barryrawlins/



-- 
This message (and any attachments) is for the recipient ...{{dropped:8}}

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On Sep 13, 2011, at 12:31 PM, Rawlins, Barry G. wrote:
> Hello
>
> I wish to extract the row indices from a data.frame in which a  
> column contains numeric data by calculating the minimum value, but  
> grouped on another column factor:
>
> An example data.frame:
>
> Code                absdiff
> NY14/3070         2
> NY14/3070         4
> NY14/3070         1
> NY14/5459         5
> NY14/5459         7
>
> So in this case the factor is Code and the numeric vector is absdiff
 > df$grp.min <- ave(df$absdiff, df$Code, FUN=min)
 > rownames(df[df$absdiff ==df$grp.min,])
[1] "3" "4"

Or if you didn't want that column clutterng up your data.frame it  
could be just aclulated within hte loggical comparison:

 > rownames(df[
         df$absdiff ==ave(df$absdiff, df$Code, FUN=min),
              ])
[1] "3" "4"

Now it should be noted that if there are multiple values of the group  
minimum, then they will be returned.

>
> The query would return row indices 3 and 4 which have the minimum  
> values for the factor.
> I am sure it would involve some combination of which and tapply but  
> cannot figure it out?
>
> Many thanks, Barry
>
> Dr Barry Rawlins
> British Geological Survey

David Winsemius, MD
West Hartford, CT