R help - Jan 2010 - Applying function to parts of a matrix based on a factor

R 2.9
Windows XP

I have a matrix, Data, which contains a factor Sex and a continuous variable
Age.
I want to get mean age by sex. I know I can do this with two statements,
mean(Data["Age,Data[,"Sex"]=="Male") and  
mean(Data["Age,Data[,"Sex"]=="Female")

I know this can be done in a single command, but I can remember how. There is a
function that allows another function work within factors, something like
magicfunction(Data,Factor=Sex). n.b. I know the function I am looking for is not
in the lapply, sapply etc. family

Please put me out of my misery (and senior moment) and remind me what function I
should be using.




John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)

Confidentiality Statement:
This email message, including any attachments, is for th...{{dropped:6}}

with(yourdataframe, tapply(age,sex,mean))

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of John Sorkin
Sent: Wednesday, January 13, 2010 12:11 PM
To: r-help at r-project.org
Subject: [R] Applying function to parts of a matrix based on a factor

R 2.9
Windows XP

I have a matrix, Data, which contains a factor Sex and a continuous variable
Age.
I want to get mean age by sex. I know I can do this with two statements,
mean(Data["Age,Data[,"Sex"]=="Male") and  
mean(Data["Age,Data[,"Sex"]=="Female")

I know this can be done in a single command, but I can remember how. There is a
function that allows another function work within factors, something like
magicfunction(Data,Factor=Sex). n.b. I know the function I am looking for is not
in the lapply, sapply etc. family

Please put me out of my misery (and senior moment) and remind me what function I
should be using.




John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)

Confidentiality Statement:
This email message, including any attachments, is for th...{{dropped:9}}

try this:

with(Data, tapply(Age, Sex, mean))


I hope it helps.

Best,
Dimitris


John Sorkin wrote:
> R 2.9
> Windows XP
> 
> I have a matrix, Data, which contains a factor Sex and a continuous
variable Age.
> I want to get mean age by sex. I know I can do this with two statements,
> mean(Data["Age,Data[,"Sex"]=="Male") and  
> mean(Data["Age,Data[,"Sex"]=="Female")
> 
> I know this can be done in a single command, but I can remember how. There
is a function that allows another function work within factors, something like
> magicfunction(Data,Factor=Sex). n.b. I know the function I am looking for
is not in the lapply, sapply etc. family
> 
> Please put me out of my misery (and senior moment) and remind me what
function I should be using.
> 
> 
> 
> 
> John David Sorkin M.D., Ph.D.
> Chief, Biostatistics and Informatics
> University of Maryland School of Medicine Division of Gerontology
> Baltimore VA Medical Center
> 10 North Greene Street
> GRECC (BT/18/GR)
> Baltimore, MD 21201-1524
> (Phone) 410-605-7119
> (Fax) 410-605-7913 (Please call phone number above prior to faxing)
> 
> Confidentiality Statement:
> This email message, including any attachments, is for th...{{dropped:6}}
> 
> ______________________________________________
> 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.
> 
-- 
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus University Medical Center

Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands
Tel: +31/(0)10/7043478
Fax: +31/(0)10/7043014

If your matrix were a data.frame, it could work like this:

df <- data.frame(age=1:100, sex=rep(1:2, 50))
with(df, by(age, sex, mean))

without the "lapply, sapply etc. family".

h

At 18:16 13.01.2010, Doran, Harold wrote:
>with(yourdataframe, tapply(age,sex,mean))
>
>-----Original Message-----
>From: r-help-bounces at r-project.org 
>[mailto:r-help-bounces at r-project.org] On Behalf Of John Sorkin
>Sent: Wednesday, January 13, 2010 12:11 PM
>To: r-help at r-project.org
>Subject: [R] Applying function to parts of a matrix based on a factor
>
>R 2.9
>Windows XP
>
>I have a matrix, Data, which contains a factor Sex and a continuous 
>variable Age.
>I want to get mean age by sex. I know I can do this with two statements,
>mean(Data["Age,Data[,"Sex"]=="Male") and
>mean(Data["Age,Data[,"Sex"]=="Female")
>
>I know this can be done in a single command, but I can remember how. 
>There is a function that allows another function work within 
>factors, something like
>magicfunction(Data,Factor=Sex). n.b. I know the function I am 
>looking for is not in the lapply, sapply etc. family
>
>Please put me out of my misery (and senior moment) and remind me 
>what function I should be using.
>
>
>
>
>John David Sorkin M.D., Ph.D.
>Chief, Biostatistics and Informatics
>University of Maryland School of Medicine Division of Gerontology
>Baltimore VA Medical Center
>10 North Greene Street
>GRECC (BT/18/GR)
>Baltimore, MD 21201-1524
>(Phone) 410-605-7119
>(Fax) 410-605-7913 (Please call phone number above prior to faxing)
>
>Confidentiality Statement:
>This email message, including any attachments, is for t...{{dropped:9}}