R help - Feb 2017 - Selecting rows and columns of a data frame using relational operators

Consider a data frame named data. data contains 4 columns and 1000
rows. Say the aim is to bring together columns 1, 2, and 4, if the
values in column 4 is equal to 1. We could use the syntax

data(data[,4] == 1, c(1 2 4))

for this purpose. Suppose now that the aim is to bring together
columns 1, 2, and 4, if the values in column 4 is equal to 1, for the
first 20 rows of column 4. We could use the syntax

data(data[1:20,4] == 1, c(1 2 4))

for this purpose. However, this does not produce the desired result.
This is surprising at least for someone coming from MATLAB because
MATLAB produces what is desired.

Question 1: The code makes sense but why does it not produce what we
expect it to produce?

Question 2: What code is instead suitable?

Hi Tunga,

The function subset() is probably what you are looking for. You might also
want to look at a tutorial to understand the R syntax.

In addition, calling your data data is not a good idea because of the name
clash with the function data().

Hope this helps,
Ulrik

On Mon, 27 Feb 2017 at 13:10 Tunga Kantarc? <tungakantarci at gmail.com>
wrote:
> Consider a data frame named data. data contains 4 columns and 1000
> rows. Say the aim is to bring together columns 1, 2, and 4, if the
> values in column 4 is equal to 1. We could use the syntax
>
> data(data[,4] == 1, c(1 2 4))
>
> for this purpose. Suppose now that the aim is to bring together
> columns 1, 2, and 4, if the values in column 4 is equal to 1, for the
> first 20 rows of column 4. We could use the syntax
>
> data(data[1:20,4] == 1, c(1 2 4))
>
> for this purpose. However, this does not produce the desired result.
> This is surprising at least for someone coming from MATLAB because
> MATLAB produces what is desired.
>
> Question 1: The code makes sense but why does it not produce what we
> expect it to produce?
>
> Question 2: What code is instead suitable?
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.
>
	[[alternative HTML version deleted]]

The answer is simple

data[,4] == 1 produces a logical vector of length nrow(data)
and the subsetting mechanism for data frames in R needs a vector of the same
length
as the data frame has rows.

data[1:20,4] == 1
produces a data frame of length 20, and if this is not the length of data.
So R applies its standard procedure, it repeats this vector as often as needed
to get
a vector of length == nrow(data)


Th following code illustrates what is happening

data <-
data.frame(x=rnorm(100),y=rnorm(100),z=rnorm(100),a=rep(c(1,2,1,2),c(2,48,2,48)))

vec1 <- data[,4]==1
vec2 <- data[1:20,4]==1

> On 27 Feb 2017, at 13:07, Tunga Kantarc? <tungakantarci at gmail.com>
wrote:
> 
> Consider a data frame named data. data contains 4 columns and 1000
> rows. Say the aim is to bring together columns 1, 2, and 4, if the
> values in column 4 is equal to 1. We could use the syntax
> 
> data(data[,4] == 1, c(1 2 4))
> 
> for this purpose. Suppose now that the aim is to bring together
> columns 1, 2, and 4, if the values in column 4 is equal to 1, for the
> first 20 rows of column 4. We could use the syntax
> 
> data(data[1:20,4] == 1, c(1 2 4))
> 
> for this purpose. However, this does not produce the desired result.
> This is surprising at least for someone coming from MATLAB because
> MATLAB produces what is desired.
> 
> Question 1: The code makes sense but why does it not produce what we
> expect it to produce?
> 
> Question 2: What code is instead suitable?
> 
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.

Hi

Above what was said

data(data[,4] == 1, c(1 2 4))
Error: unexpected numeric constant in "data(data[,4] == 1, c(1 2"

is not valid syntax as it assume that data is a function and c(1 2 4) is also
not correct syntax (I wonder if commands in MATLAB are such free form).

And I am a little puzzled what means to bring 3 columns together. Calculate sum?
Paste respective values in mentioned 3 columns.

Anyway, if you want to select only first n rows you should use something like

data[1:20,][data[1:20,4]==1,c(1,2,4)]
or
data[which(data[1:20,4]==1), c(1,2,4)]

You should also be careful with "==" if you compare numbers,
especially fractions and to select without "with" if your data contain
NA values in respective columns.

Cheers
Petr
> -----Original Message-----
> From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Erich
> Subscriptions
> Sent: Monday, February 27, 2017 1:30 PM
> To: Tunga Kantarc? <tungakantarci at gmail.com>
> Cc: R-help <r-help at r-project.org>
> Subject: Re: [R] Selecting rows and columns of a data frame using
relational
> operators
>
> The answer is simple
>
> data[,4] == 1 produces a logical vector of length nrow(data) and the
> subsetting mechanism for data frames in R needs a vector of the same length
> as the data frame has rows.
>
> data[1:20,4] == 1
> produces a data frame of length 20, and if this is not the length of data.
> So R applies its standard procedure, it repeats this vector as often as
needed
> to get a vector of length == nrow(data)
>
>
> Th following code illustrates what is happening
>
> data <-
> data.frame(x=rnorm(100),y=rnorm(100),z=rnorm(100),a=rep(c(1,2,1,2),c(2,4
> 8,2,48)))
>
> vec1 <- data[,4]==1
> vec2 <- data[1:20,4]==1
>
>
> > On 27 Feb 2017, at 13:07, Tunga Kantarc? <tungakantarci at
gmail.com>
> wrote:
> >
> > Consider a data frame named data. data contains 4 columns and 1000
> > rows. Say the aim is to bring together columns 1, 2, and 4, if the
> > values in column 4 is equal to 1. We could use the syntax
> >
> > data(data[,4] == 1, c(1 2 4))
> >
> > for this purpose. Suppose now that the aim is to bring together
> > columns 1, 2, and 4, if the values in column 4 is equal to 1, for the
> > first 20 rows of column 4. We could use the syntax
> >
> > data(data[1:20,4] == 1, c(1 2 4))
> >
> > for this purpose. However, this does not produce the desired result.
> > This is surprising at least for someone coming from MATLAB because
> > MATLAB produces what is desired.
> >
> > Question 1: The code makes sense but why does it not produce what we
> > expect it to produce?
> >
> > Question 2: What code is instead suitable?
> >
> > ______________________________________________
> > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> > 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.
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.
________________________________
Tento e-mail a jak?koliv k n?mu p?ipojen? dokumenty jsou d?v?rn? a jsou ur?eny
pouze jeho adres?t?m.
Jestli?e jste obdr?el(a) tento e-mail omylem, informujte laskav? neprodlen? jeho
odes?latele. Obsah tohoto emailu i s p??lohami a jeho kopie vyma?te ze sv?ho
syst?mu.
Nejste-li zam??len?m adres?tem tohoto emailu, nejste opr?vn?ni tento email
jakkoliv u??vat, roz?i?ovat, kop?rovat ?i zve?ej?ovat.
Odes?latel e-mailu neodpov?d? za eventu?ln? ?kodu zp?sobenou modifikacemi ?i
zpo?d?n?m p?enosu e-mailu.

V p??pad?, ?e je tento e-mail sou??st? obchodn?ho jedn?n?:
- vyhrazuje si odes?latel pr?vo ukon?it kdykoliv jedn?n? o uzav?en? smlouvy, a
to z jak?hokoliv d?vodu i bez uveden? d?vodu.
- a obsahuje-li nab?dku, je adres?t opr?vn?n nab?dku bezodkladn? p?ijmout;
Odes?latel tohoto e-mailu (nab?dky) vylu?uje p?ijet? nab?dky ze strany p??jemce
s dodatkem ?i odchylkou.
- trv? odes?latel na tom, ?e p??slu?n? smlouva je uzav?ena teprve v?slovn?m
dosa?en?m shody na v?ech jej?ch n?le?itostech.
- odes?latel tohoto emailu informuje, ?e nen? opr?vn?n uzav?rat za spole?nost
??dn? smlouvy s v?jimkou p??pad?, kdy k tomu byl p?semn? zmocn?n nebo p?semn?
pov??en a takov? pov??en? nebo pln? moc byly adres?tovi tohoto emailu p??padn?
osob?, kterou adres?t zastupuje, p?edlo?eny nebo jejich existence je adres?tovi
?i osob? j?m zastoupen? zn?m?.

This e-mail and any documents attached to it may be confidential and are
intended only for its intended recipients.
If you received this e-mail by mistake, please immediately inform its sender.
Delete the contents of this e-mail with all attachments and its copies from your
system.
If you are not the intended recipient of this e-mail, you are not authorized to
use, disseminate, copy or disclose this e-mail in any manner.
The sender of this e-mail shall not be liable for any possible damage caused by
modifications of the e-mail or by delay with transfer of the email.

In case that this e-mail forms part of business dealings:
- the sender reserves the right to end negotiations about entering into a
contract in any time, for any reason, and without stating any reasoning.
- if the e-mail contains an offer, the recipient is entitled to immediately
accept such offer; The sender of this e-mail (offer) excludes any acceptance of
the offer on the part of the recipient containing any amendment or variation.
- the sender insists on that the respective contract is concluded only upon an
express mutual agreement on all its aspects.
- the sender of this e-mail informs that he/she is not authorized to enter into
any contracts on behalf of the company except for cases in which he/she is
expressly authorized to do so in writing, and such authorization or power of
attorney is submitted to the recipient or the person represented by the
recipient, or the existence of such authorization is known to the recipient of
the person represented by the recipient.