# [R] Selecting rows and columns of a data frame using relational operators

PIKAL Petr petr.pikal at precheza.cz
Mon Feb 27 16:14:11 CET 2017

```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 Kantarcı <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
>
>
> 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 Kantarcı <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
> > 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
> guide.html
> and provide commented, minimal, self-contained, reproducible code.

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