[R] About change columns and specific rows in R

[Ivan Calandra]

Hi,

Actually, you don't need to create the column first, nor to use which:
DF[DF$month==1, "product1_1"] = DF[DF$month==1, "product1"] * 3.1

The "[" is a great tool that you need to learn. In this case, you don't 
need to combine "[" and $: within the square brackets, the vector before 
the comma indexes the rows and the one after the comma indexes the columns.

The other thing you were missing correctly referencing the rows. You 
have to specify the data.frame you want to look into.

And last, learn to use dput() to provide a nice reproducible example.

HTH,
Ivan


--
Dr. Ivan Calandra
TraCEr, Laboratory for Traceology and Controlled Experiments
MONREPOS Archaeological Research Centre and
Museum for Human Behavioural Evolution
Schloss Monrepos
56567 Neuwied, Germany
+49 (0) 2631 9772-243
https://www.researchgate.net/profile/Ivan_Calandra

On 23/05/2017 08:45, William Michels via R-help wrote:
> Hi Lily,
>
> You're on the right track, but you should define a new column first
> (filled with NA values), then specify the precise rows and columns on
> both the left and right hand sides of the assignment operator that
> will be altered. Luckily, this is pretty easy...just remember to use
> which() on the right hand side of the assignment operator to get the
> index of the rows you want. Example below for "product1":
>
>> DF$product1_1 <- NA
>> DF[DF$month == 1, "product1_1"] <- DF[which(DF$month == 1), "product1"]*3.1
>>
>
> HTH,
>
> Bill.
>
> William Michels, Ph.D.
>
>
>
>
> On Mon, May 22, 2017 at 9:56 PM, lily li <chocold12 at gmail.com> wrote:
>> Hi R users,
>>
>> I have a question about manipulating the dataframe. I want to create a new
>> dataframe, and to multiply rows with different seasons for different
>> constants.
>>
>> DF
>> year   month   day   product1   product2   product3
>> 1981     1          1         18              56            20
>> 1981     1          2         19              45            22
>> 1981     1          3         16              48            28
>> 1981     1          4         19              50            21
>> 1981     2          1         17              49            25
>> 1981     2          2         20              47            23
>> 1981     2          3         21              52            27
>>
>> For example, how to multiply product1 in month1 by 3.1, and to multiply
>> product3 in month2 by 2.0? I wrote the code like this but does not work.
>> Thanks for your help.
>>
>> DF['month'==1, ]$product1_1 = DF['month'==1, ]$product1 * 3.1;
>> DF['month'==2, ]$product3_1 = DF['month'==1, ]$product3 * 2.0;
>>
>>          [[alternative HTML version deleted]]
>>
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> ______________________________________________
> 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.
>