[R] by function with sum does not give what is expected from by function with print

[Bert Gunter]

by() chooses **data frame** subsets -- sum() is acting on these frames,
adding up everything in them.
Try this instead:

> by(mydata,list(mydata$sex,mydata$status),function(x)sum(x$values))
: 0
: 0
[1] 224
-----------------------------------------------------------
: 1
: 0
[1] 5227
-----------------------------------------------------------
: 0
: 1
[1] 11
-----------------------------------------------------------
: 1
: 1
[1] 552

Bert Gunter

"The trouble with having an open mind is that people keep coming along and
sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Thu, Jul 23, 2020 at 3:25 PM Sorkin, John <jsorkin using som.umaryland.edu>
wrote:

> Colleagues,
>
> The by function in the R program below is not giving me the sums
> I expect to see, viz.,
> 382+170=552
> 4730+170=4900
> 5+6=11
> 199+25=224
> ###################################################
> #full R program:
> mydata <- data.frame(covid=c(0,0,0,0,1,1,1,1),
> sex=(rep(c(1,1,0,0),2)),
> status=rep(c(1,0),2),
> values=c(382,4730,5,199,170,497,6,25))
> mydata
> by(mydata,list(mydata$sex,mydata$status),sum)
> by(mydata,list(mydata$sex,mydata$status),print)
> ###################################################
>
> More complete explanation of my question
>
> I have created a simple dataframe having three factors:
>  mydata <- data.frame(covid=c(0,0,0,0,1,1,1,1),
>  sex=(rep(c(1,1,0,0),2)),
>  status=rep(c(1,0),2),
>  values=c(382,4730,5,199,170,497,6,25))
>
>  > mydata
>   covid sex status values
> 1     0   1      1    382
> 2     0   1      0   4730
> 3     0   0      1      5
> 4     0   0      0    199
> 5     1   1      1    170
> 6     1   1      0    497
> 7     1   0      1      6
> 8     1   0      0     25
>
> When I use the by function with a sum as an argument, I don’t
> get the sums that I would expect to
> receive based either on the listing of the dataframe above,
> or from using by with print as an argument:
>
> > by(mydata,list(mydata$sex,mydata$status),sum)
> : 0
> : 0
> [1] 225
> -------------------------------------------------------------------------------
>
> : 1
> : 0
> [1] 5230
> -------------------------------------------------------------------------------
>
> : 0
> : 1
> [1] 14
> -------------------------------------------------------------------------------
>
> : 1
> : 1
> [1] 557
>
> I expected to see the following sums:
> 382+170=552
> 4730+170=4900
> 5+6=11
> 199+25=224
> Which as can be seen by the output above, I am not getting.
>
> Using print as an argument to the by function, I get the values
> grouped as I would expect, but for some reason I get a double
> printing of the values!
>
> > by(mydata,list(mydata$sex,mydata$status),print)
>   covid sex status values
> 4     0   0      0    199
> 8     1   0      0     25
>   covid sex status values
> 2     0   1      0   4730
> 6     1   1      0    497
>   covid sex status values
> 3     0   0      1      5
> 7     1   0      1      6
>   covid sex status values
> 1     0   1      1    382
> 5     1   1      1    170
> : 0
> : 0
>   covid sex status values
> 4     0   0      0    199
> 8     1   0      0     25
> -------------------------------------------------------------------------------
>
> : 1
> : 0
>   covid sex status values
> 2     0   1      0   4730
> 6     1   1      0    497
> -------------------------------------------------------------------------------
>
> : 0
> : 1
>   covid sex status values
> 3     0   0      1      5
> 7     1   0      1      6
> -------------------------------------------------------------------------------
>
> : 1
> : 1
>   covid sex status values
> 1     0   1      1    382
> 5     1   1      1    170
>
> What am I doing wrong, or what don’t I understand
> About the by function?
>
> Thank you
> John
>
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>
> John David Sorkin M.D., Ph.D.
>
> Professor of Medicine
>
> Chief, Biostatistics and Informatics
>
> University of Maryland School of Medicine Division of Gerontology and
> Geriatric Medicine
>
> 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)
>
>
>
> ______________________________________________
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