On Wed, 11 Dec 2024, Sorkin, John writes:
> I am trying to use the aggregate function to run a function, catsbydat2, that
> produces the mean, minimum, maximum, and number of observations of the values
> in a dataframe, inJan2Test, by levels of the dataframe variable MyDay. The
> output should be in the form of a dataframe.
>
> #my code:
> # This function should process a data frame and return a data frame
> # containing the mean, minimum, maximum, and number of observations
> # in the data frame for each level of MyDay.
> catsbyday2 <- function(df){
> # Create a matrix to hold the calculated values.
> xx <- matrix(nrow=1,ncol=4)
> # Give names to the columns.
> colnames(xx) <- c("Mean","min","max","Nobs")
> cat("This is the matrix that will hold the results\n",xx,"\n")
>
> # For each level of the indexing variable, MyDay, compute the
> # mean, minimum, maximum, and number of observations in the
> # dataframe passed to the function.
> xx[,1] <- mean(df)
> xx[,2] <- min(df)
> xx[,3] <- max(df)
> xx[,4] <- length(df)
> cat("These are the dimensions of the matrix in the function",dim(xx),"\n")
> print(xx)
> return(xx)
> }
>
> # Create data frame
> inJan2Test <- data.frame(MyDay=rep(c(1,2,3),4),AveragePM2_5=c(10,20,30,
> 11,21,31,
> 12,22,32,
> 15,25,35))
> str(inJan2Test)
> cat("This is the data frame","\n")
> inJan2Test
>
> xx <-
> aggregate(inJan2Test[,"AveragePM2_5"],list(inJan2Test[,"MyDay"]),catsbyday2,simplify=FALSE)
> xx
> class(xx)
> str(xx)
> names(xx)
>
> # Create a data frame in the format that I expect aggregate would return
> examplar <-
> data.frame(mean=c(12,22,32),min=c(10,20,30),max=c(15,25,35),length=c(4,4,4))
> examplar
> str(examplar)
>
>
> While the output is correct (the mean, mean etc. are correctly calculated),
> the format of the output is not what I want.
>
> (1) Although the returned object appears to be a data frame, it does appear
> to be a "normal" data frame. (see the output of
> (2) The column names I define in the function are not part of the data frame
> that is created.
> (3) The returned values on each row are separated by commas. I would expect
> them to be separated by spaces.
> (4) When I run str() on the output it appears that the output dataframe
> contains a list.
>> str(xx)
> 'data.frame': 3 obs. of 2 variables:
> $ Group.1: num 1 2 3
> $ x :List of 3
> ..$ : num [1, 1:4] 12 10 15 4
> .. ..- attr(*, "dimnames")=List of 2
> .. .. ..$ : NULL
> .. .. ..$ : chr [1:4] "Mean" "min" "max" "Nobs"
> ..$ : num [1, 1:4] 22 20 25 4
> .. ..- attr(*, "dimnames")=List of 2
> .. .. ..$ : NULL
> .. .. ..$ : chr [1:4] "Mean" "min" "max" "Nobs"
> ..$ : num [1, 1:4] 32 30 35 4
> .. ..- attr(*, "dimnames")=List of 2
> .. .. ..$ : NULL
> .. .. ..$ : chr [1:4] "Mean" "min" "max" "Nobs"
>
> I want it to simply be a numeric dataframe:
>
> mean min max length
> 12 10 15 4
> 22 20 25 4
> 32 30 35 4
>
> which should return the following str
>
> examplar <-
> data.frame(mean=c(12,22,32),min=c(10,20,30),max=c(15,25,35),length=c(4,4,4))
> examplar
> str(examplar)
>
> 'data.frame': 3 obs. of 4 variables:
> $ mean : num 12 22 32
> $ min : num 10 20 30
> $ max : num 15 25 35
> $ length: num 4 4 4
You'll no doubt get answers that use 'aggregate', but
for such calculations I find 'tapply' much easier/clearer:
res <- tapply(inJan2Test$AveragePM2_5, ## what to compute on
inJan2Test$MyDay, ## what to group by
function(x) c(mean = mean(x), ## what to do for each group
min = min(x),
max = max(x),
length = length(x)))
The result will be a list of vectors, which you can
bind together:
do.call(rbind, res)
## min max mean length
## 1 10 15 12 4
## 2 20 25 22 4
## 3 30 35 32 4
(Though the result is a numeric matrix. But that is
only one 'as.data.frame' away from a data.frame, if it
has to be one.)
kind regards
Enrico
> John David Sorkin M.D., Ph.D.
> Professor of Medicine, University of Maryland School of Medicine;
> Associate Director for Biostatistics and Informatics, Baltimore VA Medical
> Center Geriatrics Research, Education, and Clinical Center;
> PI Biostatistics and Informatics Core, University of Maryland School of
> Medicine Claude D. Pepper Older Americans Independence Center;
> Senior Statistician University of Maryland Center for Vascular Research;
>
> Division of Gerontology and Paliative Care,
> 10 North Greene Street
> GRECC (BT/18/GR)
> Baltimore, MD 21201-1524
> Cell phone 443-418-5382
--
Enrico Schumann
Lucerne, Switzerland
https://enricoschumann.net
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