[R] Utilizing column names to multiply over all columns

[David Winsemius]

On Aug 16, 2011, at 3:37 PM, Sam Albers wrote:

> ## Hello there,
> ## I have an issue where I need to use the value of column names to
> multiply with the individual values in a column and I have many
> columns to do this over. I have data like this where the column names
> are numbers:
>
> mydf <- data.frame(`2.72`=runif(20, 0, 125),
>                  `3.2`=runif(20, 50, 75),
>                  `3.78`=runif(20, 0, 100),
>                  yy= head(letters,2), check.names=FALSE)

 > mydf
         2.72      3.2      3.78 yy
1   31.07874 74.48555 89.306591  a
2  123.68290 74.30030 11.943576  b
3   89.64024 68.26378 97.627211  a
4   81.46604 59.79607 91.005217  b

>
> ## I had been doing something like this but this seems rather tedious
> and clunky. These append the correct values to my dataframe but is
> there any way that I can do this generally over each column, also
> using each column name as the multiplier for that column?
>
> mydf$vd2.72 <- mydf$'2.72'*2.72
> mydf$vd3.2 <- mydf$'3.2'*3.2
> mydf$vd3.78 <- mydf$'3.78'*3.78
>
> ## So can I get to this point with a more generalized solution? For
> now, I would also prefer to keep this in wide format and I am aware
> (thanks to the list!) that I could use melt() to get the values I
> want.

You will get the warning that last last column is not "going right"  
but otherwise this returns what you asked for:

sapply(1:length(mydf), function(i) mydf[[i]]* as.numeric(names(mydf) 
[i])  )
            [,1]     [,2]      [,3] [,4]
  [1,]  84.53416 238.3538 337.57891   NA
  [2,] 336.41748 237.7610  45.14672   NA
  [3,] 243.82145 218.4441 369.03086   NA
  [4,] 221.58762 191.3474 343.99972   NA
  [5,]  81.78911 213.0770  97.90072   NA
snipped remainder

-- 

David Winsemius, MD
West Hartford, CT