R help - May 2019 - Trying to understand the magic of lm (Still trying)

A number of people have helped me in my mission to understand how lm (and other
fucntions) are able to pass a dataframe and then refer to a specific column in
the dataframe. I thank everyone who has responded. I now know a bit about
deparse(substitute(xx)), but I still don't fully understand how it works.
The program below attempts to print a column of a dataframe from a function
whose parameters include the dataframe (df) and the column requested (col). The
program works fine until the last print statement were I receive an error, 
Error in `[.data.frame`(df, , col) : object 'y' not found . I hope
someone can explain to me (1) why my code does not work, and (2) what I can do
to fix it.


Many thanks to everyone who tries to help lost souls like me!


Thank you,

John


data <- data.frame(x=c(1,2,3,4,5),y=c(5,4,3,2,1))
data

doit <- function(df,col){
  dfx <- deparse(substitute(df))
  colx<- deparse(substitute(col))

  cat("results of deparse substitute")
  print(colx)
  print (dfx)

  cat("I can print the columns using column relative reference\n")
  print(df[,1])
  print(df[,2])

  cat("I can print the entire data frame \n")
  print(df)

  cat("I can print a single columng from the dataframe using a column
name\n")
  print(df[,col])
}

doit(data,y)









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)


	[[alternative HTML version deleted]]

On 5/10/19 12:53 PM, Sorkin, John wrote:
> A number of people have helped me in my mission to understand how lm (and
other fucntions) are able to pass a dataframe and then refer to a specific
column in the dataframe. I thank everyone who has responded. I now know a bit
about deparse(substitute(xx)), but I still don't fully understand how it
works. The program below attempts to print a column of a dataframe from a
function whose parameters include the dataframe (df) and the column requested
(col). The program works fine until the last print statement were I receive an
error,  Error in `[.data.frame`(df, , col) : object 'y' not found . I
hope someone can explain to me (1) why my code does not work, and (2) what I can
do to fix it.
>
>
> Many thanks to everyone who tries to help lost souls like me!
>
>
> Thank you,
>
> John
>
>
> data <- data.frame(x=c(1,2,3,4,5),y=c(5,4,3,2,1))
> data
>
> doit <- function(df,col){
>    dfx <- deparse(substitute(df))
>    colx<- deparse(substitute(col))
>
>    cat("results of deparse substitute")
>    print(colx)
>    print (dfx)
>
>    cat("I can print the columns using column relative
reference\n")
>    print(df[,1])
>    print(df[,2])
>
>    cat("I can print the entire data frame \n")
>    print(df)
>
>    cat("I can print a single columng from the dataframe using a column
name\n")
>    #
#Try instead:

 ? ? print( df[ , colx]? # colx will be a character value, ... there is 
no `y`-object
> #print(df[,col])
> }
>
> doit(data,y)
>
>
>
>
>
>
>
>
>
> 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)
>
>
> 	[[alternative HTML version deleted]]
>
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