Cannot assign column name by indexing within function argument - r

I am learning R, and I am trying to understand the indexing properties. I cannot seem to understand why the following code to change a column name does not work:
state.all <- as.data.frame(state.x77)
head(state.all)
state.all$States <- rownames(state.all)
rownames(state.all) <- NULL
# why the following row does not work?
names(state.all["States"]) <- "Test"
colnames(state.all)
While this works:
state.all <- as.data.frame(state.x77)
head(state.all)
state.all$States <- rownames(state.all)
rownames(state.all) <- NULL
# This work
names(state.all)[which(colnames(state.all)=="States")] <- "Test"
colnames(state.all)
Shouldn't the function be able to overwrite the name of the column also in the first example? Is it something to do with the local vs. global environment?
Thanks in advance!

What you're trying to do is replacing the name of column number 9.
the expression which(colnames(state.all)=="States") results in the index if the column named "States" (if there is any) and then takes this index and replaces the value in the names vector.
the expression state.all["States"] just returns the values of this column so of course nothing will happen.
I suggest something like colnames(state.all)[which(colnames(state.all)=="States")] <- "Test".

Related

How to get an R function to have a global effect on a dataframe?

I have been trying to create a function which will permanently change a value in specific cells of my data frame. I insert the data frame name, row index I wish to change, and the new name as a string. However, the function seems to change the value name within the local environment but not global.
The function is as follows:
#change name function
name_change <- function(df, row, name) {
df[row, 1] = name
return(df[row, 1])
}
E.g. if data frame was:
Name
Column B
Mark
2
Beth
4
The function name_change(df, 2, 'Jess') would change Beth to Jess.
When inserted as raw code it does permanently change the value. But then does not work when used as a function.
df[2, 1] = 'Jess'
Thanks in advance for your time
If you change your function like this:
name_change <- function(df, row, name) {
df[row, 1] = name
return(df)
}
and then assign the result of the function back to the original df, you will get the change you are looking for:
df = name_change(df,2,'Jess')
An alternative to the solutions already provided is to use the superassignment operator <<-. The ordinary assignment <- (or '=' you used) operate in your function's environment only. The superassignment reaches beyond your function's closure and can thus modify the dataframe residing in the global environment. Note, though, this is a quick'n'dirty fix only.
That said, the code would read like this:
#change name function
dirty_name_change <- function(df, row, name) {
df[row, 1] <<- name ## note the double arrow
}
You are returning the value of the cell, not the mutated df. R passes by arguments by value so you can imagine the function modifying a copy of df passed in. The solution is to return the mutated df and reassign it.
Can you pass-by-reference in R?

Problems with renaming columns via variables in R

I'm having issues with a specific problem I have a dataset of a ton of matrices that all have V1 as their column names, essentially NULL. I'm trying to write a loop to replace all of these with column names from a list but I'm running into some issues.
To break this down to the most simple form, this code isn't functioning as I'd expect it to.
nameofmatrix <- paste('column_', i, sep = "")
colnames(eval(as.name(nameofmatrix))) <- c("test")
I would expect this to take the value of column_1 for example, and replace (in the 2nd line) with "test" as the column name.
I tried to break this down smaller, for example, if I run print(eval(as.name(nameofmatrix)) I get the object's column/rows printed as expected and if I run print(colnames(eval(as.name(nameofmatrix))) I'm getting NULL as expected for the column header (since it was set as V1).
I've even tried to manually type in the column name, such as colnames(column_1) <- c("test) and this successfully works to rename the column. But once this variable is put in the text's place as shown above, it does not work the same. I'm having difficulties finding a solution on how to rename several matrix columns after they have been created with this method. Does anyone have any advice or suggestions?
Note, the error I'm receiving on trying to run this is
Error in eval([as.name](nameofmatrix)) <- \`vtmp\` : could not find function "eval<-"
We could return the values of the objects in a list with get (if there are multiple objects use mget, then rename the objects in the list and update those objects in the global env with list2env
list2env(lapply(mget(nameofmatrix), function(x) {colnames(x) <- newnames
x}), .GlobalEnv)
It can also be done with assign
data(mtcars)
nameofobject <- 'mtcars'
assign(nameofobject, `colnames<-`(get(nameofobject),
c('mpg1', names(mtcars)[-1])))
Now, check the names of 'mtcars'
names(mtcars)[1]
#[1] "mpg1"

How to remove the first row from multiple dataframes?

I have multiple dataframes and would like to remove the first row in all of them.
I have tried using a for loop but cannot understand what I am doing wrong
for (i in cities){
i <- i[-1, ]
}
I get the following error code:
Error in i[-1, ] : incorrect number of dimensions
If we assume that the only objects in your workspace are dataframes then this might succeed:
cities <- objects() )
for (i in cities) { assign(i, get(i)[-1,])}
Explanation:
Two thing wrong with original codes:
One was already mentioned in comments. "df" is not the same as df. You need to use get to convert a character value to a "true" R name that is used to retrieve an object having that name. The result of object() is only a character value. In R the term "name" means a "language object". See the help page: ?mode. (There is potential confusion about rownames and columnnames which are always "character"-class.) It's not like SAS which is a macro language that has no such distinction.
The second error was trying to get substitution for the i on the left-hand side of <-. The would have failed even if you were working with actual R names. The assign function is designed to handle character values that are then converted to R names.
say you get a list of all the tables in your environment, and you call that list cities. You can't just iterate over each value of cities and change things, because in the list they are just characters.
Here is what you need:
for (i in cities){
tmp <- get(i) # load the actual table
tmp <- tmp[-1, ] # remove first column
assign(i, tmp) # re-assign table to original table name
}

R: add column to dataframe, named based on formula

More 'feels like it should be' simple stuff which seems to be eluding me today. Thanks in advance for assistance.
Within a loop, that's within a function, I'm trying to add a column, and name it based on a formula.
I can bind a column & its name is taken from the bound object: data<-cbind(data,bothdata)
I can bind a column & manually name the bound object: data<-cbind(data,newname=bothdata)
I can bind a column which is the product of an equation & manually name the bound object: data<-cbind(data,newname2=bothdata-1)
Or another way: data <- transform(data, newColumn = bothdata-1)
What I can't do is have the name be the product of a formula. My actual formula-derived example name is paste("E_wgt",rev(which(rev(Esteps) == q))-1,"%") & equation for column: baddata - q.
A simpler one: data<-cbind(data,paste("magic",100,"beans")=bothdata-1). This fails because cbind isn't expecting the = even though it's fine in previous examples. Same fail for transform.
My first thought was assign but while I've used this successfully for creating forumla-named objects, I can't see how to get it to work for formula-named columns.
If I use an intermediary step to put the naming formula in an object container then use that, e.g.:
name <- paste("magic",100,"beans")
data<-cbind(data,name=bothdata-1)
the column name is "name" not "magic100beans". If I assign the equation result to an formula-named object:
assign(paste("magic",100,"beans"),bothdata-1)
Then try to cbind that via get:
data<-cbind(data,get(paste("magic",100,"beans")))
The column is called "get(paste("magic",100,"beans"))". Boo! Any thoughts anyone? It occurs to me that I can do cbind then separately colnames(data)[ncol(data)] <- paste("magic",100,"beans")) which I guess I'll settle for for now, but would still be interested to find if there was a direct way.
Thanks.
Chances are that cbind is overkill for your use case. In almost every instance, you can simply mutate the underlying data frame using data$newname2 <- data$bothdata - 1.
In the case where the name of the column is dynamic, you can just refer to it using the [[ operator -- data[["newcol"]] <- data$newname + 1. See ?'[' and ?'[.data.frame' for other tips and usages.
EDIT: Incorporated #Marek's suggestion for [["newcol"]] instead of [, "newcol"]
It may help you to know that data$col1 is the same than data[,"col1"] which is the same than data[,x] if x is "col1". This is how I usually access/set columns programmatically.
So this should work:
name <- paste("magic",100,"beans")
data[,name] <- obsdata-1
Note that you don't have to use the temporary variable name. This is equivalent to:
data$magic100beans <- obsdata-1
Itself equivalent, for a data.frame, to:
data<-cbind(data, magic100beans=bothdata-1)
Just so you know, you could also set the names afterwards:
old_names <- names(data)
name <- paste("magic",100,"beans")
data <- cbind(data, bothdata-1)
data <- setNames(data, c(old_names, name))
# or
names(data) <- c(old_names, name)

Is there a more efficient/clean approach to an eval(parse(paste0( set up?

Sometimes I have code which references a specific dataset based on some variable ID. I have then been creating lines of code using paste0, and then eval(parse(...)) that line to execute the code. This seems to be getting sloppy as the length of the code increases. Are there any cleaner ways to have dynamic data reference?
Example:
dataset <- "dataRef"
execute <- paste0("data.frame(", dataset, "$column1, ", dataset, "$column2)")
eval(parse(execute))
But now imagine a scenario where dataRef would be called for 1000 lines of code, and sometimes needs to be changed to dataRef2 or dataRefX.
Combining the comments of Jack Maney and G.Grothendieck:
It is better to store your data frames that you want to access by a variable in a list. The list can be created from a vector of names using get:
mynames <- c('dataRef','dataRef2','dataRefX')
# or mynames <- paste0( 'dataRef', 1:10 )
mydfs <- lapply( mynames, get )
Then your example becomes:
dataset <- 'dataRef'
mydfs[[dataset]][,c('column1','column2')]
Or you can process them all at once using lapply, sapply, or a loop:
mydfs2 <- lapply( mydfs, function(x) x[,c('column1','column2')] )
#G.Grothendieck has shown you how to use get and [ to elevate a character value and return the value of a named object and then reference named elements within that object. I don't know what your code was intended to accomplish since the result of executing htat code would be to deliver values to the console, but they would not have been assigned to a name and would have been garbage collected. If you wanted to use three character values: objname, colname1 and colname2 and those columns equal to an object named after a fourth character value.
newname <- "newdf"
assign( newname, get(dataset)[ c(colname1, colname2) ]
The lesson to learn is assign and get are capable of taking character character values and and accessing or creating named objects which can be either data objects or functions. Carl_Witthoft mentions do.call which can construct function calls from character values.
do.call("data.frame", setNames(list( dfrm$x, dfrm$y), c('x2','y2') )
do.call("mean", dfrm[1])
# second argument must be a list of arguments to `mean`

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