How to use sprintf() function to format columns in R - r

Question

I am a newbie and learning R. Please accept my apology for stupid questions.
I am Stuck with an error and unable to understand what it is exactly?
"Error in [.data.frame: undefined columns selected"
Can anyone help me with this, please?
When I use the below:
sprintf("%s", "Return (%)")
It worked fine and render the table for me correctly.
But If I use sprintf("%.1f", "Return (%)"), it gives the error with
"Error in sprintf: invalid format '%.1f'; use format %s for character objects"
Now, I converted the column in numeric with
sprintf("%.1f", as.numeric("Return (%)")).
Then I get the below error again:
"Error in [.data.frame: undefined columns selected"

Answer 1

R uses the extract operator to access information within an object. Since a data frame is also an R object, we use the extract operator to access columns in a data frame.
The first sprintf() function fails because it interprets "Return (%)" as text, and therefore generates an invalid format error.
The second sprintf() function fails because as.numeric("Return (%)") is parsed as an incomplete reference to a column in a data frame.
To extract columns from a data frame, multiple forms of the extract operator can be used:
1. dataframe[rows,columns]
2. dataframe[,"columnName"]
3. dataframe[["columnName"]]
4. dataframe$columnName
Since columns can be reference by name or position (from left to right),
one way to solve the problem described in the question is to use the column number with the extract operator to reference the percentage return column, as follows.
textFile <- "symbol,Return (%)
AAPL,23.1
GOOG,30.5
IBM,11.8
MSFT,14"
stockReturns <- read.csv(text=textFile)
# version that works: Return (%) is second column in data frame
sprintf("%.1f",stockReturns[,2])
...and the output:
> sprintf("%.1f",stockReturns[,2])
[1] "23.1" "30.5" "11.8" "14.0"
>
Why did the error occur?
When we print the data frame, we see how R parsed the column name when reading the header via read.csv().
stockReturns
> stockReturns
symbol Return....
1 AAPL 23.1
2 GOOG 30.5
3 IBM 11.8
4 MSFT 14.0
>
Interesting! R converted the space and special characters to ..... Now let's try sprintf() again with how R interpreted the column name, and the [[ form of the extract operator.
> sprintf("%.1f", stockReturns[["Return...."]])
[1] "23.1" "30.5" "11.8" "14.0"
>
Now that we know the problem is caused by special characters in the column name, we can use the colnames() function to rename the Return (%) column name to something we can more easily reference in R code. This allows one to use multiple forms of the extract operator to access the column from a data frame.
colnames(stockReturns) <- c("symbol","return_pct")
sprintf("%.1f",stockReturns[["return_pct"]])
sprintf("%.1f",stockReturns$return_pct)
sprintf("%.1f",stockReturns[,"return_pct"])
...and the output:
> colnames(stockReturns) <- c("symbol","return_pct")
> sprintf("%.1f",stockReturns[["return_pct"]])
[1] "23.1" "30.5" "11.8" "14.0"
> sprintf("%.1f",stockReturns$return_pct)
[1] "23.1" "30.5" "11.8" "14.0"
> sprintf("%.1f",stockReturns[,"return_pct"])
[1] "23.1" "30.5" "11.8" "14.0"
>
Note: this answer references content from my blog article, Forms of the Extract Operator.