new R user has regression trouble - r

Question

I am new to R and am trying to run a regression analysis. I have constructed arbitrary vectors with the c() function to learn the plot, lm, fit, abline, and summary functions. That has worked properly, but when trying to regress imported data, I receive the following error message. I don't know what's causing the error or how to fix it. Any thought? Thanks.
library(xlsx)
Loading required package: xlsxjars
Loading required package: rJava
x <- "~/Desktop/x.xlsx"
y <- "~/Desktop/y.xlsx"
X <- read.xlsx(x,1)
Y <- read.xlsx(y,1)
dim(X)
[1] 149 1
dim(Y)
[1] 149 1
plot(X,Y)
Error in stripchart.default(x1, ...) : invalid plotting method
plot(X)
plot(Y)
Also, I don't think I understand all of the arguments accepted in the read.xlsx function. For example, if sheetindex is meant to index the sheets, wouldn't, in this example, x be 1 and y be 2? But then:
X <- read.xlsx(x,1)
Y <- read.xlsx(y,2)
Error in sheets[[sheetIndex]] : subscript out of bounds
Furthermore, the dimension is incorrect. The .xlsx file has 1 column, 150 rows, and no header.
dim(X)
[1] 149 1
When converting to a .csv file, which I don't particularly want to do b/c of the total number of .xlsx file I have, I still have the same plotting error, however the dimension seems to be correct. In this example, the number of rows and columns remain the same at 1 and 150 respectively, but there is a header.
x <- "~/Desktop/x.csv"
y <- "~/Desktop/y.csv"
X <- read.table(x, header = T)
Y <- read.table(y, header = T)
plot(X,Y)
Error in stripchart.default(x1, ...) : invalid plotting method
dim(X)
[1] 150 1

Answer 1

The problem is that X and Y are objects called data frames (?data.frame for details) and not vectors. the plot function is actually a wrapper around a family of other object-specific plot functions and in this case is trying to plot using stripchart(), which is causing the problem. This reproduces the problem, and fixes it:
X=data.frame(x=1:100)
Y=data.frame(y=rnorm(100,mean=1:100,sd=5))
plot(X,Y)
names(X)
names(Y)
plot(X$x, Y$y)
Assuming your data always consist of just one column, you could fix your code above by converting from a data.frame into an object of type numeric (i.e., the same sort of object as X=c(1,2,3,5)) which can be done a variety of ways
X <- unlist(read.xlsx(x,1))
Y <- unlist(read.xlsx(y,1))
or, alternatively, it's better to just havie read.xlsx() return a list instead of a data.frame
X <- read.xlsx(x,1, as.data.frame=FALSE)
Y <- read.xlsx(y,1, as.data.frame=FALSE)
Or you can just access the first column of the data.frame when you call plot
plot(X[,1], Y[,1])
See the help files for what all these functions return (e.g. ?as.numeric, ?unlist, ?names, etc.) and also see ?class, ?mode and ?typeof for querying object properties.

Answer 2

Furthermore, the dimension is incorrect. The .xlsx file has 1 column, 150 rows, and no header.
Header is by default TRUE, so you should specify this in your read.xlsx call.
X <- read.xlsx(x,1, header = TRUE)
Regarding the plot error:
plot(X,Y)
Error in stripchart.default(x1, ...) : invalid plotting method
read.xlsx returns data.frames, thats why the error shows up. Here is an example:
X <- data.frame(rnorm(150))
Y <- data.frame(rnorm(150))
plot(X, Y)
# Error in stripchart.default(x1, ...) : invalid plotting method
Please read carefully the read.xlsx documentation and about R object types.