Help understand the error in the function defined in R

Question

I am very new to R and just learned how to write simple functions. Can someone help me understand why the following function is not working.

fboxplot <- function(mydataframe, varx, vary) { p <- ggplot(data=mydataframe, aes(x=varx, y=vary)) p + geom_boxplot() } col1 = factor(rep(1:3, 3)) col2 = rnorm(9) col3 = c(rep(10,5), rep(20,4)) df = data.frame(col1 = col1, col2 = col2, col3 = col3) 

Now, if I call fboxplot function

 fboxplot(df, col1, col2) 

I get the error Error in eval(expr, envir, enclos): object varx not found . I also tried

 fboxplot(df, varx = col1, vary = col2) 

This gives the same error. Where am I mistaken?

Thank you for your help.

Answer 1

The aes function in ggplot2 uses library() type names, i.e. accepts the argument name as an argument. If it is an object, it does not evaluate it, but instead takes a name. Here it takes varx as an argument, not what varx also evaluates to.

It works if you use aes_string() instead and use characters as arguments in the fboxplot() call:

 fboxplot <- function(mydataframe, varx, vary) { p <- ggplot(data=mydataframe, aes_string(x=varx, y=vary)) p + geom_boxplot() } col1 = factor(rep(1:3, 3)) col2 = rnorm(9) col3 = c(rep(10,5), rep(20,4)) df = data.frame(col1 = col1, col2 = col2, col3 = col3) fboxplot(df, "col1", "col2") 

Answer 2

The problem is that you go through the varx and vary , and the aes function expects variable names (but not as strings). One way to fix this is to use the aes_string function, into which you can pass variable names as strings (still not vectors):

The following should work:

 fboxplot2 <- function(mydataframe, varx, vary) { p <- ggplot(data=mydataframe, aes_string(x=varx, y=vary)) p + geom_boxplot() } fboxplot2(df, "col1", "col2")