What is the difference between cat and print?

Question

cat and print both seem to offer a \"print\" functionality in R.

x <- \'Hello world!\\n\'
cat(x)
# Hello world!
print(x)
# [1] \         


        

Answer 1

An essential difference between cat and print is the class of the object they return. This difference has practical consequences for what you can do with the returned object.

print returns a character vector:

> print(paste("a", 100* 1:3))
[1] "a 100" "a 200" "a 300"

> class(print(paste("a", 100* 1:3)))
[1] "a 100" "a 200" "a 300"
[1] "character" 

cat returns an object of class NULL.

> cat(paste("a", 100* 1:3))
a 100 a 200 a 300

> class(cat(paste("a", 100* 1:3)))
a 100 a 200 a 300[1] "NULL"

In some cases, it is important to return the output as is in your console, for example when you want to copy-paste the output. In those cases, you really don't want to return a character vector. I found it a useful strategy in those cases to combine print and cat: Use print to create the object, use cat to print it to your console.

> output <- print(paste("a", 100* 1:3)) # use print to create the object
> cat(output) # use cat to print it *as is* to your console
a 100 a 200 a 300

Using xtable package for printing LaTeX tables in R:

> require(xtable)
> df <- data.frame(a = 1, č = 5) # dataframe with foreign characters
> output <- print(xtable(df), include.rownames = FALSE)
> output <- gsub("č", "c", output) # replace foreign characters before 
> # copying to LaTeX

> cat(output)
\begin{table}[ht]
\centering
\begin{tabular}{rr}
\hline
a & c \\
\hline
1.00 & 5.00 \\ 
\hline
\end{tabular}\end{table}

> print(output)
[1] "\\begin{table}[ht]\n\\centering\n\\begin{tabular}{rr}\n  
\hline\na & c    \\\\ \n  \\hline\n1.00 & 5.00 \\\\ \n   
\\hline\n\\end{tabular}\n\\end{table}\n"