Equivalent of "throw" in R

Question

See help(tryCatch):

Conditions are signaled by 'signalCondition'. In addition, the
'stop' and 'warning' functions have been modified to also accept
condition arguments.

and later under 'See Also':

'stop' and 'warning' signal conditions, and 'try' is essentially a simplified version of 'tryCatch'.

so you probably want stop.

Simple example:

f <- function(a, b){ 

    if (a == 0){ 
            stop("error message")
    }
 }

Beyond the base functions that Dirk mentions:

The R.oo package has additional exception handling functionality, including a throw() function which is very useful. You can catch exceptions with the usual try or trycatch functions:

> try(throw("Division by zero.")); print("It's ok!");
Error: [2009-10-22 10:24:07] Exception: Division by zero.
[1] "It's ok!"

You can read more about it here: http://www1.maths.lth.se/help/R/R.oo/

Actually the function stopifnot is very convenient to implement sanity checks in your code. It takes in several logical expressions and returns an error if any of them evaluates to false.

Example: To check if column 'c' exists in the dataframe 'df':

df <- data.frame(a = numeric(), b = numeric())
stopifnot(!is.null(df$c))

This will throw the following error:

Error: !is.null(df$c) is not TRUE

You can check if the column exists and do whatever your want.
Suppose a data.frame named df1 and checking if column col1 exists:

if(! any(grepl('^col1$',colnames(df1)))) stop("nonexistent column")

or

if(! any(grepl('^col1$',colnames(df1)))) return(-1)

For instance