Evaluate expression given as a string

Question

I'm curious to know if R can use its eval() function to perform calculations provided by e.g. a string.

This is a common case:

eval("5+5") 

However, instead of 10 I get:

[1] "5+5" 

Any solution?

Answer 1

The eval() function evaluates an expression, but "5+5" is a string, not an expression. Use parse() with text=<string> to change the string into an expression:

> eval(parse(text="5+5")) [1] 10 > class("5+5") [1] "character" > class(parse(text="5+5")) [1] "expression" 

Calling eval() invokes many behaviours, some are not immediately obvious:

> class(eval(parse(text="5+5"))) [1] "numeric" > class(eval(parse(text="gray"))) [1] "function" > class(eval(parse(text="blue"))) Error in eval(expr, envir, enclos) : object 'blue' not found 

See also tryCatch.