Evaluate expression given as a string

r eval r-faq
Federico Giorgi picture Federico Giorgi · Nov 16, 2009 · Viewed 191.3k times · Source

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

Harlan picture Harlan · Nov 16, 2009

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.

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