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

User · Answer

Nowadays you can also use lazy eval function from lazyeval package     gt  lazyeval  lazy eval  5 5    1  10

User · Answer

Alternatively  you can use evals from my pander package to capture output and all warnings  errors and other messages along with the raw results    gt  pander  evals  5 5     1    src  1   5   5    result  1  10   output  1    1  10    type  1   numeric    msg  msg messages NULL   msg warnings NULL   msg errors NULL    stdout NULL  attr   class    1   evals

User · Answer

You can use the parse   function to convert the characters into an expression   You need to specify that the input is text  because parse expects a file by default   eval parse text  5 5

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Similarly using rlang   eval parse expr  5 5

User · Answer

Sorry but I don t understand why too many people even think a string was something that could be evaluated  You must change your mindset  really  Forget all connections between strings on one side and expressions  calls  evaluation on the other side   The  possibly  only connection is via parse text         and all good R programmers should know that this is rarely an efficient or safe means to construct expressions  or calls   Rather learn more about substitute    quote    and possibly the power of using do call substitute            fortunes  fortune  answer is parse     If the answer is parse   you should usually rethink the question       -- Thomas Lumley         R-help  February 2005    Dec 2017  Ok  here is an example  in comments  there s no nice formatting    q5  lt - quote 5 5  str q5    language 5   5  e5  lt - expression 5 5  str e5    expression 5   5    and if you get more experienced you ll learn that q5 is a  call  whereas e5 is an  expression   and even that e5  1   is identical to q5   identical q5  e5  1       1  TRUE

User · Answer

The eval   function evaluates an expression  but  5 5  is a string  not an expression  Use parse   with text  lt string gt  to change the string into an expression    gt  eval parse text  5 5     1  10  gt  class  5 5    1   character   gt  class parse text  5 5     1   expression    Calling eval   invokes many behaviours  some are not immediately obvious    gt  class eval parse text  5 5      1   numeric   gt  class eval parse text  gray      1   function   gt  class eval parse text  blue     Error in eval expr  envir  enclos    object  blue  not found   See also tryCatch

User · Answer

Not sure why no one has mentioned two Base R functions specifically to do this   str2lang   and str2expression     These are variants of parse    but seem to return the expression more cleanly  eval str2lang  quot 5 5 quot        gt  10    eval str2expression  quot 5 5 quot        gt  10   Also want to push back against the posters saying that anyone trying to do this is wrong   I m reading in R expressions stored as text in a file and trying to evaluate them  These functions are perfect for this use case

[r] Evaluate expression given as a string

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