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Dynamic evaluation¶

Variables¶

Action	Code	Details
Generate valid variable names from string(s)	`make.names('a@b')`
Generate unique variable names from string(s)	`make.unique(rep('a', 2))`
Check if variable exists	`exists('var')`
Get variable name from expression	`deparse(substitute(expr))`
Fill in variable values (substitute) in expression	`substitute(expr, env)`	Useful when expression is called in another function
Force evaluation of variable	`force(arg)`

Action	Code	Details
Create new environment, inherit from global	`environment()`
Create new environment without parent	`new.env(parent = emptyenv())`	Technically has a parent, but inherits no variables
Check if variable exists in environment	`exists('var', envir=env)`
Get current environment	`sys.frame()`
Get parent environment	`parent.frame()`
Get package environment	`getNamespace('package')`
Find environment of variable	`pryr::where('var')`
Get value of variable in environment	`get('var', envir=env)`
Try get value of variable, with default return	`get0('var', envir=env, ifnotfound='default value')`

Action	Code	Details
Create call	`call = quote(A = 1)`
Create expression	`expr = expression(A = 1)`
Create expression from string	`expr = parse(text='A = 1')`
Evaluate call	`eval(call)`
Substitute a call	`do.call(substitute, list(CALL, env=ENV))`	where CALL is stored in a variable
Evaluate expression	`eval(expr)`
Evaluate expression with specified values for (undefined) variables	`a = 1 q = quote(a + b) eval(q, list(b=3))`
Get expression as string	`deparse(expr, width.cutoff=500)`
Force evaluation of variable	`force(arg)`
Lazy-evaluate a variable assignment through expression	`delayedAssign('x', 2 + 2)`
Call inline function	`{function(x) x + 1}(5)`
Call function with some arguments forced	`forceAndCall(f)`
Call function with arguments in list	`do.call(fun, list(5))`
Call function with arguments in list, ignoring unused	`R.utils::doCall(fun, n=100, args=env)`

Also referred to as piping. Snippets use the magrittr package

Action	Code	Details
Chain functions	`y = x %>% table %>% prop.table`
Chain function calls, using the former input as a later argument	`z = data.table(…) %>% .[, mean(x)]`	call must be placed in brackets for embedded function calls
Call consecutive function, but return the former value	`y = x %>% table %T>% print`
Call functions and update the LHS	`x %<>% sort %>% abs`

Action	Code	Details
Get all arguments, including defaults	`mget(names(formals()),sys.frame(sys.nframe()))`
Get all specified arguments, including ellipsis	`as.list(match.call()[-1])`
Pass all specified arguments to a nested function	`match.call() %>% eval()`
Pass all specified arguments to a nested function	`call = match.call() call$extra = 'test' eval(call)`
Pass all specified arguments to a nested function	`call = match.call() call[[1]] = as.symbol('newFunction') eval(call)`
Get name of the parent calling function	`parentCall = sys.calls()[[sys.nframe()-2]] as.character(parentCall[[1]])`