Evaluation
- Variables
- Environment
- Dynamic evaluation
- Chained evaluation
- Functions
Variables
| What | How | 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 in expression | substitute(expr, env) | Useful when expression is called in another function |
| Force evaluation of variable | force(arg) | |
Environment
| What | How | Details |
| Create new environment | environment() | |
| 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') | |
Dynamic evaluation
| What | How | 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) | |
| Evaluate expression | eval(expr) | |
| Evaluate expression with undefined terms | a = 1
q = quote(a + b)
eval(q, list(b=3)) | |
| 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) | |
| Substitute a call | do.call(substitute, list(CALL, env=ENV)) | where CALL is stored in a variable |
| Get expression as string | deparse(expr, width.cutoff=500) | |
Chained evaluation
| What | How | Details |
| Chain function calls | 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 | |
Functions
| What | How | 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 another 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]]) | |