For example, call by value / call by reference specifies that a function application evaluates the argument before it proceeds to the evaluation of the function's body and that it passes two capabilities to the function, namely, the ability to look up the current value of the argument and to modify it via an assignment statement.
12.
In case of call by value, one can think of the argument as a value ( properly, as the " value " of the argument ) for example, f ( 2 ) and a = 2; f ( a ) are equivalent calls while in call by reference one can think of the argument as a variable in the calling context.
13.
In the most common case, call by value, a parameter acts within the subroutine as a variable initialized to the value of the argument ( a local ( isolated ) copy of the argument if the argument is a variable ), but in other cases, e . g . call by reference, the argument supplied by the caller can be affected by actions within the called subroutine ( as discussed in evaluation strategy ).
