3/23/2009
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Chapter 9 – continued
Abstraction and User-Defined Functions
Multiple Inputs
if a function has more than one input,
parameters in the function definition are separated by
commas
input values in the function call are separated by commas
values are matched to parameters by order
1st input value in the function call is assigned to the 1st
parameter in the function
2nd input value in the function call is assigned to the 2nd
parameter in the function
. . .
Multiple Inputs
function OldMacVerse(animal, sound)
// Assumes: animal and sound are strings
// Results: displays corresponding Old
MacDonald verse
{
. . .
}
--------------------------------------------
OldMacVerse("cow", "moo");
OldMacVerse("moo", "cow");
Parameters and Locals
parameters play an important role in functions
they facilitate the creation of generalized computatio ns
i.e., the function defines a formula, but certain values w ithin the
formula can differ each time the function is call ed
technically, a parameter is a
local variable,
meaning it
exists only inside its particular function
when the function is called, memory cells are allo cated for the
parameters and each input from the call is assigned to its
corresponding parameter
once a parameter has been assigned a value, you can refer to that
parameter within the function just as you would any o ther variable
when the function terminates, the parameters “go away,” and
their associated memory cells are freed
Parameters and Locals
by default, variables other than parameters are
considered global, meaning they exist and can be
accessed by JavaScript code anywhere in the page
note: it is possible to use the same name to refer to a local
variable and a global variable
within the function, the local variable is acc essible
outside that function, the global variable is access ible
Declaring Local Variables
we have seen that variables are useful for storing
intermediate steps in a complex computation
within a user-defined function, the programmer is free to create
new variables and use them in specifying the function’s
computation
however, by default, new variables used in a function are glo bal
but what if the same variable name is already used elsewhere?
to avoid name conflicts, the programmer should declare
temporary variables to be local
a variable declaration is a statement that lists all local variables
to be used in a function (usually the first statement in a function)
general form: var LOCAL_1, LOCAL_2, . . ., LOCAL_n;
