Lecture #16: Functions
November 8, 1999
Bob Pitha
bpitha@cs.ucsd.edu
http://www-cse.ucsd.edu/classes/fa99/cse5a
whats a function?
A group of declarations and statements that is assigned a name
effectively, a named statement block
A sub-program
we
write our program by writing a function named main
inside which we can use other functions
which can themselves use other functions, and so on
why use functions?
They can break your problem down into smaller sub-tasks
easier to solve complex problems
They make a program much easier to read
abstraction
like printf, we just have to know what a function does without worry about the details of how it works
Generalize a repeated set of instructions
we dont have to keep writing the same thing over and over
characteristics of functions
Name
Arguments
Body
Return value
function name
Is an identifier
This is how you refer to the function elsewhere in the program
function arguments
These are variables whose values you specify every time you invoke the function
so they can have different values each time
function body
These are the declarations and statements that actually do the work
Specified in the function definition
return value
A function can optionally return a value.
This value becomes the value of the function call expression
If a function doesnt return a value, the return type
should be void
function declaration
Just as with variables, functions can be declared
The declaration contains:
the function name
the data types of the arguments
the data type of the return value
function declaration vs. definition
The definition is the actual function
it has the function name
it has the argument declarations
it specifies the return type
it has the statements that comprise the function body
The declaration is just a reference to the function
it has the function name
it has the argument data types
it specifies the return type
it has no code
no statements or declarations
what does the function declaration do?
it enables you to call a function without having to know the definition
remember when we used printf?
we didnt know how the function worked
but the compiler needed to know that printf was actually a function
the file stdio.h contains a declaration for printf
function definition is a declaration
You need to declare a function before you can call it
at least you do in order to adhere completely to the ANSI standard
You can declare the function by:
including a function prototype
including the function definition
function prototype
This is a fancy name for a function declaration that isnt the function definition
It must include:
the return type
the function name
the data types of the arguments in parentheses
optionally with names, but theyre ignored
and a semicolon after the right parenthesis
an analogy
When you buy a TV, you get a users manual
that manual tells you how to change channels
in case youve been living in a cave for the past 50 years
once youve read the manual, I can tell you, change to channel 7 and you know what to do
or I can say, change to channel 4
In this analogy:
change the channel is the function
the channel number is the argument
another analogy
You know (or can learn) how to calculate the geometric area of an ellipse
given the major and minor axes
Once you know, all I have to do is give you the two values, and you give me back a number
In this analogy:
calculate the area of an ellipse is the function
the lengths of the 2 axes are the arguments
the area is the return value
function example
what does this function do?
It takes a single argument
an int
It returns the value of that argument multiplied by two
also an int
how might we call this function?
note this
Variable names
inside MultByTwo, the argument is named x
when we call it, we pass a variable named i
there is no correspondence between the name of what we pass in and what we call it inside the function
function call expression
We now have a new expression: the function call
the expression is evaluated by executing the statements in the function
the value of the function call expression is determined by the first return statement encountered in the function
function call expression example
So given the code:
i
= MultByTwo (37) + 4;
the statement is an assignment expression statement
operator
is =
operands
are i
and the expression MultByTwo(37)
+ 4
the sub-expression is an arithmetic expression
operator
is +
operands
are the expression MultByTwo(37)
and 4
MultByTwo(37) is an expression
it
is evaluated by calling the function MultByTwo, with the argument 37
function call expression example continued
When the computer evaluates the expression
MultByTwo (37)
it invokes the function:
function call
The local variable x is set to the value of the argument
which in this case was a constant, 37
retval is set to 74
the value 74 is returned, which means the value of the expression MultByTwo(37) becomes 74
expression evaluation
i
= MultByTwo (37) + 4;
So the value of the expression
MultByTwo (37) + 4
becomes 78
And this is what i gets set to
return statement
This is the statement that causes the function to return a value
it must return a value that can be converted into the data type specified as the function return value
A function may have any number of return statements, but the first one encountered will end the execution of the function and return the value
return statement example
function arguments
Each argument declares a local variable that is only accessible from inside the function
its value is set using the value that is specified in the call
it ceases to exist after the return statement is executed
usually
Important concept here:
the local variable inside the function is totally distinct from the variable whose value was used to call the function
so if the function changes the value of one of its arguments, it doesnt affect the variable used to call the function
pass-by-value
This is the term that means that function arguments are passed into the function by copying their values
rather than giving the function direct access to the actual variables
