Lecture #17: Functions 2

November 10, 1999

Bob Pitha

bpitha@cs.ucsd.edu

http://www-cse.ucsd.edu/classes/fa99/cse5a

let’s review functions

• A function is like a small program within the program

• You pass data into the function via its arguments

• You get data back from the function via its return value

• You call the function using a function call expression

function example

function example – arguments

arguments

• The functions argument define variables for the function

– these variables are only available from inside the function

– their values are set to whatever values are “passed in” when the function is called

– these are copies of the values that are passed. Changing the value of an argument does not change the value of the variable whose value was passed in

function example – return value

return value

• The function can return a single value

– its data type is defined as part of the function declaration

– if the function is declared as returning a value, it must return a value

– if you don’t want to return a value, the return type should be void

function call expression

• You invoke a function when the computer evaluates a function call expression

– function name followed by arguments in parentheses

– commas separate multiple arguments

– if no arguments, just empty parentheses

let’s look at an example

example

Function 1: main()

Function 2: print_heading()

Function 3: print_table()

Function 4: power()

function declaration issues

• We must declare a function before we can call it

– we can use a function prototype

– or we can use the function definition as a declaration

• but only so long as the definition comes before the call

• so in our example, we used prototypes because our function definitions came after the function calls

example

function definition order

• But if we had introduced the functions in a different order, we wouldn’t need to use prototypes

– because the definition also declares the function

example (rearranged)

prototype issues

• There are some issues here…

– this can be harder to read

• since main now comes at the end

• it’s kind of Reverse Polish Notation for programs

– it only works within a single file

• we’ve only seen programs that fit in a single file

• but we can use multiple .cpp files to write our program

• every function must be defined in a single file

• but each function could be in a different file

example

variable declarations

• We can now revisit variable declarations, and discuss some issues we passed over the first time

variable concepts

• scope

• visibility

• extent

scope

• The region of the program code in which declaration has effect

– Local variables: declaration through to end of block

– Global variables: declaration through to end of file

– Parameters: through to end of function

what does this mean?

• It means that when you declare a variable, it doesn’t mean the variable is available everywhere

example – local scope

example – parameter scope

example – global scope