Functions - Call By Value

1
Functions - Call By Value

• overloading
• return values part 2

2
Functions, again!

• are self-contained blocks of code, the inner
  workings of which are invisible to the remainder
  of the program.

• are subprograms in C .
• perform a specific task.
• can act on data and return a value.
• Every C program has at least one function
  main().

3
Functions

• Why use functions?
• make programs easier to write, debug and maintain
  - divide and conquer!

• Two main types of functions
• predefined -- found in the header files
• user-defined -- todays topic

4
The NOT rule for functions

• The arguments in the function prototype, the
  function call, and the function header must agree
  in number, order, and type
• Remember this rule!

5
Function Overloading

• Two or more distinct functions may have the same
  name.
• The data types of the arguments in the function
  calls must match those in the prototypes and in
  the definitions.
• The same function is given multiple definitions
  or implementations. The correct one is chosen by
  the compiler, not the programmer.

6
Function Overloading

• The functions must differ in their parameter
  lists. The type and/or number of parameters must
  be different.

Examples
double myFunction(int, int, int) int
myFunction(double, int, int) int myFunction
(double, double) void myFunction(double)

7
Function Overloading
.
// a is used // c is used // b is used // d is
used

8
Returning Values

• A function can receive many values
• Only one value can be directly returned

9
Returning Values

• The return statement
• tells the function which value to send back to
  the calling program
• terminates the function call and returns
  immediately to the calling program

10
Return Statement

• Syntax
• return expression
• Examples
• return c
• return hypotenuse

11
Return Statement

• int find_max(int x, int y)
• int maximum
• if (x gt y) maximum x else maximum
  y
• return maximum

12
Passing Data

• passing by reference may give back several
  values accomplished by using references (next
  topic) using pointers (much later)

13
Passing Data - by Value

• passing by valueA copy of a value is passed
  from the calling function to the called function.

double Pythagorus(double a, double b) a
a a b b b double c sqrt(aa
bb) return c
double Pythagorus(double a, double
b) double c c sqrt(aa bb) return
c

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Storing Values into Parameters
call to find_max
find_max(firstnum, secnum)
find_max(x, y)
x
y
865
9090

15
Passing Data - by Value

• void main(void)
• double height 4.0, base 3.0
• double Pythagorus(double, double)
• cout ltlt Hypotenuse ltlt Pythagorus(height,
  base)ltltendl
• . . .
• double Pythagorus(double a, double b)
• double c c sqrt(aa bb)
• return c

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Passing Data - by Value

• double Pythagorus(double a, double b)
• double c
• a
• b
• c sqrt(aa bb)
• return c

back in main cout ltlt height cout ltlt base

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Passing Data - by Value

• void print_val(int) // function prototype
• void main(void)
• int w 3
• cout ltlt"w before the function call is
  "ltltwltlt\n
• print_val(w)
• cout ltlt"w after the function call is
  "ltltwltlt\n
• void print_val(int q)
• coutltlt"Value passed to the function is
  "ltltqltltendl
• q q 2 // doubles the value
• coutltlt"Value at the end of the function is "ltlt q
  ltltendl

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Passing Data - by Value

• Output
• w before the function call 3
• Value passed to the function is 3
• Value at the end of the function is 6
• w after the function call is 3