Function with Output Parameters

1
Function with Output Parameters

• We have seen that functions can return a single
  value or no value (void return type)
• It is quite often useful to be able to return
  more than one value
• In this case, we use output parameters to pass
  back the additional information
• The argument in this case must specify a location
  to put the value in, not a value

2
Function with Output Parameters

• When we want to specify the location to store
  (e.g.) an integer, we must declare a pointer to
  an integer
• void separate(double num, char signp,
• int wholep, double fracp)
• double magnitude
• if (num lt 0) signp -
• else if (num 0) signp 0
• else signp
• magnitude fabs(num)
• wholep floor(magnitude)
• fracp magnitude - wholep

3
Function with Output Parameters

• Now, if we want to call this function, we have to
  supply a value for the first argument and
  variables for the second, third, and fourth
• int main(void)
• double value
• char sn
• int whl
• double fr
• printf(Enter a value to analyzegt )
• scanf(lf, value)
• separate(value, sn, whl, fr)

4
Function with Output Parameters

• Notice how we specify the address of a variable -
  with the operator (as in scanf)
• In the called function, we must use var in
  expressions - otherwise we will be calculating
  with the address of the variable, not the value!
• A declaration such as int var declares var as a
  pointer to an integer variable (which is declared
  somewhere else)

5
Function with Output Parameters

• Note that we can pass a number (e.g. 5.24) as the
  first argument to the function, but we cant pass
  numbers (or characters) for the other arguments
  since they expect addresses not values
• What happens if we omit the operator when
  calling the function?

6
Meaning of the Symbol

• The symbol has three separate meanings in C
• The simple one is the binary multiplication
  operator var1 var2
• In a declaration it means that the variable is a
  pointer to an element of the given type char
  signp
• In the body of a function (e.g. in an
  expression), it means follow the pointer signp
  - myvar ptrvar 1

7
Arguments Used for Both I/O

• We have seen arguments used for input or for
  output
• We can also use a single argument for both input
  (pass information to the called function) and
  output (return information to the calling
  function)
• To do this we must use a pointer to a variable
• Lets write a function to add switch the values
  of two variables

8
Arguments Used for Both I/O

• void switch(int first, int second)
• int holder
• holder first
• first second
• second holder
• int main(void)
• int one 1, two 2 / Initialized! /
• printf(One d Two d\n, one, two)
• switch(one, two)
• printf(One d Two d\n, one, two)

9
Scope of Names

• The scope of a name refers to the region of a
  program where a particular meaning of a name is
  visible (can be referenced)
• We need to understand the scope of functions,
  variables, and constants
• For a constant - define PI 3.14 - we can use the
  constant only in the file in which it is declared
• Arguments are visible only in the function in
  which they are declared

10
Scope of Names

• Local variables (variables defined in a function)
  are visible only in that function
• Global variables (variables defined outside of
  and before all functions) are visible to all
  functions in the file
• Functions are visible to all other functions in
  the file
• Arguments and local variables can be declared
  with the same name as global functions or
  variables. In this case, they hide the globals

11
Scope of Names

• Functions cannot be nested in C (in many other
  languages they can, resulting in more complicated
  scope rules)
• More complications arise when we use more than
  one file to implement a program