Functions Parameters

1
Functions Parameters Variable Scope
Chapter 6
2
Overview

• Using Function Arguments and Parameters
• Differences between Value Parameters and
  Reference Parameters
• Using Local Variables in a Function
• Variable Scope and Duration

3
Functions

• Every C program must have a main function
• execution starts by looking for a main
• All other functions are called directly from
  main, or indirectly through a chain of function
  called from main
• Function Calls
• One function calls another by using the name of
  the called function next to ( ) enclosing an
  argument list.
• ex. strlen(FirstName)
• A function call temporarily transfers control
  from the calling function to the called function.

4
Weve been using value parameters

• Simple types (like int, char, float, double) are
  value parameters by default
• you can specify reference parameters with
• To create a reference parameter place an after
  the type in both the function heading and
  prototype
• void Cube(int x)

5
When to Use Reference Parameters

• If you want your function to
• Assign a value to a variable from where it was
  called
• Change the value of a variable permanently
• Using a reference parameter
• is like giving someone the key to your home
• the key can be used by the other person to change
  the contents of your home!

6
Anatomy of a function call
myByRef(int a) a 10 myByVal(int a) a
100 int main() int age 25
myByVal(age) myByRef(age)
7
Anatomy of a function call
myByRef(int a) a 10 myByVal(int a) a
100 int main() int age 25
myByVal(age) myByRef(age)
8
Anatomy of a function call
myByRef(int a) a 10 myByVal(int a) a
100 int main() int age 25
myByVal(age) myByRef(age)
9
Anatomy of a function call
myByRef(int a) a 10 myByVal(int a) a
100 int main() int age 25
myByVal(age) myByRef(age)
10
Anatomy of a function call
myByRef(int a) a 10 myByVal(int a) a
100 int main() int age 25
myByVal(age) myByRef(age)
11
Anatomy of a function call
myByRef(int a) a 10 myByVal(int a) a
100 int main() int age 25
myByVal(age) myByRef(age)
12
Anatomy of a function call
myByRef(int a) a 10 myByVal(int a) a
100 int main() int age 25
myByVal(age) myByRef(age)
13
Example Pass-by-Reference

• We want to find 2 real roots for a quadratic
  equation with coefficients a,b,c. Write a
  prototype for a bool function named GetRoots( )
  with 5 parameters.
• Return value is true if real roots exist, false
  otherwise
• The first 3 (value) parameters are type float.
  The last 2 are reference parameters of type
  float.
• Need to pass 2 values back

14
GetRoots prototype

• Bool GetRoots(float, float, float, float,
  float)
• This function uses 3 incoming values
• It calculates 2 outgoing values, returning the 2
  real roots of the quadratic equation with
  coefficients a, b, c
  

15
Function Definition
bool GetRoots( float a, float b, float c,
float root1, float root2) float temp //
local variable temp b b - 4.0 a c
if (temp lt 0) return false else
root1 (-b sqrt(temp))/(2.0 a)
root2 (-b - sqrt(temp))/(2.0 a)
return true
16

• include ltiostreamgt
• include ltcmathgt
• bool GetRoots(float, float, float, float,
  float)
• using namespace std
• int main ( )
• float c1, c2, c3, first, second
• int count 0
• ...
• while ( count lt 5 )
• cin gtgt c1 gtgt c2 gtgt c3
• if (GetRoots(c1, c2, c3, first, second))
• cout ltlt first ltlt second ltlt endl
• else
• cout ltlt "No real solution"
• count
• ...

17
Data Flow ? Passing Mechanism
18
Functions with reference parameters

• Write a prototype and definition for a void
  function called GetGrade( ) with one reference
  parameter of type char
• The function repeatedly prompts the user to
  enter a character at the keyboard until one of
  these has been entered A, B, C, D, F

19
void GetGrade(char letter) // prototype

• void GetGrade(char letter)
• cout ltlt Enter employee rating. ltlt endl
• cout ltlt Use A, B, C, D, F
• cin gtgt letter
• while ((letter ! 'B') (letter ! 'C')
• (letter ! 'A') (letter ! 'D')
• (letter ! 'F'))
• cout ltlt "Invalid. Enter again "
• cin gtgt letter

20
Using a Parameter Passing Mechanism

• Pass-by-Reference
• use only if the design of the called function
  requires that it be able to modify the value of
  the parameter
• Pass-by-Constant-Reference
• use if the called function has no need to modify
  the value of the parameter, but the parameter is
  very large
• guarantees that the called function cannot modify
  the variable passed in
• Pass-by-Value
• use when none of the reasons given above apply