User-Defined Functions (cont’d) Reference Parameters
Outline
 Exercise
 Passing by value
 Scope of variables
 Reference parameters (section 6.1)
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Tracing Exercise
#include <iostream>
using namespace std;
Execution
int Blend( int red, int green );
// prototype
void main()
{
int red = 5, blue = 3;
blue = Blend(blue, red);
cout << red << ' ' << blue << '\n';
blue = Blend(red, blue);
cout << red << ' ' << blue << '\n';
}
int Blend( int red, int green )
{
int yellow;
// parameters passed by value
// local variable in Blend function
cout << “enter Blend “ << red <<‘ ‘ << green << ‘\n’;
yellow = red + green;
cout << “leave Blend “ << red <<‘ ‘ << green << ‘\n’;
return (yellow + 1);
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}
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Passing by Value
main function data area
actual arguments:
red
5
blue
3
Blend function data area
(1st call)
formal arguments:
red
3
green
5
Local arguments:
yellow
8
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Tracing Exercise (cont’d)
main
red blue
5
3
9
15
Blend (1st call)
red green yellow
3
5
8
Output:
enter Blend 3 5
leave Blend 3 5
59
enter Blend 5 9
leave Blend 5 9
5 15
Blend (2nd call)
red green yellow
5
9
14
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Scope of Variables
 Scope - where a particular meaning of a
variable identifier is visible or can be referenced
 Local - can be referred to only within a
program segment or function

In a program segment (localized declarations of
variables)
for (int i = 1; i <= 10; ++i)
cout << “*”;
• Commonly used for loop control variables
• Declared at point of first reference
• Value has meaning (i.e. can be referenced) only
inside loop segment.
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Scope of Variables (cont’d)

In a function this applies to
• formal argument names
• constants and variables declared within the function
 Global - can be referred to within all functions


useful especially for constants
must be used with care
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Listing 3.15
Outline of program for studying
scope of variables
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Scope of Variables (cont’d)
 Global variables MAX & LIMIT are visible
to main, one, funTwo functions
 Local variable localVar in main function
visible only to main function
 Local variables anArg, second & oneLocal
in one function are visible only to one
function
 Local variables one, anArg & localVar in
funTwo function are visible only to funTwo
function
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Passing by Reference Example
#include <iostream>
using namespace std;
int Blend( int& red, int green );
// prototype
void main()
{
int red = 5, blue = 3;
Blend(blue, red);
cout << red << ' ' << blue << '\n';
Blend(red, blue);
cout << red << ' ' << blue << '\n';
}
void Blend(int& red, int green) // green parameter passed by value,
// while red parameter passed by reference
{
int yellow;
// local variable in Blend function
cout << “enter Blend “ << red <<‘ ‘ << green << ‘\n’;
yellow = red + green;
cout << “leave Blend “ << red <<‘ ‘ << green << ‘\n’;
red = yellow + 1;
}
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Passing by Reference
main function data area
actual arguments:
red
5
blue
3
Blend function data area
(1st call)
formal arguments:
red
address
of blue
green
5
Local arguments:
yellow
8
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Passing by Reference Example
main
(cont’d)
red blue
5
3
9
15
Blend (1st call)
red green yellow
5
8
Output:
enter Blend 3 5
leave Blend 3 5
59
enter Blend 5 9
leave Blend 5 9
15 9
Blend (2nd call)
red green yellow
9
14
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User Defined Functions Can:
 return no value

type void
 return exactly one value
function type
 return statement

 return more than one value
type void or function type
 reference parameters

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Reference Parameters
 Formal parameter data type directly
followed by & indicate a parameter
passed by reference
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Listing 6.1
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Function to compute sum and
average
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Listing 6.1
Function to compute sum and
average (continued)
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User-Defined Function
computeSumAve
 Two function input parameters

num1, num2
 Two function output parameters

sum, average
 & indicates function output parameters
 Function call
computeSumAve(x, y, sum, mean);
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Argument Correspondence
Actual Argument
x
y
sum
mean
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Corresponds to
Formal Argument
num1 (fn. input)
num2 (fn. input)
sum (fn. output)
average (fn. output)
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Call-by-Value and Call-byReference Parameters
 & between type and identifier defines a
parameter as function output mode (pass
by reference)
 no & in a parameter’s declaration identifies
parameter as fuction input mode (pass by
value)
 Compiler uses information in parameter
declaration list to set up correct
argument-passing mechanism
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Figure 6.1
Data areas after call to
computeSumAve (before execution)
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Figure 6.2
Data areas after execution of
computeSumAve
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Notes on Call-by-Reference
 Place the & only in the formal parameter
list - not in the actual parameter list
 Place the & also in the prototype:
void computeSumAve(float, float, float&, float&);
 Note that this is a void function
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When to Use a Reference or a
Value Parameter
 If information is to be passed into a function and
doesn’t have to be returned or passed out of the
function, then the formal parameter representing
that information should be a value parameter
(function input parameter).
 If information is to be returned to the calling
function through a parameter, then the formal
parameter representing that information must be
a reference parameter (function output
parameter).
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When to Use a Reference or a
Value Parameter (cont’d)
 If information is to be passed into a
function, perhaps modified, and a new
value returned, then the formal parameter
representing that information must be a
reference parameter (input/output
parameter)
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Next lecture we will talk more about
Value and Reference Parameters
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