FUNCTIONS - II
Chapter 9 and 10
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Today we will continue with functions
covering….
Passing by Reference 
Scope
Sorting variables
Function Structure Charts (Ch10)
2
This function changes its parameter
void countdown(float n)
{
while (n>0) {
cout <<n;
n=n-1;}
cout << "...ZERO!!!“<<endl;
}
int main( )
{ float x=5;
countdown(x);
cout<<“after countdown(), x is “<<x;
}
Q: What does x hold after fn call?
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Program Output
54321…ZERO!!!
after countdown(), x is 5
(WHY?)
Pass-by-Value:
Only a copy of the argument x is given to the
function
Even though n changes, x retains original value
Sometimes you need the function to change
the argument. Thus we also have …
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Passing by Reference
This is done in situations where a
function needs to change the value of
the argument passed to it
To pass an argument by reference,
simply append an ampersand ‘&’, to the
type specifier in the function’s
parameter list
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Passing by Reference
(contd..)
Then the local variable is only a
reference to the argument passed to it
The argument is read-write instead of
read-only(as in the case of pass by
value)
Any change to the local variable inside
the function will cause the same change
to the argument that was passed to it
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This function uses PBR
void countdown(float& n)
{
while (n>0) {
Added Only 1 Character!
cout <<n;
n=n-1;}
cout << "...ZERO!!!“<<endl;
}
int main( )
{ float x=5;
countdown(x);
cout<<“after countdown(), x is “<<x;
}
Q: What does x hold after fn call?
7
New Program Output
54321…ZERO!!!
after countdown(), x is 0
(WHY?)
Pass-by-Reference:
Direct access to argument x is given to the
function (Read/Write)
When it changes “n”, it really changes x
Here n is another name for the argument, x
Often called a reference or “alias” to x
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Demonstrate politic.cpp in Ch09
PBR Functions can “return” more than 1 value
Reference parameters can be same name or
different name than arguments
Draw lines between arguments and parameters
Arguments to Reference parameters must be
variables of the same type (no constants)
Arguments to Value parameters can be
constants, variables, or expressions, and type is
more lenient
Illustrate by modifying politic.cpp
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Demonstrate circle.cpp, ch09
PBR Functions can calculate 2 or more
quantities and return them
Some parameters are Value, some are
Reference
Draw lines between arguments and
parameters
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Typical Test Question….output?
void f(float x, float& y)
{ // changes reference argument to 99:
x = 88; y = 99;
}
int main()
{
float a = 22, b = 44;
cout<< "a = “<< a << ", b = "<< b << endl;
f(a,b);
cout<< "a = "<< a << ", b = "<< b << endl;
}
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The call f(a,b) passes a by value to x and it
passes b by reference to y
x is a local variable that is assigned a’s value
of 22
y is an alias for the variable b whose value is
44
The function assigns 88 to x but that has no
effect on a
When it assigns 99 to y it is really assigning 99
to b, because y is an alias for b
Finally, a still has its original value 22 (readonly), while b has the new value 99(read-write)
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Summing up…
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Your Turn
What does this program display?
void func1 (int &x, int &y, int z);
int main()
{ int a=2, b=2, c=2;
func1 ( a, b, c);
cout <<"a="<<a<<"b="<<b<<"c="<< c <<endl;
}
void func1 (int &x, int &y, int z)
{ x=x+1;
y=y-1;
z=z+2;
}
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How do you know when to use &?
Don’t overdo it because you are not sure
Ask…will the function need to initialize or
update an argument you are giving it?
YES…then use a reference parameter
NO…then use a value parameter
See circle.cpp again to study the value and
reference parameters
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Time for Break !!
Download Lab8Functions.cpp
Do problems 1-4
16
Today we will continue with functions
covering….
Passing by Reference
Scope 
Sorting variables
Function Structure Charts (Ch10)
17
Remember scope ?
The scope of a name consists of that
part of the program where it can be
used
It begins where the name is declared
The scope extends to the end of the
innermost block that contains the
declaration
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void f(); // f() is global
void g(); // g() is global
float x = 11; // this x is global
int main()
{ float x = 22;
{ float x = 33;
cout<<"In block inside main(), x = "<<x<<endl;
} // end scope of internal block
cout<<"In main(), x = "<<x<< endl;
cout<<"In main(), ::x = "<<::x<<endl; //global x
f();
g();
} // end scope of main()
void f()
{ float x = 44;
cout << "In f(), x = " << x << endl;
} // end scope of f()
void g()
{ cout << "In g(), x = " << x << endl;
} // end scope of g()
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In the previous example…
f() and g() and the first x are global –
their scope includes the entire file
The second x is declared inside main() ,
it is accessible only from within the
main()
The third x is declared inside an internal
block, so its scope is restricted to that
internal block
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PROGRAM OUTPUT for Previous
Example
In
In
In
In
In
block inside main(), x = 33
main(), x = 22
main(), ::x = 11
f(), x = 44
g(), x = 11
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Scope and You: the Rules!!
Don’t use global variables for now. You
don’t need them.
Lazy programmers try to skip writing
parameter lists by making all function
variables global—0 points if you do!!
Best is to declare all variables needed at
beginning of main/other function
Advice—use different names for function
call arguments and function parameters
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Today we will continue with functions
covering….
Passing by Reference
Scope
Sorting variables 
Function Structure Charts (Ch10)
23
Sorting Data
Arranging data in increasing order
A real “bread and butter” CS/IS task
Sorted data are easier to search
Usually a slow process for long lists of data
Efficiency of advanced sort methods can
result in tremendous gains in processing
time (CSIS 10B)
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Sort works by using Swap
One of the most useful algorithms
Swap two variables:
float x=3, y=5, temp;
temp=x;
x=y;
y=temp;
x
y
temp
3
5
5
5
3
?
3
3
3
Used so much we want to make a function
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void swap(float& x, float& y)
{ // exchanges the values of x , y:
float temp = x;
x = y;
y = temp;
}
int main()
{
float a = 22.2, b = 44.4;
cout<<"a = "<<a<<",b = "<<b<<endl;
swap(a,b);
cout<<"a = "<<a<<",b = "<<b<<endl;
}
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Let’s understand that program..
The formal parameters x and y are
declared as reference variables :
float& x, float& y
The reference operator & makes x and
y synonyms for the arguments passed
to the function
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Basic Sort
If two adjacent data are out of order, swap
float a, b;
cin>>a>>b;
if (a>b) swap(a, b);
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Extending Sort for 3 variables
cin>>a>>b>>c;
If any two adjacent data are out of order, swap
if (a>b) swap(a, b);
if (b>c) swap(b, c);
Recheck 1st two in case c was smallest
if (a>b) swap(a, b);
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Demonstrate alpha3.cpp, ch09
Modify to simplify, only swap w1 and w2 and
show changes
What if you want to sort integers?
Function overloading…
make another swap(int &x, int &y)
Compiler can tell which one to go to during execution
by looking at the arguments you are using
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Making a Sort3( ) function…revA
//Sort3( ) Sorts three params in numeric order
//IN/OUT: a, b, and c are returned in numeric order
// currently only a stub
void Sort3(float &a, float &b, float &c)
{ cout<<“values passed to Sort():”<<endl;
This is called a
cout<< a <<“ ”<< b <<“ ”<< c << endl;
“function stub”
a=1.0;
b=2.0;
It just tests
c=3.0;
parameter
}
passing
int main( )
{ float r, s, t;
cin>>r>>s>>t;
Sort3(r, s, t);
cout<<“in order:” << r <<“ ”<< s <<“ ”<< t << endl;
}
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// Add swap definition here
//Sort3( ) Sorts three params in numeric order
//IN/OUT: a, b, and c are returned in numeric order
void Sort3(float &a, float &b, float &c)
{ if (a>b) swap(a, b);
if (b>c) swap(b, c);
if (a>b) swap(a, b);
}
int main( )
{ float r, s, t;
cin>>r>>s>>t;
Sort3(r, s, t);
cout<<“in order:” << r <<“ ”<< s <<“ ”<< t << endl;
}
Making a sort3( )
function…revB
32
Today we will continue with functions
covering….
Passing by Reference
Scope
Sorting variables
Function Structure Charts (Ch10) 
33
Structure Charts (see p226 +)
Show the large scale layout of your program
Each block is a function
Linkage between functions shown
value and reference parameters
Demonstrate structure charts for
Politic.cpp
Circle.cpp
Class.cpp (ch10 folder)
Will help you visualize Project 2 ATM
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Demonstrate class.cpp
Notice the documentation of in and out
parameters
Also notice the passing of a filestream to
find_mark function
ofstream& fout
This will be important later in Project 2
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THE END !!!
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