Functions
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
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
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User-defined functions
Top-Down Design
Abstraction
Local Variables (Scope of variables)
Library Functions
Library Functions
Library Functions
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CS3369 Real Time Control
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 Top-Down Design:


break the task of the program to subtasks.
solve each subtask by a sequence of statements.
In C++ a sequence of statements can be implemented by
a function
• One can use one statement to represent a sequence of
statements.
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#include<iostream.h>
#include<dos.h>
#include<conio.h>
void main()
{
int i, x=1;
for(x=1; x<=72; x++)
{
clrscr();
for(i=1; i<=x;i++)
cout<<" ";
cout<<" * \n ";
for(i=1; i<=x; i++)
cout<<" ";
cout<<" * \n";
for(i=1; i<=x; i++)
cout<<" ";
cout<<"*********\n";
for(i=1; i<=x; i++)
cout<<" ";
cout<<" * \n";
for(i=1; i<=x; i++)
cout<<" ";
cout<<" *
\n";
delay(300);
}
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}
#include<iostream.h>
void draw_plane( int x)
{
int i;
void draw_plane(int y); for(i=1; i<=x;i++)
cout<<" ";
/* The prototype */
cout<<" * \n ";
for(i=1; i<=x; i++)
void main()
cout<<" ";
{ int x;
cout<<" * \n";
for(x=1; x<=72; x++)
for(i=1; i<=x; i++)
{ clrscr();
cout<<" ";
draw_plane(x);
cout<<"*********\n";
delay(300);
for(i=1; i<=x; i++)
}
cout<<" ";
cout<<" * \n";
for(i=1; i<=x; i++)
cout<<" ";
cout<<" *
\n";
}
#include<dos.h>
#include<conio.h>
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Call the same function
twice, we can show two
air planes.
#include<iostream.h>
#include<dos.h>
#include<conio.h>
void draw_plane(int y);
/* The prototype */
void main()
{ int x;
for(x=1; x<=72; x++)
{ clrscr();
draw_plane(x);
if(x>=19)
draw_plane(x-18);
delay(300);
void draw_plane( int x)
{
int i;
for(i=1; i<=x;i++)
cout<<" ";
cout<<" * \n ";
for(i=1; i<=x; i++)
cout<<" ";
cout<<" * \n";
for(i=1; i<=x; i++)
cout<<" ";
cout<<"*********\n";
for(i=1; i<=x; i++)
cout<<" ";
cout<<" * \n";
for(i=1; i<=x; i++)
cout<<" ";
cout<<" *
\n";
}
/*1st plane is 18 columns away */
}
5/29/2016 }
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 Some Discussion about Functions
1. In C++ a sequence of statements can be implemented
by a function
2. User can use (call) a function many times so that
the programs become shorter.
3. A function provides a tool to use/repeat a sequence of
statements flexibly, i.e., each use (call) of the same
function does similar but different thing. (See Slide 4 for
an example.)
4. A function corresponds to some subtask in top down
design so that the design of the whole program becomes
easier.
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User Defined Functions
 Function prototypes:(what are the inputs and outputs)

return_type
function_name (parameter_list);
• here parameter_list is either empty or
– type_1 parameter_1; parameter_list;
– //this is a recursive definition
• the function prototypes should be given before main().
(see examples)
 Function definitions:


return_type function_name (parameter_list)
{statements; return a_value_of_return_type;}
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Function Calls
 Whenever one needs to use a function, one calls
the function:

assign values to the variables in parameter_list
• Example: draw_plane(x-18);

match return_type in function call assignment.
• If your function returns a value, assignment this value to a
variable of the same type.
Example: Write a program that asks the user to input two
integers from the keyboard and output the maximum of
the two integers.
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An Example

 #include <iostream.h>

 int max(int x, int y);

 int main() {

int a, b;

int x;

cout <<“input two integers\n”;

cin >> a >> b;

x = max(a,b);

cout <<“the maximum is” <<max(a,b);

return 0;

}

 int max (int x, int y) {



int tmp=x;
if (tmp<y) tmp=y;
return tmp;}
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//input&output library functions
//prototype of function max;
//the main function

//define two integers

//define another integer

//ask for two integers

//input two integers

call the function max()

//output their maximum

//return 0 for main()

//end of main
//definition of max(x,y)

//initialize tmp to be x

//if y is bigger, let tmp=y

//return tmp at the end of max
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 A function definition is like a small program
and a function call is the same thing as running
this program.
 A function uses formal parameters, instead of
“cin”, for input. The arguments to the function
are the input and they are plugged in for the
formal parameters.
 The output of a function is send back to the
program which calls this function, via the return
value. The function uses the return statement
for output purpose.
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Procedure Abstraction
 Blackbox analogy: Others should know how to use
a function by looking at the function prototype and
its accompanying comment without looking at the
body of the function definition.


Prototype comment should include all the conditions
required of the arguments to the function and should
describe the value that is returned.
All variables used in the function body should be
declared in the function body (the formal parameters
do not need to be declared in the body since they are
listed in the function prototype.)
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Shooting a Bullet using functions
#include <math.h>
#include<dos.h>
#include<stdio.h>
#include <graphics.h>
#include<conio.h>
void shoe(int i);
void erease(int i);
void main(void)
{
int driver =DETECT,mode;
int i,j;
int x=1,y=1;
initgraph(&driver,&mode,"a:\\bgi");
setcolor(WHITE);
line(1,400,400,400);
for ( i = 0; i < 80; i++ )
{ show(i);
delay(300);
erease(i);
}
closegraph();
}
void show(int i)
{
x=5*i;
/* x=i; */
y=9*i-0.15*i*i; /* y=i; */
setcolor(RED);
circle(400-x,400-y,2);
}
void erease(int i)
{
x=5*i;
/* x=i; */
y=9*i-0.15*i*i; /* y=i; */
setcolor(BLACK);
circle(400-x,400-y,2);
}
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Shooting Bullets using functions
#include <math.h>
#include<dos.h>
#include<stdio.h>
#include <graphics.h>
#include<conio.h>
void shoe(int i);
void erease(int i);
void main(void)
{
int driver =DETECT,mode;
int i,j;
int x=1,y=1;
initgraph(&driver,&mode,"a:\\bgi");
setcolor(WHITE);
line(1,400,400,400);
for ( i = 0; i < 80; i++ )
{ show(i);
if (i>=11) show(i-10);
delay(300);
erease(i);
if (i>=11) erease(i-10);
void show(int i)
{
x=5*i;
/* x=i; */
y=9*i-0.15*i*i; /* y=i; */
setcolor(RED);
circle(400-x,400-y,2);
}
void erease(int i)
{
x=5*i;
/* x=i; */
y=9*i-0.15*i*i; /* y=i; */
setcolor(BLACK);
circle(400-x,400-y,2);
}
}
closegraph();
}
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CS3369 Real Time Control
Software/WANG Lusheng
Shooting Bullets in different
directions/speeds
#include <math.h>
#include<dos.h>
#include<stdio.h>
#include <graphics.h>
#include<conio.h>
void shoe(int i, float h, float v);
void erease(int i, float h, float v);
void main(void)
{
int driver =DETECT,mode;
int i,j;
int x=1,y=1;
initgraph(&driver,&mode,"a:\\bgi");
setcolor(WHITE);
line(1,400,400,400);
for ( i = 0; i < 80; i++ )
{ show(i,5.0, 9.0);
if (i>=11) show(i-10, 10.0, 8.0);
delay(300);
erease(i, 5.0, 9.0);
if (i>=11) erease(i-10, 10.0, 8.0);
void show(int i, float h, float v)
{
x=h*i;
/* x=i; */
y=v*i-0.15*i*i; /* y=i; */
setcolor(RED);
circle(400-x,400-y,2);
}
void erease(int i, float h, float v )
{
x=h*i;
/* x=i; */
y=v*i-0.15*i*i; /* y=i; */
setcolor(BLACK);
circle(400-x,400-y,2);
}
}
closegraph();
}
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Shooting Bullets in different
directions/speeds
#include <math.h>
#include<dos.h>
#include<stdio.h>
#include <graphics.h>
#include<conio.h>
void shoe(int i, float h, float v);
void erease(int i, float h, float v);
int vertical(int i, float v, float acc);
void main(void)
{
int driver =DETECT,mode;
int i,j;
int x=1,y=1;
initgraph(&driver,&mode,"a:\\bgi");
setcolor(WHITE);
line(1,400,400,400);
for ( i = 0; i < 80; i++ )
{ show(i,5.0, 9.0);
if (i>=11) show(i-10, 10.0, 8.0);
delay(300);
erease(i, 5.0, 9.0);
if (i>=11) erease(i-10, 10.0, 8.0);
}
closegraph();
}
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void show(int i, float h, float v)
{
x=h*i;
/* x=i; */
y=v*i-0.15*i*i; /* y=i; */
setcolor(RED);
circle(400-x,400-y,2);
}
void erease(int i, float h, float v )
{
x=h*i;
y=vertical (i, v, 0.15); /* y=v*i-0.15*i*i; */
setcolor(BLACK);
circle(400-x,400-y,2);
}
int vertical(int i, float v, float acc)
{ int y;
y= v*i-acc*i*i;
return y;
}
A defined function can call another defined function.
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 Top-Down Design:


break the task of the program to subtasks.
solve each subtask by a sequence of statements.
Shooting a Bullet
Shooting two bullets
1. Show a bullet at a specified
position
1. Show a bullet at a specified position.
2. Hold the figure for a while
(delay)
3. Erase the bullet at the same
position
4. Repeat Steps 1-3 for 80
times, each with a different
position.
2. Show another bullet if the 1st bullet is far
enough.
3. Hold the figure for a while
4. Delete the two bullets
5. Repeat Steps 1-4 for 80 times.
Remarks: Logically, the programs are very
simple, though they look like sophisticated.
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