Transcript Chapter 2 Writing and Reading C++ Programs

Announcements
 Homework 1 is due on July 8th, this Wednesday, at 19:00
 Submit to SUCourse
 Strictly follow the submission guideline provided in the homework
document
 About the homework:
 If you run under the debugger (Ctrl+F5), you will get this prompt:
 Press any key to continue . . .
 If you run with F5 only, you will NOT get this prompt, so it is OK.
 You can add the following at the end of your code to get the same
effect:
cin.ignore();
cin.get();
 Write comments in your program
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Announcements
 Midterm exam will be on
Monday, July 27th, 2015, 19:40 – 21:20
 Final Exam to be schedule between August 12-14
 For those who have exam collisions with another
course: talk with your other instructor immediately!
 I cannot change the exam dates anymore but I can
work out a solution with them.
Summary of what we learned yesterday
 Basics of C++
 Our first program: “Hello world”
 Format of a program
 Syntax of literals, keywords, symbols, variables
 Data types
 int (short, long, unsigned)
 float/double
 char
 Arithmetic operations
 Assignment (compound) operators
 Standard input (cin) and output (cout)
 More complex arithmetic operations and operator
precedence
Functions (Chapter 2 – Continued)
 Main function – will soon get big
 Solution: Divide and Conquer
 divide your problem into small ones
 write one function for each
 combine them in the main program
 Advantages:
 Main is organized into modular pieces
 When you need to change something, you only
change that function and not main
 Reuse the same code piece several times
 e.g. all programs use cin and cout
Functions
Without function
With function
#include <iostream>
using namespace std;
#include <iostream>
using namespace std;
/* traditional first
program */
/* traditional first
program */
int main()
{
cout << "Hello world"
<< endl;
return 0;
}
void Hello()
{
cout << "Hello world"
<< endl;
}
int main()
{
Hello();
return 0;
}
Syntax of Functions
 A program is a collection of functions and classes
 main and other programmer defined functions
 only main is executed automatically when the program starts
 other functions must be called to be executed
 Programmer-defined functions
 piece of code to do a specific job
 must be declared before using (calling)
return-type function-name(parameters)
{
local variables
statements
}
 to execute a function just write its name (and arguments
for parameters)
 When a function is called, execution order temporarily jumps to
function
 after the function ends, execution order goes back to the caller
Syntax for Functions
#include <iostream>
using namespace std;
void Hello()
{
cout << "Hello world"
<< endl;
}
int main()
{
Hello();
return 0;
}
return-type function-name(parameters)
{
local-variables
statements
}
• “void” means returns nothing.
• no parameters (we will see
parametric functions today)
• no local variables
Drawing Head – Without functions
#include <iostream>
using namespace std;
int main()
{
cout << " |||||||||||||||| "
cout << " |
| "
cout << " |
o
o
| "
cout << " _|
|_ "
cout << "|_
_|"
cout << " |
|______|
| "
cout << " |
| "
return 0;
}
<<
<<
<<
<<
<<
<<
<<
endl;
endl;
endl;
endl;
endl;
endl;
endl;
 Draws a head – but not so modular
 To change head style, main needs to change.
 Is this so bad?
 What if you want to draw several heads like a totem?
 You have to duplicate code 
Drawing Head – With functions
parts.cpp
#include <iostream>
using namespace std;
// functions appear here
int main()
{
Hair();
Sides();
Eyes(); Ears(); Smile();
Sides();
return 0;
}
 This one is more complicated, but what are the advantages?
 Modularity
 Change head style  just change functions
 What about eyeglasses?
 Code re-use (not duplicated)
 Multiple heads  let’s see totem.cpp
Functions with Parameters
 You may need to pass data into a function when you
call it
 Consider a generic function to calculate the area of
any circle and display it
 how will the function know the radius?
 solution: parameters
 function is defined without knowing the value of the radius
 value of radius is passed to the function when it is called
 parameters are defined similar to variables
type name
Example: double radius
return-type func-name(type param1,…,type param<n>)
{
local variables
statements
}
Area calculation with parameterized function
#include <iostream>
using namespace std;
// area calculation program that employs functions
void calculate_area(double radius)
{
double area;
area = 3.14 * radius * radius;
cout << "The area of a circle with radius " << radius << " is "
<< area << endl;
}
int main()
{
double r;
r = 3;
calculate_area(r);
calculate_area(2.5);
calculate_area(r / 2);
return 0;
}
Area calculation with parameterized function
#include <iostream>
using namespace std;
// area calculation program that employs functions
void calculate_area (double radius)
{
double area;
radius is 3
area = 3.14 * radius * radius;
cout << "The area of a circle with radius " << radius << " is "
<< area << endl;
}
Output Screen
int main()
{
double r;
r = 3;
calculate_area(r);
calculate_area(2.5);
calculate_area(r / 2);
return 0;
}
The area of a circle with radius 3 is 28.26
Area calculation with parameterized function
#include <iostream>
using namespace std;
// area calculation program that employs functions
void calculate_area (double radius)
{
double area;
radius is 2.5
area = 3.14 * radius * radius;
cout << "The area of a circle with radius " << radius << " is "
<< area << endl;
}
Output Screen
int main()
{
double r;
r = 3;
calculate_area(r);
calculate_area(2.5);
calculate_area(r / 2);
return 0;
}
The area of a circle with radius 3 is 28.26
The area of a circle with radius 2.5 is 19.625
Area calculation with parameterized function
#include <iostream>
using namespace std;
// area calculation program that employs functions
void calculate_area (double radius)
{
radius is 1.5
double area;
area = 3.14*radius*radius;
cout << "The area of a circle with radius " << radius << " is "
<< area << endl;
}
Output Screen
int main()
The area of a circle with radius 3 is 28.26
The area of a circle with radius 2.5 is 19.625
The area of a circle with radius 1.5 is 7.065
{
double r;
r = 3;
calculate_area(r);
calculate_area(2.5);
calculate_area(r / 2);
return 0;
Functions with Parameters
 Parameters and Arguments
 parameter is the generic name used in function
 radius in the calculate_area function
 argument is the value passed to function while it is called
 2.5
 r
(actually current value of r is passed)
 r / 2 (actually current value of r / 2 is passed)
 Parameter list provides type and name of the parameter
 Argument type must match parameter type
 Functions may have multiple parameters
 corresponding arguments are separated by commas
 in the same order of parameter list
 be careful about the type matching
 Functions may call other functions (have seen in the totem
example and will see in the next example)
Functions with Parameters
 Parameters versus Local Variables
 Parameters are defined and used locally, but their initial value
comes from the caller function via arguments
 Local variables are defined and used within the function
 Initial value does not come from the caller function
 While designing your functions, think carefully about local
variables and parameters
 If you need to pass the initial value from the caller function, then
it should be a parameter.
 If you won’t pass the initial value, then it should be a local
variable.
 Example: in calculate_area function
 area is a local variable since we do not pass its initial value from main
 radius is a parameter since we need to pass its initial value from main
 Unnecessary parameters may cause problems and grade
reduction (for homework)
Old McDonald’s Farm
 Goal is to have a modular program (with functions) to display
the song
 each verse repeats using a different animal
 refrain parts repeat within verses
 a partial output (with two animals only) below:
Old MacDonald had a farm, Ee-igh, Ee-igh, oh!
And on his farm he had a cow, Ee-igh, Ee-igh, oh!
With a moo moo here
And a moo moo there
Here a moo, there a moo, everywhere a moo moo
Old MacDonald had a farm, Ee-igh, Ee-igh, oh!
Old MacDonald had a farm, Ee-igh, Ee-igh, oh!
And on his farm he had a pig, Ee-igh, Ee-igh, oh!
With a oink oink here
And a oink oink there
Here a oink, there a oink, everywhere a oink oink
Old MacDonald had a farm, Ee-igh, Ee-igh, oh!
Old McDonald’s Farm
(Design of Functions)
 Which functions do we need?
 a function for only “Ee-igh, Ee-igh, oh!”
 a function for the refrain (nakarat)
 a function for “And on his farm ...” line
 animal is parameter
 a function for “With a ...”, “And a ...”, “Here a ...” lines
 noise of the animal is a parameter
 a function for the whole verse
Old McDonald’s Farm (Program 1)
 See oldmac1.cpp
 What is the problem?
 We need a new function for a new animal (e.g. chicken)
 Cow and Pig functions are duplicates of each other.
 Solution: Put the common code into a new function for
the whole verse
Old McDonald’s Farm (Program 2)
 See oldmac2.cpp
 Verse does the common work with 2 parameters – first
one is for animal, second is for noise
 therefore it is called using 2 arguments – first one is for
animal, second is for noise
 order of arguments is important
 Functions call functions
 Refrain calls EiEio
 Verse calls Refrain
 animal is used as parameter name in two functions
 Should we need to use the same parameter name in both cases?
Or can we use different names in two different functions?
 Same questions are valid for noise as well.
 Answer is “Yes we can use different names”.
 Can we input animal and noise?
Mac Donald’s farm with user input
 We want the user to enter/input values for animal and
noise
Enter the name of an animal: sheep
Enter noise that a sheep makes: baah
Old MacDonald had a farm, Ee-igh, Ee-igh, oh!
And on his farm he had a sheep, Ee-igh, ee-igh, oh!
With a baah baah here
And a baah baah there
Here a baah, there a baah, everywhere a baah baah
Old MacDonald had a farm, Ee-igh, Ee-igh, oh!
 We’ll pass the user-entered values to the Verse function
 The input stream cin takes input from the keyboard using
operator >>
 Values that are input are stored in variables and then
passed to function verse as arguments
 see macinput2.cpp
Mac Donald’s farm with user input
// other functions goes here (see macinput2.cpp)
void Verse(string animal, string noise)
{ // this function doesn’t change
// see the source code for the function code
}
int main()
{
string animal;
string noise;
// variable for name of animal
// variable for noise it makes
cout << "Enter the name of an animal: ";
cin >> animal;
cout << "Enter noise that a " << animal << " makes: ";
cin >> noise;
Verse(animal,noise);
return 0;
}
Analysis of the Run
1. input value “sheep” is stored in variable animal
2. input value “baah” is stored in variable noise
3. “sheep” and “baah” values are passed to function
Verse as arguments as well as used in cout in main
Variables (review from previous lectures)
 Variables are used to store values in memory
 memory locations that are accessed using a name
 Each variable has a type, a name (identifier), and a value
 Methods to give values to variables
 Assignment, using
 Input, using
=
cin
 Definition:
type variable_names_separated_by_comma;
Where to define variables
 You can define variables anywhere within a function
 as long as it is defined before its first use
 Two common places of variable definition
 At the beginning of the function in which they’re used:
{
string animal, noise;
cout << "enter animal ";
cin >> animal;
cout << "enter noise a " << animal << " makes ";
cin >> noise;
}
 Just before the first place they’re used:
{
cout << "enter animal ";
string animal;
cin >> animal;
cout << "enter noise a " << animal << " makes ";
string noise;
cin >> noise;
}
Where to define variables
 NO GLOBAL VARIABLES
 A global variable is a variable defined outside the function bodies
#include <iostream>
using namespace std;
int global_number;
int main()
{
cout << "Hello world“ << endl;
return 0;
}
 They can be used in all functions without definition
 That is why it is the #1 enemy of parameters
 If you use global variables, you violate the independence property of
functions and may lose your control in big programs
 Thus, IT IS FORBIDDEN TO USE GLOBAL VARIABLES
 If you use a global variable in hw and exams, your grade is reduced!
 We will revisit the global variable concept towards the end of the
course.
Variable Initialization
 Variables have garbage (junk values) until
 they are assigned a value using assignment operator or
 myint = 5;
 an input is stored into them (using cin statement)
 cin >> myint;
 Variables must be given a value before being used for
the first time in an expression or an output statement or
as an argument to a function
 idea behind this rule: you can never know what is inside of
an uninitialized variable !
 not a syntax error, compiler may or may not warn you!
 You may initialize variables at the declaration
int mynumber = 5;
 After initialization, you may change the value stored in a
variable several times
 that is why they are named as “variable”
Scope of a Variable and Parameter
 Not explained in the book in this way
 RULE 1: A variable or parameter can be referred only
within the function in which it is declared
 e.g. you cannot refer the variable “animal” in function Eieio
 RULE 2: A specific identifier can be used several times in
different functions as variable or parameter names.
Those are actually different variables/parameters.
 e.g. animal and noise are used both in main (as variable) and
Verse (as parameter)
 RULE 3: A specific identifier must be unique within a
function
 e.g. you cannot define a local variable “animal” in Verse since
there is a parameter named “animal”
 Detailed scope rules will be given later in this course
Reading Assignment
 Section 3.3 (page 83)
 Case Study: Pizza Slices
 Similar to circle area calculation program
 Run the program pizza.cpp