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HW1 will be assigned tomorrow or Wednesday
Due Wednesday October 15, 19:30
Homeworks will be assigned at SUCourse
You will submit there too
Homework will be explained in recitations
Moreover, there will be important explanations about the
homework submission procedure in recitations
Submission is a tricky process, please do attend the
recitations in order not to make a mistake
SUCourse is ready
You may reach the course web site
http://people.sabanciuniv.edu/levi/cs201/ from SUCourse as
well
Lecture notes are at course web site.
Assistant office hours are determined
See details at course web site
CS201 Introduction to Computing @ Sabancı University 1
Chapter 2
Writing and Understanding C++
Writing programs in any language requires understanding
the syntax and semantics of the programming language
Syntax is similar to rules of spelling and grammar:
• After main() you should put {
Semantics is what a program means
Approaches of learning programming languages
Template based
• Examine example programs and make analogies
• Like a child learns how to speak
First learn syntax and semantics, then start by writing
small programs, ...
• Like learning a foreign language
Which one do you prefer?
We will follow the second method
CS201 Introduction to Computing @ Sabancı University 2
Towards Understanding of C++
“Hello World” program. No biggie!
#include <iostream>
using namespace std;
// traditional first program
int main()
{
cout << "Hello world" << endl; /* display */
return 0;
}
This program must be
typed
compiled (hello.cpp => hello.obj)
linked (Build) (hello.obj and any other object modules linked
together => project.exe)
executed
CS201 Introduction to Computing @ Sabancı University 3
Anatomy of a C++ Program
Not all functions are defined in the core C++ language; some
statements are in libraries
In order to use them, you have to explicitly say so
#include statements make libraries of functions accessible to
the program
Utility functions and tools that make the programmer’s life
easier are defined in libraries
Helps programmers develop code independently in a
standard way
Compiler needs access to interface, what the functions
look like, but not to implementation of those functions
• This is in the #included file
• e.g. #include <iostream>
for input/output functions
All programs that use standard C++ libraries should have
using namespace std;
CS201 Introduction to Computing @ Sabancı University 4
Anatomy of a C++ Program
Comments make programs readable by humans (and by assistants!)
Easier maintenance
Try to use natural language, do not repeat the code!
• Bad example
area = pi*r*r; /* area is pi*r*r */
• Better example
area = pi*r*r; /* calculate area */
Two ways of commenting
• Using // make the line comment line
• Between /* and */
Compiler disregards
comments
Comments in your homework
affect your grades
In VC++, comments are in green
CS201 Introduction to Computing @ Sabancı University 5
Anatomy of a C++ Program
Execution of the program begins with main
Each program must have a main function
Execution of C++ programs is organized as a sequence of
statements
Unless otherwise stated, statements execute
sequentially one after another
• Branching, repetition are possible (we will see them later)
The main function returns a value to the operating
system or the environment in which it is executed
• return 0
• Why 0? Because 0 means no problems encountered!
CS201 Introduction to Computing @ Sabancı University 6
Rules Rules Rules
Now some syntax rules and definitions
ABC of C++
What is an “identifier”?
Reserved words
Symbols and compound symbols
What is a “literal”?
Types of literals
Variables and basic types
Where to use blanks, line breaks?
Where to use semicolon?
Basic Input/Output
CS201 Introduction to Computing @ Sabancı University 7
Identifiers
Names of elements in a program
Names of variables, functions, classes, etc.
Sequence of letters (a .. z, A ..Z), digits (0 ..9) underscore _
Cannot start with a digit
number1 valid
number_1 valid
Me_myself valid
1number not valid
Do not start with an underscore
Case sensitive
Number1 and number1 are not the same
Pick meaningful names in the context of your program and improve the
self-readability of the code (do not use your boy/girlfriend’s name!)
First occurrence of an identifier must be its declaration
Standard declarations are in #include files
We will see how to declare user-defined variables, functions, etc. as
time goes by
CS201 Introduction to Computing @ Sabancı University 8
Reserved Words (Keywords)
Special and fixed meanings
built-in in C++ language
no need to have libraries to use them.
Cannot be changed by programmer
int
return
Full list is Table 2.1 of the textbook
Full list also in MSDN:
http://msdn.microsoft.com/en-us/library/aa245310(VS.60).aspx
You cannot use a reserved word as a user-defined identifier
In MS VC++, reserved words are automatically blue
CS201 Introduction to Computing @ Sabancı University 9
Symbols
Non-digit and non-letter characters with special meanings
Mostly used as operators (some examples are below, full list later)
+ addition, sign
- subtraction, minus
= assignment
/ division
* multiplication
% modulus
Compound symbols (two consecutive symbols – one meaning),
examples below, full list later
/*
*/
<<
>>
== equality comparison
CS201 Introduction to Computing @ Sabancı University 10
Literals
Meaning: Fixed values
Different format and rules for different types
Integer
3 454 -43 +343
Real
3.1415 +45.44 -54.6 1.2334e3
Last one is 1.2334 times 10 to the power 3 (scientific notation)
String
• Sequences of characters
• Within double quotes (quotes are not part of the string)
• Almost any character is fine (letters, digits, symbols)
"Hello my friend"
"10 is bigger"
"10 > 22 ?"
CS201 Introduction to Computing @ Sabancı University 11
Variables and types
Variables are used to store data that can change during program
Memory locations of certain sizes
Data input must be stored in a variable
Results are stored in variables
generally defined at the beginning of functions (no strict rule
on the place of definition except being before the first use)
Basic types (more to follow)
Integer
Memory
int list of identifiers separated by comma ; number1 age
int number1, age, deniz, selin;
deniz selin
int num2;
Real num2
float list of identifiers separated by comma ;
float area, circumference; area
• There is another real type called double
that we will see later circumference
String
string list of identifiers separated by comma ; myname
string myname;
CS201 Introduction to Computing @ Sabancı University 12
Arithmetic Operations
Operators: * - + / %
Operands: values that operator combines
variables or literals
Combination of operators and operands is called expression
Syntax and semantics for arithmetic operations
Addition, subtraction: + and –, 4
for int, real (float and double)
23 + 4 x + y d – 14.0 + 23 5 - 3 + 2
-1
Multiplication: *, for int, real (float and double)
23 * 4 y * 3.0 d * 23.1 * 4 5 – 3 * 2
Division: /, different for int and real number types (double,
float)
• for int operands result is the integer part of division
21 / 4 is 5 21 / 4.0 is 5.25 x / y
Modulus: %, remainder of division, only for int
21 % 4 is 1 18 % 2 is 0 x % y
CS201 Introduction to Computing @ Sabancı University 13
Assignment Operator
Memory
variable = expresion;
To store a new value in a variable
The value of expression becomes name a b
the value of variable
Previous value of variable is lost 45 99
value
int a, b;
a = 45;
b = a+54;
a*b = 332; wrong syntax
Be careful about the types of left and right hand sides
they must match
compiler may or may not warn you
CS201 Introduction to Computing @ Sabancı University 14
Example Program
Write a program to calculate the area of a circle
program first input a name and print a greeting
input the radius
calculate and display area
identify literals, identifiers, symbols, variables and expressions in this program
#include <iostream>
#include <string>
using namespace std;
// area calculation program
int main()
{
int radius;
float area;
string myname;
cout << "Please enter your name: ";
cin >> myname;
cout << "Hello " << myname << "! Welcome to my area calculation program" << endl;
cout << "Please enter the radius of your circle: ";
cin >> radius;
area = 3.14*radius*radius;
cout << "the area is: " << area << endl;
return 0;
}
CS201 Introduction to Computing @ Sabancı University 15
Issues
What happens if the user enters a real number for radius?
wrong result
solution: real radius
Can we combine
cout << "Hello " << myname << "! Welcome to my area calculation program" << endl;
cout << "Please enter the radius of your circle: ";
Yes
Can we eliminate the variable area?
Yes
CS201 Introduction to Computing @ Sabancı University 16
Where to use Blanks
You must have at least one blank
between two words (identifiers or keywords)
int count;
between a word and numeric literal
return 0;
You cannot have a blank
within a word (e.g. float cannot be written as flo at)
within a compound symbol
• e.g. != cannot be written as ! =
within a literal
• e.g. 3.14 cannot be written as 3 . 14
• except string literals, in string literals blanks are blanks
At all other places
blanks are optional. E.g. a = 5 ; is the same as a=5;
Several blanks are functionally same as single blank
except within string literals
CS201 Introduction to Computing @ Sabancı University 17
Where to use Line Breaks and Semicolon
As a general rule, line breaks are possible whenever blank is
possible
exception: a string literal must start and finish on the
same line
Where to use a semicolon?
general rule: at the end of a statement
but there are exceptions
• variable declaration: variable declaration is not a statement
but you have to use semicolon at the end of each
declaration
it may be hard to identify the statements
• is #include <iostream> a statement?
an ability that will be gained over time
CS201 Introduction to Computing @ Sabancı University 18
Stream output
Output is an essential part of our programs
cout (read as “see-out”) is the standard output stream (monitor),
Accessible via #include<iostream>
Objects are inserted onto stream with insertion operator <<
Objects are literals, variables or expressions
expressions are evaluated before output
whatever exists within a string literal between " " are displayed
Different objects separated by insertion operator <<
cout << "What a wonderful" << endl << "world" << endl;
cout << "gross = " What a wonderful
<< 12*12 << endl; world
cout << 5 << " in. = "; gross = 144
cout << 5*2.54 << " cm. " << endl; 5 in. = 12.7 cm.
endl means “end of line” (defined in iostream)
it causes the next output displayed at the beginning of the next line
Line breaks in the program do not finish the output line in the
output screen
CS201 Introduction to Computing @ Sabancı University 19
Stream Input
cin (read as “see-in”)
standard input stream (from keyboard)
you can input only into variables
extraction operator >> is used between cin and variables
cin >> variable1 >> variable2 >> variable3 .......... ;
int a, b, anynum;
cin >> b >> anynum >> a;
Data entry must be in the same order of variables in cin statement
first the value for b, then the value for anynum, then the value
of a must be entered by the user using the keyboard
CS201 Introduction to Computing @ Sabancı University 20
Stream Input
You have to have at least one blank between any two input
entry
Multiple blanks are OK
You may input values at several lines for a single cin
statement
You cannot display something using cin statement
Type match between variable and the corresponding input
value
If mismatch then the input entry fails for the rest of the
program
But the values read up to that point are kept in the
variables
Important note on terminology
“reading a value” means such an input
CS201 Introduction to Computing @ Sabancı University 21
Stream Input – Example showing the
operation
CS201 Introduction to Computing @ Sabancı University 22
Another Example Program
Conversion from hours to minutes and seconds
We will see two programs for the same job
time.cpp uses three variables (for hours, minutes and
seconds)
time2.cpp uses just one variable
CS201 Introduction to Computing @ Sabancı University 23
