Transcript Lecture 16 - ODU Computer Science

CS149D Elements of Computer
Science
Ayman Abdel-Hamid
Department of Computer Science
Old Dominion University
Lecture 16: 10/24/2002
Lecture 16: 10/24/2002
CS149D Fall 2002
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Outline
•Variables and Data types
•Assignment statement
•Arithmetic operators
•Increment/Decrement operators
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Variables Names
•Program variables correspond to memory spaces reserved for storage
•A variable name is called an identifier
•An identifier in C++ can be up to 255 characters long
Can not begin with a digit
(Invalid: 1First)
Can not contain blanks
(Invalid: num elements)
Can not contain a hyphen, underscore is OK
(Invalid: num-elements)
Special symbols are not allowed
(Invalid: cost$, cost!)
Reserved words can not be used as identifiers
(Invalid: int, const, float)
A reserved word is a word that has a special meaning in C++. It can not be used as a
programmer-defined identifier
•C++ is case sensitive, so count is different than Count
•To declare a variable, need to identify its data type
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Data Types
C++ Data types (built-in data types)
•Integers
•Floating-point numbers
•Characters
•And more
•The data type determines how data is represented in the computer and the
kind of processing that the computer can perform on it
•The number of bytes that a data type occupies in memory is system
dependent
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Variable Declaration
•DataType identifier;
int x;
•DataType identifier, identifier, …;
int x,y,z;
•The initial value stored in a variable is not know unless you initialize it with
the declaration
int x = 10;
•Can mix declaration with declaration/initialization
int x=10, y, z = 5;
•Use const keyword to indicate that the value of this variable can not change
const float PI = 3.141593f;
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Numeric Data Types
•Integer numbers
short, int, long
Can use unsigned keyword (unsigned short)
•Numbers with fractions
float, double, long double (double is “double precision”)
Can use scientific notation float x = 5.0e6; which means x  5 * 106
const float x = 2.3;
//x is considered a double constant
const float x = 2.3f;
//or const float x = 2.3F;
•Number of bytes and range of values is system dependent. In Microsoft VC++
Data Type
#of Bytes
Smallest value
Largest Value
int
4
-2,147,483,648
2,147,483,647
short
2
-32768
32767
float
4
3.4 * 10-38
3.4 * 1038
double
8
1.7 * 10-308
1.7 * 10308
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The textbook lists 2
bytes for int on a
Borland Turbo C++
3.0 compiler
See Table 2.2 on
page 29
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Scientific Notation
•Rewrite the floating-point number as a mantissa times a power of 10
•Mantissa: absolute value greater than or equal to 1.0 and less than 10.0
25.6  2.56 * 101
•Letter e is used to separate mantissa from exponent
25.6  2.56e1
•Precision: number of digits allowed for decimal portion of mantissa
•Exponent range: number of digits allowed for for exponent
Float
Precision
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double long double
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see Table 2.2
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Alphanumeric Data Types
•Single alphanumeric character
char x = `a`;
char y = `1`; is different than int y =1;
Most programming languages use ASCII to represent the
English alphabet and other symbols (1 byte/character)
•Sequence of characters
string name = “CS149 Lecture”;
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Assignment statement
Identifier = expression
Where expression can be a constant, another variable, or the result of an
operation
int x, y;
x = 10;
// x  10, x is assigned the value of 10
y = x;
x = x –10;
•Multiple assignments
x = y = z = 0;
•Look out for the data types of both sides of the assignment
int a;
a = 12.8;
// actually a is assigned 12
•Numeric conversion with possibility of information loss
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Arithmetic Operators
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Operators
Unary operator: One operand
Binary operator: two operands
+
Unary Plus
+ Addition
-
Unary minus
- Subtraction
*
Multiplication
/ floating-point division or integer division (no fractional part)
% modulus operator (remainder of integer division)
Examples
Area_triangle = 0.5 *base * height;
Y = -x;
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Arithmetic Operators
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Examples of integer division and modulus
7/2 = 3
7%2 = 1
2/7 = 0
2%7 = 2
7.0/2.0 = 3.5
5 % 2.3 = Error (both operands must be integers)
•Note that the result of integer division is a truncated result, not a rounded result.
5/3 = 1
5.0/3.0 = 1.66666
•Operation between different types is a mixed operation
Value with lower type is converted or promoted to the higher type
int sum = 18, count = 5;
float average;
average = sum/count;
// average assigned 3.0 and not 3.6 (Why?)
The correct way to compute average would be
average = (float) sum / (float) count;
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// explicit type casting
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Arithmetic Operators
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Practice problems page 34
int a = 27, b= 6;
int b = 6;
float c;
float a, c = 18.6;
c = a/(float)b;
a = (int) c/b;
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Increment and Decrement Operators
•Unary operators for incrementing and decrementing variables ++, --
•Can be used in a prefix (++count) or postfix (count++) position
int x;
x++; is equivalent to x = x +1; is equivalent to ++x;
•How about
x = y++ * 3;
x = ++y *3; //is the value assigned to x the same in both cases: NO
•Will see what is the difference next lecture when we talk about operator
precedence and compound arithmetic expressions
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