Transcript Variable types

Types
What they do
Copyright © 2007-2010 – Curt Hill
Introduction
• People are smart, computers are
dumb
• This is obvious when we speak
about types
• The type is usually understood when
people work in algebra
• We do not care if the number is an
integer, fraction or real – it is just a
number
• Not so in C++ – we must explicitly
state the type
Copyright © 2007-2010 – Curt Hill
Standard types in C
programs
• A type determines legal values and
legal operations
• Most of the standard types are
reserved words
• In the rest of this presentation the
sizes and precisions given are for
our machines
• Others compilers may be different
Copyright © 2007-2010 – Curt Hill
Integer types
• int - a standard integer
– Integers in range + or - 2.14 billion
– Occupies four bytes
• short - half the size
– Range is -32768 to +32767
• long – In our case same as int
Copyright © 2007-2010 – Curt Hill
Integer Operators
• The integer types allow the standard
mathematical operators:
–
–
–
–
Addition +
Subtraction Multiplication *
Integer division /
• 5/2=2
– Integer remainder %
• 5%2=1
Copyright © 2007-2010 – Curt Hill
Integer division
• Integer operators always give
integer results
• Remembering division and
remainder
–
–
–
–
Do long division
Do not go past the decimal point
Quotient is obtained with /
Remainder with %
• Integer constants are strings of
digits with or without a sign
Copyright © 2007-2010 – Curt Hill
Division operators
4
5 ) 22
r 2
• If a is 22 and b is 5
• a/b gives 4
• a%b gives 2
Copyright © 2007-2010 – Curt Hill
Floating point types
• Also known as real numbers
• Consist of exponent and mantissa
– Similar to scientific notation
• Float
– Regular floating point numbers
– About seven decimal digits
• Double
– Twice as many mantissa digits as float
– About 14 decimal digits
Copyright © 2007-2010 – Curt Hill
Constants
• There are two forms for a real
constant
• Floating point constants have either
a decimal point or an exponent or
both
• Scientific notation is something like
this: 3.2 × 104
• Since the exponent is hard to type
we use E notation:
3.2E4
Copyright © 2007-2010 – Curt Hill
Double Preferred
• Our machines have plenty of
memory so saving space is not the
issue
• The Pentium only does double
arithmetic
• The standard math library accepts
only double parameters
• Therefore we routinely use double
instead of float
Copyright © 2007-2010 – Curt Hill
Floating Point Operators
• The real types allow the standard
mathematical operators:
–
–
–
–
Addition +
Subtraction Multiplication *
Real division /
• 5.0 / 2.0 = 2.5
– No remainder operator
Copyright © 2007-2010 – Curt Hill
Overloaded operators
• The / operator is overloaded, which
means that it has different meanings
depending on what operands it has
• With two integers it means integer
division, with two floating point
numbers it means real division
• We will later consider what it means
when the types do not match
Copyright © 2007-2010 – Curt Hill
Character types
•
•
•
•
Represents an ASCII character
char
One character
Constants are enclosed in single
quotes
– As opposed to string constants which
are in double quotes
• Small integers can be used to
specify as well
Copyright © 2007-2010 – Curt Hill
Characters as numbers?
• There is some interchangability
between characters and small
integers
• Each character is represented on
the machine by an integer in the
range -128 to +127
• The character set of a machine is
the convention that connects the
number to the character
– We will use ASCII on Windows
Copyright © 2007-2010 – Curt Hill
Booleans
• Type name is bool
• Holds a true or false
– Both are reserved words
• Will discuss this one more seriously
later
Copyright © 2007-2010 – Curt Hill
Modifiers
• Before types we can apply several
modifiers that alter some of the
characteristics of the type
• long
– Usually means that the item is twice as
long as normal for more precision
– Not applicable to characters
• short
– Usually means that the item is half the
length
– Not applicable to characters
Copyright © 2007-2010 – Curt Hill
More modifiers
• signed
– Explicitly must have a sign
• unsigned
– May not have a sign
– Not allowed on floating point types
– Gives one more bit of precision for
integers or characters
Copyright © 2007-2010 – Curt Hill
Examples
• A double is a long float
• Sometimes there is a long double
• Characters, may be by default either
signed or unsigned
– Ours are signed
• Any particular compiler may ignore
some of these
Copyright © 2007-2010 – Curt Hill
For our purposes we will
only consider the following:
•
•
•
•
char
int
double
bool
Copyright © 2007-2010 – Curt Hill
Mixed type expressions
• When an arithmetic expression has
two types in it, we call this a mixed
type expression
• In general the weaker type is
converted to the stronger type and
then the operation proceeds
• Shorter items are weaker than
longer
• Integer items are weaker than
floating
Copyright © 2007-2010 – Curt Hill
Examples:
• Add a short int to an int
– The short int is converted to an int and
the operation proceeds
• Add an int to a float
– Convert the int to a float and continue
• To convert:
– The original form and value of the
variable are unchanged
– For the purpose of the expression
change the value from one type to
another Copyright © 2007-2010 – Curt Hill