Data Types

Copyright © 2006 Patrick McDermott

College of Alamedapmcdermott@peralta.edu

Fact CRC Cards: (Attribute) ERD: Attribute Relational: Attribute Database: Column Programmer: Variable UML: Property Etc., etc., etc.…: Field, Item, Element

The Rules C++

• Allowed/Forbidden characters– Letters, Numerals, _– NO Spaces

• Break_character: Underscore _

– No initial numeral

• Length, Very Long• Case SeNsItiVe• Cannot be a Keyword (Keywords are Reserved Words)

Nouns for Variables/ClassesVerbs for Functions/Methods

The Rules Java

• Allowed/Forbidden characters– Letters, Numerals, $, _– NO Spaces

• Break_character: Underscore _

– No initial numeral

• Length, Very Long• Case SeNsItiVe• Cannot be a Keyword (Keywords are Reserved Words)

Nouns for Variables/ClassesVerbs for Functions/Methods

Numbers, Text & Truth

• Numbers– int or double

• Text– char or string– C#: char is 2 bytes in C++ wchar_t

• Booleans– Logical: true or false.

• We might not have beauty, but it does have truth.

• Decimal (.NET)– ± $792,281,625,142,643,375,935,439,503.35m – ± 79,228,162,514,264,337,593,543,950,335M

2 Kinds of NumbersIntegers

• int• A counting number• A whole number• Discrete• No fraction• No decimal• 2 3 9

Floating Point

• float, but use double• A measuring number• A fractional number• Continuous• Fraction• Decimal• 2½ , 3.14159, 99.99• 3.0 too!

Numbers• Integers: int

@ 4 billion

–2,147,483,648 to +2,147,483,647• 0-4.3 billion if unsigned

Not enough bils for Bill’s fortune

• Double-Precision Floating-Point: double– Range: Big enough

• You can enumerate all the atoms in the Universe.

– Precision: at least 15 decimal digits• Just enough for $Trillion to the cent.• $1,234,567,890,123.45• Not quite enough for a credit card number

I Declare!String s;String s = "Hi";int x = 0;double x, y;int x = 0, y = 1;final int MYNUMBER = 0;

The Unchanging constant

ALLUPPERCASE or ALL_UPPER_CASE

Number versus Character Sort

As numbers, one & two & three are binary 1 and 10 and 11.As text, 1 & 2 & 3 are ASCII characters 49 & 50 & 51.

Numbers sort by value.Text sorts character by character.

You can be richer than Bill Gates!As text, $9 is greater than $80,000,000,000!So, just keep $9, and sort net worth as text!

# Raw Numbers Text Text Sort1 1 1 1 12 111 2 11 113 2 11 111 1114 49 17 123 1235 423 22 17 176 11 49 2 27 22 111 22 228 313 123 313 3139 17 313 423 423

10 123 423 49 49

Precision• Values of type float have seven digits of

precision.• Values of type double have 15-16 digits

of precision.• Values of type decimal are represented as

integer values that are scaled by a power of 10. Values between -1.0 and 1.0 are represented exactly to 28 digits.

Size not Standard

Each computer system is different.Types are based on the system Word.We are on a 32-bit (4-byte) system,So int is a word (4 bytes), double is 2 words (8 bytes)…

Type Size

char, unsigned char, signed char 1 byte

short, unsigned short 2 bytes

int, unsigned int 4 bytes

long, unsigned long 4 bytes

float 4 bytes

double 8 bytes

long double 8 bytes

The incredible Growing int

Windows 3.11(and earlier)

The 16-bit WindowsA 2-byte system word

Windows 95(and later)

The 32-bit WindowsA 4-byte system DWord

C Standard for int:The short must not be longer than the intThe long must not be shorter than the int

short

int

long

short

int

long

Windows 7 & Java(and later)

The 64-bit WindowsA 8-byte system QWord

short

int

long

Alphanumeric Text Strings

• Single character or a string of characters?

'a' is the single character known as a

"a" is a character string consisting of the letter a• A C string is actually a null-terminated array

It needs a space for the null (binary zeroes)

"a" is 2 characters long: a + null• lpsz “long pointer to a null-terminated string” [yuck!]

• String TheoryStrings have been a sore point, with me & others

Unnecessarily overcomplicated

What were they thinking???