Variables, Types, Values 14 January 2019 OSU CSE 1 Variables A - - PowerPoint PPT Presentation

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Variables, Types, Values 14 January 2019 OSU CSE 1 Variables A - - PowerPoint PPT Presentation

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Variables, Types, Values 14 January 2019 OSU CSE 1 Variables A variable is the name of a location that stores a value of a particular type We might say the variable has that value We might say the variable

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SLIDE 1

Variables, Types, Values

14 January 2019 OSU CSE 1

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Variables

  • A variable is the name of a “location” that

“stores” a value of a particular type

– We might say the variable “has” that value – We might say the variable “has” that type or “is of” that type

14 January 2019 OSU CSE 2

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Examples

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true 'y' 13 3.14 "Go"

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Examples

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This is a boolean variable b whose value is true, i.e., b = true

  • r, more simply, just

b true 'y' 13 3.14 "Go"

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SLIDE 5

Examples

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This is a char variable c whose value is 'y', i.e., c = 'y' true 'y' 13 3.14 "Go"

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Examples

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This is a int variable i whose value is 13, i.e., i = 13 true 'y' 13 3.14 "Go"

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SLIDE 7

Examples

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This is a double variable d whose value is 3.14, i.e., d = 3.14 true 'y' 13 3.14 "Go"

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SLIDE 8

Examples

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This is a String variable s whose value is "Go", i.e., s = "Go" (or s = <'G', 'o'>) true 'y' 13 3.14 "Go"

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SLIDE 9

Types

  • A type is the name of the set of all

possible values that a variable might have

  • Examples:

– A variable of type String might have values like "foo", "Hello World", etc. – A variable of type int might have values like –1, 18, etc. – A variable of type double might have values like 3.1416, 10.0, etc.

14 January 2019 OSU CSE 9

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Program vs. Mathematical Variables

  • A program variable has a particular value

at any one time during program execution, and that value (generally) may change at

  • ther times
  • A mathematical variable stands for an

arbitrary but fixed value

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Program vs. Mathematical Types

  • A program type has a corresponding

mathematical type that models it

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Program vs. Mathematical Types

  • A program type has a corresponding

mathematical type that models it

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When reasoning about a program variable of a given program type, treat its value at any given time as if it were a mathematical variable of the corresponding mathematical type.

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Mathematical Models

Program type Mathematical type String string of character boolean boolean char character int integer (-2147483648 through 2147483647) double real (about ±10±308, 15 significant digits)

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Mathematical Models

Program type Mathematical type String string of character boolean boolean char character int integer (-2147483648 through 2147483647) double real (about ±10±308, 15 significant digits)

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String is built-in to Java; boolean, char, int, and double are among the 8 primitive (and also built-in) types of Java; differences later.

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Mathematical Models

Program type Mathematical type String string of character boolean boolean char character int integer (-2147483648 through 2147483647) double real (about ±10±308, 15 significant digits)

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All these mathematical types are “built-in” to mathematics!

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Mathematical Models

Program type Mathematical type String string of character boolean boolean char character int integer (-2147483648 through 2147483647) double real (about ±10±308, 15 significant digits)

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Program code is shown in a blue fixed-width font, with keywords in bold.

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Mathematical Models

Program type Mathematical type String string of character boolean boolean char character int integer (-2147483648 through 2147483647) double real (about ±10±308, 15 significant digits)

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Mathematics is shown in a green fixed-width italic font, with keywords in bold.

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Declaring a Variable

  • When you declare a program variable,

you both provide a name for a location to store its value, and indicate its program type

– Recall: the program type determines the mathematical type, which in turn determines the possible values the variable can have

int j;

14 January 2019 OSU CSE 18

?

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SLIDE 19

Declaring a Variable

  • When you declare a program variable,

you both provide a name for a location to store its value, and indicate its program type

– Recall: the program type determines the mathematical type, which in turn determines the possible values the variable can have

int j;

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? The standard Java convention for naming variables is to use camel case: start with a lower case letter and only capitalize the first letter of each following word, e.g., myLuckyNumber

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Declaring a Variable

  • When you declare a program variable,

you both provide a name for a location to store its value, and indicate its program type

– Recall: the program type determines the mathematical type, which in turn determines the possible values the variable can have

int j;

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This is an int variable j whose value is undefined. ?

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Initializing a Variable

  • To initialize a variable, you assign it a

value

– Recall: the program type determines the mathematical type, which in turn determines the possible values the variable can have

int j = 13;

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13

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SLIDE 22

Initializing a Variable

  • To initialize a variable, you assign it a

value

– Recall: the program type determines the mathematical type, which in turn determines the possible values the variable can have

int j = 13;

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This is an int variable j whose value is 13, i.e., j = 13 13

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Reasoning: Tracing Tables

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Code State

x = 1.414 int j = 13; x = 1.414 j = 13

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Reasoning: Tracing Tables

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Code State

x = 1.414 int j = 13; x = 1.414 j = 13 Every other row in the left column contains some program statement(s).

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Reasoning: Tracing Tables

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Code State

x = 1.414 int j = 13; x = 1.414 j = 13 Every other row in the right column contains some mathematical sentences ("facts").

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Reasoning: Tracing Tables

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Code State

x = 1.414 int j = 13; x = 1.414 j = 13 This equal sign, in code, means assignment of a value to a program variable.

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Reasoning: Tracing Tables

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Code State

x = 1.414 int j = 13; x = 1.414 j = 13 This equal sign, in mathematics, means equality of two values.

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Reasoning: Tracing Tables

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Code State

x = 1.414 int j = 13; x = 1.414 j = 13 There is no value for mathematical variable j in this state because program variable j hasn’t been declared yet.

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Reasoning: Tracing Tables

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Code State

x = 1.414 int j = 13; x = 1.414 j = 13 There is a value for j in this state because j has been declared before this state.

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Literals

  • A data value appearing, literally, in a

program is called a literal

String fileName = "foo.txt"; boolean found = false; char win = 'W'; int j = 13; double ht = 9.27;

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SLIDE 31

Literals

  • A data value appearing, literally, in a

program is called a literal

String fileName = "foo.txt"; boolean found = false; char win = 'W'; int j = 13; double ht = 9.27;

14 January 2019 OSU CSE 31

This is a String literal; written as characters between double-quote marks: "…"

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SLIDE 32

Literals

  • A data value appearing, literally, in a

program is called a literal

String fileName = "foo.txt"; boolean found = false; char win = 'W'; int j = 13; double ht = 9.27;

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This is a boolean literal; must be either true or false.

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SLIDE 33

Literals

  • A data value appearing, literally, in a

program is called a literal

String fileName = "foo.txt"; boolean found = false; char win = 'W'; int j = 13; double ht = 9.27;

14 January 2019 OSU CSE 33

This is a char literal; normally written as a single character between single-quote marks: '…'

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SLIDE 34

Literals

  • A data value appearing, literally, in a

program is called a literal

String fileName = "foo.txt"; boolean found = false; char win = 'W'; int j = 13; double ht = 9.27;

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This is an int literal; normally written (as in mathematics) as a decimal constant.

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SLIDE 35

Literals

  • A data value appearing, literally, in a

program is called a literal

String fileName = "foo.txt"; boolean found = false; char win = 'W'; int j = 13; double ht = 9.27;

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This is a double literal; normally written (as in mathematics) as a decimal constant with a decimal point.

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Forms of Literals

Program type Literal examples String "I\'m" "at OSU" boolean true false char 'A' '\t' '\"' '\u03c0' int 29

  • 13

035 0x1a double

  • 18. 18.0

8E-4 6.022E23

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Forms of Literals

Program type Literal examples String "I\'m" "at OSU" boolean true false char 'A' '\t' '\"' '\u03c0' int 29

  • 13

035 0x1a double

  • 18. 18.0

8E-4 6.022E23

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escaped special character: single-quote

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Forms of Literals

Program type Literal examples String "I\'m" "at OSU" boolean true false char 'A' '\t' '\"' '\u03c0' int 29

  • 13

035 0x1a double

  • 18. 18.0

8E-4 6.022E23

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non-printing character: tab

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Forms of Literals

Program type Literal examples String "I\'m" "at OSU" boolean true false char 'A' '\t' '\"' '\u03c0' int 29

  • 13

035 0x1a double

  • 18. 18.0

8E-4 6.022E23

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Unicode character: small Greek π

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Forms of Literals

Program type Literal examples String "I\'m" "at OSU" boolean true false char 'A' '\t' '\"' '\u03c0' int 29

  • 13

035 0x1a double

  • 18. 18.0

8E-4 6.022E23

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  • ctal integer

(base-8): 29 in decimal

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Forms of Literals

Program type Literal examples String "I\'m" "at OSU" boolean true false char 'A' '\t' '\"' '\u03c0' int 29

  • 13

035 0x1a double

  • 18. 18.0

8E-4 6.022E23

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hexadecimal integer (base-16): 26 in decimal

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Forms of Literals

Program type Literal examples String "I\'m" "at OSU" boolean true false char 'A' '\t' '\"' '\u03c0' int 29

  • 13

035 0x1a double

  • 18. 18.0

8E-4 6.022E23

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scientific notation: 8 x 10–4

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Constants

  • A variable whose value is initialized and

never changed is called a constant

int myLuckyNumber = 13; double avogadro = 6.022E23;

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Constants

  • A variable whose value is initialized and

never changed is called a constant

final int myLuckyNumber = 13; final double avogadro = 6.022E23;

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The keyword final indicates to the compiler your intent that a variable is actually a constant.

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Constants

  • A variable whose value is initialized and

never changed is called a constant

final int myLuckyNumber = 13; final double avogadro = 6.022E23;

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When constants are declared inside a method, the standard Java convention is to use camel case just like for variables.

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Constants

  • A variable whose value is initialized and

never changed is called a constant

final int MY_LUCKY_NUMBER = 13; final double AVOGADRO = 6.022E23;

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When constants are declared at the class level, the standard Java convention is to use all upper case letters and to separate the words with '_'.

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Resources

  • Java for Everyone, Chapter 2

– http://osu.worldcat.org/title/java-for-everyone-late-objects/oclc/808511232

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