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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

  1. Variables, Types, Values 14 January 2019 OSU CSE 1

  2. 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

  3. Examples true 'y' 13 3.14 "Go" 14 January 2019 OSU CSE 3

  4. Examples true 'y' 13 3.14 "Go" This is a boolean variable b whose value is true , i.e., b = true or, more simply, just b 14 January 2019 OSU CSE 4

  5. Examples true 'y' 13 3.14 "Go" This is a char variable c whose value is 'y' , i.e., c = 'y' 14 January 2019 OSU CSE 5

  6. Examples true 'y' 13 3.14 "Go" This is a int variable i whose value is 13 , i.e., i = 13 14 January 2019 OSU CSE 6

  7. Examples true 'y' 13 3.14 "Go" This is a double variable d whose value is 3.14 , i.e., d = 3.14 14 January 2019 OSU CSE 7

  8. Examples true 'y' 13 3.14 "Go" This is a String variable s whose value is "Go" , i.e., s = "Go" (or s = <'G', 'o'> ) 14 January 2019 OSU CSE 8

  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

  10. 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 other times • A mathematical variable stands for an arbitrary but fixed value 14 January 2019 OSU CSE 10

  11. Program vs. Mathematical Types • A program type has a corresponding mathematical type that models it 14 January 2019 OSU CSE 11

  12. Program vs. Mathematical Types • A program type has a corresponding mathematical type that models it 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 . 14 January 2019 OSU CSE 12

  13. 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) 14 January 2019 OSU CSE 13

  14. Mathematical Models Program type Mathematical type String string of character boolean boolean String is built-in to Java; char character boolean , char , int int , and double are integer among the 8 primitive (and ( -2147483648 through also built-in) types of Java; 2147483647 ) differences later. double real (about ± 10 ± 308 , 15 significant digits) 14 January 2019 OSU CSE 14

  15. Mathematical Models Program type Mathematical type String string of character boolean boolean All these mathematical types char character are “built-in” to mathematics! int integer ( -2147483648 through 2147483647 ) double real (about ± 10 ± 308 , 15 significant digits) 14 January 2019 OSU CSE 15

  16. Mathematical Models Program code is shown in a Program type Mathematical type blue fixed-width font, with keywords in bold . String string of character boolean boolean char character int integer ( -2147483648 through 2147483647 ) double real (about ± 10 ± 308 , 15 significant digits) 14 January 2019 OSU CSE 16

  17. Mathematical Models Mathematics is shown in a Program type Mathematical type green fixed-width italic font, with keywords in bold . String string of character boolean boolean char character int integer ( -2147483648 through 2147483647 ) double real (about ± 10 ± 308 , 15 significant digits) 14 January 2019 OSU CSE 17

  18. 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

  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 The standard Java convention for mathematical type, which in turn determines naming variables is to use camel the possible values the variable can have case : start with a lower case letter and only capitalize the first letter of int j ; ? each following word, e.g., myLuckyNumber 14 January 2019 OSU CSE 19

  20. 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 This is an int mathematical type, which in turn determines variable j whose the possible values the variable can have value is undefined . int j ; ? 14 January 2019 OSU CSE 20

  21. 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 ; 13 14 January 2019 OSU CSE 21

  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 ; This is an int variable j whose value is 13 13 , i.e., j = 13 14 January 2019 OSU CSE 22

  23. Reasoning: Tracing Tables Code State x = 1.414 int j = 13; x = 1.414 j = 13 14 January 2019 OSU CSE 23

  24. Reasoning: Tracing Tables Code State x = 1.414 Every other row in the left column contains some program int j = 13; statement(s). x = 1.414 j = 13 14 January 2019 OSU CSE 24

  25. Reasoning: Tracing Tables Code State x = 1.414 Every other row in the right column contains some mathematical int j = 13; sentences ("facts"). x = 1.414 j = 13 14 January 2019 OSU CSE 25

  26. Reasoning: Tracing Tables This equal sign, in Code State code, means assignment of a value to a program x = 1.414 variable. int j = 13; x = 1.414 j = 13 14 January 2019 OSU CSE 26

  27. Reasoning: Tracing Tables Code State This equal sign, in mathematics, means equality of two x = 1.414 values. int j = 13; x = 1.414 j = 13 14 January 2019 OSU CSE 27

  28. Reasoning: Tracing Tables There is no value for Code State mathematical variable j in this state because program variable j hasn’t x = 1.414 been declared yet. int j = 13; x = 1.414 j = 13 14 January 2019 OSU CSE 28

  29. Reasoning: Tracing Tables There is a value for j Code State in this state because j has been declared x = 1.414 before this state. int j = 13; x = 1.414 j = 13 14 January 2019 OSU CSE 29

  30. 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 30

  31. Literals • A data value appearing, literally, in a program is called a literal String fileName = "foo.txt"; boolean found = false ; char win = 'W' ; This is a String literal; int j = 13 ; written as characters double ht = 9.27 ; between double-quote marks: "…" 14 January 2019 OSU CSE 31

  32. Literals • A data value appearing, literally, in a program is called a literal String fileName = "foo.txt"; boolean found = false ; char win = 'W' ; This is a boolean literal; int j = 13 ; must be either double ht = 9.27 ; true or false . 14 January 2019 OSU CSE 32

  33. Literals • A data value appearing, literally, in a program is called a literal String fileName = "foo.txt"; boolean found = false ; char win = 'W' ; This is a char literal; int j = 13 ; normally written as a double ht = 9.27 ; single character between single-quote marks: '…' 14 January 2019 OSU CSE 33

  34. Literals • A data value appearing, literally, in a program is called a literal String fileName = "foo.txt"; This is an int literal; boolean found = false ; normally written (as in char win = 'W' ; mathematics) as a int j = 13 ; decimal constant. double ht = 9.27 ; 14 January 2019 OSU CSE 34

  35. Literals • A data value appearing, literally, in a program is called a literal String fileName = "foo.txt"; boolean found = false ; This is a double literal; char win = 'W' ; normally written (as in int j = 13 ; mathematics) as a decimal constant with a double ht = 9.27 ; decimal point. 14 January 2019 OSU CSE 35

  36. 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 14 January 2019 OSU CSE 36

  37. Forms of Literals Program type Literal examples String "I\'m" "at OSU" boolean true false escaped special char 'A' '\t' '\"' character: '\u03c0' single-quote int 29 -13 035 0x1a double 18. 18.0 8E-4 6.022E23 14 January 2019 OSU CSE 37

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