Data Types & Arrays

1. Data Types & Arrays

2. Goals • By the end of this unit, you should understand … • … why we use data types. • … the different types of primitive data that are available to programmers. • … how to use arrays. • … how we can use different algorithms to sort arrays.

3. Data in Memory • All programming languages include a way to reference data stored in memory. • Typically we store such data using places in memory called variables. In a program we reference a variable's name (it's identifier) to either assign data to a variable or to read data from a variable.

4. Variable Identifiers • Variable identifiers are the way we reference variables in a program. They are also referred to as variable names. • In most languages, identifiers must follow the rules below: • Identifiers must begin with a letter. • Subsequent identifier characters may be letters, numbers or the underscore character, "_". • Identifiers may not contain spaces. • Identifiers must be unique and cannot conflict with a reserved word (keyword).

5. VALID IDENTIFIERS userName user_name USERNAME userName2 uSeRnAmE INVALID IDENTIFIERS _userName user'sName user name user-name 2UserName Identifier Examples

6. Identifiers are Case-Sensitive! • Identifiers are case-sensitive, meaning that uppercase letters and lowercase letters are seen as different characters. • Therefore: • userName <> USERNAME • UserName <> username • userName <> UsErNaMe • etc …

7. Identifier Conventions • Although not "hard and fast" rules, programmers have developed some common conventions for creating identifiers: • Create variable identifiers using a meaningful name. • Some programmers like to use prefixes to an identifier, indicating data type (more on that later). • Use camel-casing for identifiers that are multi-word phrases.

8. More on Camel-Casing • Camel-Casing is a convention some programmers use to make identifiers easy to read. If an identifier has more than 1 word in it, you can use camel-casing. • To use camel-casing: • Begin the identifier name with a lowercase letter. • Capitalize the first letter of each subsequent word in an identifier's name.

9. Camel-Casing Examples • userName • monthlyStatement • loanAmount • selectedCourse • studentID • instructorOfRecord • yearlySalary

10. Data Typing • When we create a variable, not only do we give a variable an identifier, we also declare its type (sometimes referred to as "dimensioning" a variable). • Among other purposes, a variable type tells a computer how much memory to set aside for a particular variable. • Why do we type variables? Conservation of resources.

11. Primitive Types • Data primitives, or primitive types, are data types a computer inherently understands. • Most languages break primitive types into four categories (if not more): • Integers • Floating Point Number (Real Numbers) • Characters/Strings • Booleans

12. Integer Numbers • Programmers use integer data types to store positive and negative whole numbers and the number zero. • Require small amount of space in memory. • We would declare in pseudocode like this:Declare variableName as Integer

13. Floating Point Numbers • Programmers use the “float” data type to store numbers with fractional values. • Floats require larger amounts of space in memory. • You can also store integers as floats. • Pseudocode Example:Declare variableName as Float

14. Characters • A characteris a single number, letter or symbol not intended to be used in calculations. • Each character is represented internally by a numeric code in a given encoding scheme, like ASCII, ANSI or Unicode (related link). • The encoding schemes represent lower- and upper-case letters using different numbers. • Even a space or blank is represented by a number! • We enclose character values in quotes.

15. Strings • Some programming languages using the string data type both for true strings and characters, while others recognize only characters as a data type and strings as special arrays made of characters. A stringis a sequence of characters. • Just like with characters, we enclose string values in quotes. • The null string consists of two quotes : “” • Strings have a length in memory that is equal to the number of characters in the string, including blanks.

16. Creating a String in Pseudocode • Basic form:Declare variableName as String • Example:Declare userName as String

17. Comparing Strings • Two strings are equal if they have the same length and the same characters, in the same order. • “Blue” and “BLUE” are not equal. • “c%$$” and “c%$$” are equal. • Other comparisons are based on the ASCII/ANSI/Unicode value: • “B” is less than “b” because “B” has an encoding value of 66, while “b” has a value of 98.

18. Numbers as Strings • We can “tell” the computer to treat a number as a string, if we don’t intend to use that number in a calculation, by enclosing it in quotes. The code below will not display 3, but the character string “1 + 2”. Declare myString as String myString = “1 + 2” Write myString

19. Booleans • Programmers use the Boolean data type to store true/false values. • Booleans require very small amounts of space in memory. • When assigning values to a Boolean, we use the words true or false, without quotes. • Pseudocode Example:Declare variableName as Boolean

20. Converting Data Types • Sometimes, we need to convert a value to another data type in order to use it. In most languages, we can convert a smaller data type to a larger one without trouble. For instance, converting an integer to a float: Declare firstNumber as Integer Declare secondNumber as Float firstNumber = 8675309 secondNumber = firstNumber • secondNumber gets the value 8675309.0

21. Casting • When programmers need explicitly convert data types (like converting from a float to an integer), they use something called casting or parsing. • We need to be careful when casting, as it can be a destructive operation. For instance, casting the value 5.745 to an in integer number will result in value 5, completely different from the original value.

22. Casting • Casting differs from language to language (some languages use casting operators, some use functions, etc.). • Pseudocode (uses a casting operator): Declare userNumber as String Delcare actualNumber as Integer Prompt “Enter an integer number: ”, userNumber actualNumber = (Integer) userNumber

23. Introducing Arrays • Sometimes, it is quite cumbersome to declare multiple variables. Consider having to declare 35 different variables to represent students in class (70 if you separate first and last names!). • To solve this problem, programming languages include another data structure called an array.

24. What is an Array? • An array is a data structure that contains related items that are of the same data type and where each item in the array shares the same name. • In memory, array values occupy contiguous locations. • For instance, we could use an array to store the names of students in a class ...

25. Common Array Terms • An element is a value stored in an array. • Each element is stored in a different position in an array. We can reference a position using a subscript (a.k.a. index). When we reference an element, we indicate the array name and the subscript of the particular element we want: Write myStringArray[5]

26. Array Numbering & Length • We number subscripts, or indexes, starting with the number zero (0). We increment by 1 for each subsequent index. • The array’s length refers to the total number of indexes used in an array. • The length is always one more than the last index number. Conversely, the last index number is always one less than the array’s length.

27. A one-dimensional array is a single list of values (called “elements”) and their addresses in the array (called “indexes” or “subscripts”) Think of a seating chart in elementary school – a single row of students is an array 0 Janie Bobby 1 Sally 2 Joey 3 Mary 4 Row0 One-Dimensional Arrays Mary is located in Row0[4]

28. Two-Dimensional Arrays • Two dimensional arrays contain other arrays as elements • To reference an element of a “child” array, you would need to reference both the index number of the child array and the index number of the element in the child array. • See the next slide for details …

29. 0 0 0 Terri Katie Janie Cindy Jim Bobby 1 1 1 Mike Alex Sally 2 2 2 Joey Ravi Jacob 3 3 3 Mary Jamal Kirby 4 4 4 2 0 1 Two-Dimensional Array Example Mary is located in Class[0,4] Class Array

30. Parallel (Corresponding) Arrays • Parallel arrays give the programmer the ability to store multiple pieces of information about a single entity in an application. • The indexes of each array should correspond to one another for individual entities: • Student ID • Student Name • Student Exam Score

31. Parallel Arrays Example Array Name = SID Array Name = n201Class Array Name = examScr

32. Searching and Sorting Arrays • If we have multiple, related arrays, with the elements ordered correctly, we can find values in one array based on on a search key value in another array. • A search key is a known value for one of the arrays. In the text example, a specific value of ‘Flight’ can be used to find the Origin, Time, and Gate of that flight. It can be used as a search key. • This is called a ‘table lookup’ or ‘serial search’.

33. Searching and Sorting Arrays • 3 steps: • Load, or populate, the arrays. • Search the array that contains the key, to find a match. • Display the key and corresponding values in the non-key arrays with the same subscript as the one with the key value, if a match is found. • Display an ‘unsuccessful search’ message if no matches were found.

34. Searching and Sorting Arrays • Use a flag, or switch, to decide which message to print. • A flag is a variable that is set to zero or one, depending on the results of an operation. • The value of the flag can be checked later in the program with an ‘If’ statement to determine which action to take.

35. The Bubble Sort Technique • To sort data means to arrange it in a particular numerical or alphabetical order, ascending or descending. • To sort small amounts of data, use the ‘bubble sort’ technique. • Make several passes through the data. • Compare adjacent pairs of data. • If the pairs are not in order, swap them. • Continue until all data is processed.

36. Sorting Examples • http://www.cs.iupui.edu/~aharris/n301/alg/tmcm-java-labs/labs/xSortLabLab.html

37. Questions?

38. Resources • Venit, Stewart. Extended Prelude to Programming: Concepts and Design. Scott/Jones, Inc., 2002.