Introduction to Variables

1. Introduction to Variables • Remember that memory is a long series of boxes referred to by numerical addresses. • We could allow any numeric memory location to be used in our algorithms to store data in memory and then retrieving the data when we needed it. • The problem with using numeric memory addresses directly is twofold. • Programs with more than a few pieces of data would drive you crazy trying to keep track of where you put it. • Different real world machines have different memeory structures. 1 8 x y

2. Introduction to Variables • We use the concept of a variable to assign a name to the memory location. • Lets call our data "x" and "y". • We use a data declaration to tell the computer that we are using variable names to help us store data. • Declarations need to specify the variable NAME, TYPE, and STRUCTURE. • The computer will assign memory locations to each variable: • Here, “x” is associated with memory location 1. • Here, “y” is associated with memory location 8. 1 8 x y

3. Data Types and Data Structures • When specifying data in a program we need to describe its name, type and its structure. • Data's type impose meaning onto data (semantics) and data's structure impose organization (syntax) onto data. • Data Type (definition): A label applied to data that tells the computer how to interpret and manipulate data. • Type tells the computer how much space to reserve for variables and how to interpret operations on them. • Data Structure (definition): The way data is organized logically. • Describes how different pieces of data are organized.

4. Data Types and Structures • Usually data type and structure are not related. • Example: Integer list[n] is an array of n integer values, while Boolean blist[n] is an array of n Boolean values. • The rule is that a structure can contain any type. • Data types typically have specific operations associated with them • Example: Operations assocaited with integers are add, subtract, multiply, divide. • Most data structures also have specific operations associated with them • Typically involve storing and retrieving data from the structure • programming languages that support the structure will provide you with these operations as functions built into the language. • If not provided, you can build the functions yourself.

5. Data Types • The computer always needs to know the kind, or TYPE of data you expect it to work with. • The type tells the computer how to store and manipulate the data. • Basic data types include: • Integers: The counting numbers. 1, 2, 3, 4, … etc • 1, 16, -34, 1024 are integers. • Numbers: All other numbers (sometimes called “floating point”) • 3.14159, -62.0, .125 are numbers. • Characters: The letters of the alphabet, plus punctuation, plus the numerals 0, 1, 2, …, 9. Usually designated using single-quotes. • ‘a’, ‘b’, ‘c, ‘8’, ‘(‘ are characters, but ’12’ is not. • Strings: Groups of characters, designated with double quotes. • “Hello, World!” is a string; “1024” is a string, but 1024 is not. • Booleans: True and False are the only booleans.

6. Data Types • Programming languages that require you to specify the types of data you are using are called TYPED LANGUAGES. • The act of specifying the type is called DECLARING the type. • Programming languages that do not require you to specify the types of data you are using are called UNTYPED LANGUAGES. • The computer tries to guess the type of data you want to use. • JavaScript is an untyped language. • Untyped languages can be easier for the programmer to write code with, but programmers are always concerned with data types when writing algorithms. • We will declare our data types when writing algorithms.

7. Data Types - Integers • There are two basic ways to classify numeric data: Integers, and Numbers. • Integers: The counting numbers. These are used when it is clear that the values required are whole numbers only. They can be negative or positive. • Example uses: • Used for math operations • Used to count distinct items (you can't have 4.5 people in class) • Used to count through loop iterations (you cannot enter a loop 4.5 times) • Used to index arrays (example: list[i] requires i to be an integer) • Typical operations apply: addition, multiplication, subtraction, division.

8. Data Types - Numbers • Numbers (Floating Point): The real numbers (also known as floating point because of the way a real number is stored in a computers memory). • Used for math operations. • Used for any value not limited to integer form. • Numbers: Examples: • Areas, lengths and measures. • Temperatures. • Fiscal ( $100.35 ) • Typical operations apply: addition, multiplication, subtraction, division. • Special operations are often provided by programming languages: modulo, square, square root, etc.

9. Data Types - Characters and Strings • Character: A single alphabetic letter or punctuation mark. • Uses ASCII code, which is a 1 byte in length (1 byte = 8 bits). • This means there are 28 = 256 distinct values allowed. • 52 values are taken up by small and capital letters. • 10 values taken up by digits 0 - 9. • The rest are punctuation and special characters. • String: A list (or array) of characters. • It actually a structure (array) but so commonly used that most languages provide it as a type. • Most languages provide some utility functions to manipulate strings, such as concatenate, print, slice, or get_character.

10. Data Types - Boolean • Named for mathemitican/logician Bool. • Boolean: Used for any situation where only two values allowed. • Used for “True-False” situations: tests for Do-Loops and If-Then-Else blocks. • Standard Boolean, or logical, operations allowed: Comparisons (equals, greater than, less than), logical AND, logical OR, and logical NOT.

11. Data Types - Boolean • Example: IF-THEN-ELSE - "test" must return a Boolean value IF (test) THEN foo1(); ELSE foo2(); ENDIF • Example: LOOPs - "test" must return a Boolean value. WHILE (test) DO foo1(): ENDDO

12. Data Types - Pointers • Pointer: A data type who's value is a memory address. • Pointers are used to "point" to places in the memory. • Used to allow you to make new variables "on the fly" and refer to the new variable by its location rather than its name. • You can assign pointers values just like any other variable. • Pointer: An integer pointing to a memory address. • Used to point to complex data structures. • Used to access memory quickly. • Since it is an integer, standard math operations apply.

13. Data Types