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1 CS 106 Computing Fundamentals II Chapter 22 “Data Types” Herbert G. Mayer, PSU CS Status 7/10/2013 Initial content copied verbatim from CS 106 material.

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Presentation on theme: "1 CS 106 Computing Fundamentals II Chapter 22 “Data Types” Herbert G. Mayer, PSU CS Status 7/10/2013 Initial content copied verbatim from CS 106 material."— Presentation transcript:

1 1 CS 106 Computing Fundamentals II Chapter 22 “Data Types” Herbert G. Mayer, PSU CS Status 7/10/2013 Initial content copied verbatim from CS 106 material developed by CS professors: Cynthia Brown & Robert Martin

2 2 Syllabus Define Data Type Define Data Type Information Content in Bits Information Content in Bits Storing Text Storing Text Integers Integers Floating Point Numbers Floating Point Numbers Logical Values Logical Values Date Type Date Type Variables Variables

3 3 Define Data Type A computer data type is the representation of a particular range of valuesA computer data type is the representation of a particular range of values Such values can be numeric, textual, or otherSuch values can be numeric, textual, or other Values constitute an integer subrange type Values from ‘A’.. ‘Z’ constitutes a character subrange We’ll look at the most common ones, such as character, integer, floating-point, character, booleanWe’ll look at the most common ones, such as character, integer, floating-point, character, boolean For each one we learn:For each one we learn: What is the type typically used for in a program The potential range of values we can store What kinds of operations we can do on it

4 4 Information Content in Bits One bit can have two values, 0 and 1. You cannot find a way to represent more than two different values with one bitOne bit can have two values, 0 and 1. You cannot find a way to represent more than two different values with one bit Two bits can represent 4 values: 00, 01, 10, 11Two bits can represent 4 values: 00, 01, 10, 11 Three bits gives 8 values: 0 followed by each of the 4 values for 2 bits, or 1 followed by each of the four values for 2 bitsThree bits gives 8 values: 0 followed by each of the 4 values for 2 bits, or 1 followed by each of the four values for 2 bits In general, adding a bit doubles the number of values. With n bits you can represent 2 n valuesIn general, adding a bit doubles the number of values. With n bits you can represent 2 n values

5 5 Storing Text One kind of data we often want to store is textOne kind of data we often want to store is text Text is made up of characters. To let computers of different kinds interact with each other, standard character sets are usedText is made up of characters. To let computers of different kinds interact with each other, standard character sets are used The ASCII character set (the old standard) uses 8 bits per character: American Standard Code for Information InterchangeThe ASCII character set (the old standard) uses 8 bits per character: American Standard Code for Information Interchange Another common one is EBCDIC: Extended Binary Coded Decimal Interchange Code; use by IBMAnother common one is EBCDIC: Extended Binary Coded Decimal Interchange Code; use by IBM 8 bits can encode 2 8 = 256 different characters8 bits can encode 2 8 = 256 different characters

6 6 Text 256 characters is enough for upper and lower case English letters, punctuation, the 10 digits, space and new line, and some odd characters like letters with umlauts256 characters is enough for upper and lower case English letters, punctuation, the 10 digits, space and new line, and some odd characters like letters with umlauts But there are many other alphabets in the world, not to mention Chinese characters!But there are many other alphabets in the world, not to mention Chinese characters! The new standard, called Unicode, uses 16 bits per character. With 2 16 possible characters, there are plenty to go around, or 65,536 different valuesThe new standard, called Unicode, uses 16 bits per character. With 2 16 possible characters, there are plenty to go around, or 65,536 different values Yet CDC supercomputers used 6-bit character set, no lower- AND uppercase letters!Yet CDC supercomputers used 6-bit character set, no lower- AND uppercase letters!

7 7 String Type VBA uses data type String for strings of text, including single characters. Example: “hello”VBA uses data type String for strings of text, including single characters. Example: “hello” Each type, including String, has appropriate operations associated with itEach type, including String, has appropriate operations associated with it Note that there is no specification and thus limitation on the length of a string in VBA! Very convenient for userNote that there is no specification and thus limitation on the length of a string in VBA! Very convenient for user

8 8 Operations on Types (Strings) With the string type, most operations use built-in functions. The & string operator concatenates two strings. For example: varA = “Hello ”--note space at the end: ‘ ‘ varB = “There” varC = varA & varB gives varC the value “Hello There” We’ll look at the string functions later

9 9 Numbers Computer hardware is usually set up to handle two distinct types of numbers, integer and floating pointComputer hardware is usually set up to handle two distinct types of numbers, integer and floating point There is often separate hardware for doing arithmetic on each type, and they are used in different waysThere is often separate hardware for doing arithmetic on each type, and they are used in different ways Floating point number representation is defined via specific standards, the most common being IEEE 754Floating point number representation is defined via specific standards, the most common being IEEE 754

10 10 Integers God invented integers, man invented floating point numbers!God invented integers, man invented floating point numbers! Integers are used for counting, e.g. indexingIntegers are used for counting, e.g. indexing Excel/VBA has two integer types, called Integer and Long. Integer is 2 bytes and Long is 4 bytesExcel/VBA has two integer types, called Integer and Long. Integer is 2 bytes and Long is 4 bytes Signed integer numbers range from to Since there can be more than rows in a worksheet, it’s best to always use Long unless you know for sure your numbers are smallerSigned integer numbers range from to Since there can be more than rows in a worksheet, it’s best to always use Long unless you know for sure your numbers are smaller

11 11 Floating Point Numbers This is the kind of number we use for general computations, which can have a fractional partThis is the kind of number we use for general computations, which can have a fractional part Internally they are stored in a form that uses a mantissa and an exponent, allowing representations similar to scientific notation ( E 23)Internally they are stored in a form that uses a mantissa and an exponent, allowing representations similar to scientific notation ( E 23) Excel/VBA has two floating point types, Single- Precision (4 bytes) and Double-Precision (8 bytes)Excel/VBA has two floating point types, Single- Precision (4 bytes) and Double-Precision (8 bytes) Double has way more precision and we shall use it in CS 106Double has way more precision and we shall use it in CS 106

12 12 Operations on Types (Numbers) Numbers have the usual arithmetic operations, with the usual precedence. SoNumbers have the usual arithmetic operations, with the usual precedence. So * 4 means 2 + (3 * 4), not (2 + 3) * 4 If you want the last version, use parentheses to change the precedence. You can always use parentheses to be sure the expression means what you want it toIf you want the last version, use parentheses to change the precedence. You can always use parentheses to be sure the expression means what you want it to Most arithmetic operations use the same notation when applied to integers or floating point numbersMost arithmetic operations use the same notation when applied to integers or floating point numbers

13 13 Mixing Integers and Floats If you write an expression that uses both integers and floating point numbers, the result will be a floating point numberIf you write an expression that uses both integers and floating point numbers, the result will be a floating point number Operations like exponentiation or division also yield a floating point result when applied to two integersOperations like exponentiation or division also yield a floating point result when applied to two integers Types can be converted to related, other typesTypes can be converted to related, other types Such conversions can generate unexpected results, or else requires a user with intimate understanding of all conversion rules!Such conversions can generate unexpected results, or else requires a user with intimate understanding of all conversion rules!

14 14 Logical (Boolean) Values Another data type is for logical values, called Boolean in honor of logician George BooleAnother data type is for logical values, called Boolean in honor of logician George Boole Booleans have one of two values: True or FalseBooleans have one of two values: True or False Booleans in VBA Excel are two bytes – efficient?Booleans in VBA Excel are two bytes – efficient? Why two bytes for information that could be encoded in one bit? The rationale is speed, since it would take longer for a computer to extract the value of a single bit from a group and the wasted bits are cheapWhy two bytes for information that could be encoded in one bit? The rationale is speed, since it would take longer for a computer to extract the value of a single bit from a group and the wasted bits are cheap

15 15 Operations on Boolean Boolean values are manipulated via operations such as: AND, OR, XOR, and NOT We’ll look at these closely in our module on conditionals

16 16 Date Type Dates are their own data type in VBA/ExcelDates are their own data type in VBA/Excel They take up 8 bytes, and can range from January 1, 0100 to December 31, 9999They take up 8 bytes, and can range from January 1, 0100 to December 31, 9999 The basic display format for dates is MM/DD/YYYY, though you can choose other formatsThe basic display format for dates is MM/DD/YYYY, though you can choose other formats Internally, starting with Jan. 1, 1900, dates are represented by a number which is 1 on 1/1/1900 and goes up from thereInternally, starting with Jan. 1, 1900, dates are represented by a number which is 1 on 1/1/1900 and goes up from there

17 17 Operations on Dates You can add and subtract numbers from dates, as long as you are using dates after January 1, E.g.:You can add and subtract numbers from dates, as long as you are using dates after January 1, E.g.: 2/12/ is 2/12/19 Excel is smart enough to go into the next month or year if need beExcel is smart enough to go into the next month or year if need be

18 18 Other Data Types VBA has plenty of other data types, but the ones we discussed we’ll mainly useVBA has plenty of other data types, but the ones we discussed we’ll mainly use VBA includes a Range type, to define to a range of cells. These can be useful in programming; we’ll introduce them as we need themVBA includes a Range type, to define to a range of cells. These can be useful in programming; we’ll introduce them as we need them There is also a Variant data type that is kind of a grab- bag of all types together. It uses a lot of extra space and we will avoid it; too advanced!There is also a Variant data type that is kind of a grab- bag of all types together. It uses a lot of extra space and we will avoid it; too advanced! However, Excel cells do automatically have the type Variant, else you could not arbitrarily assign a cell all kinds of valuesHowever, Excel cells do automatically have the type Variant, else you could not arbitrarily assign a cell all kinds of values

19 19 Declaring Variables When we start to use variables, we will make it a practice to declare them: state up front what their data type isWhen we start to use variables, we will make it a practice to declare them: state up front what their data type is The alternative, which might seem attractive, is to let Excel deduce their data typeThe alternative, which might seem attractive, is to let Excel deduce their data type The problem with this is that it can make a typo in a variable name into a serious, hard to find error as Excel just thinks it is a new variableThe problem with this is that it can make a typo in a variable name into a serious, hard to find error as Excel just thinks it is a new variable Very common source of problems in old programming languages, such as Fortran and PL/IVery common source of problems in old programming languages, such as Fortran and PL/I We will use Option Explicit as the first line in every VBA module we write to avoid this problemWe will use Option Explicit as the first line in every VBA module we write to avoid this problem

20 20 The Dim Statement To declare a variable we use a statement such asTo declare a variable we use a statement such as Dim varA As Long Dim is a keyword that tells VBA that we are getting ready to declare a variableDim is a keyword that tells VBA that we are getting ready to declare a variable varA is the name of the variable – a user defined namevarA is the name of the variable – a user defined name You must use the special keyword As, between the variable name and the name of the typeYou must use the special keyword As, between the variable name and the name of the type Use Const instead of Dim to declare a constant, as inUse Const instead of Dim to declare a constant, as in Const varA As Long = 1 A constant can’t be changed by the code; must be initializedA constant can’t be changed by the code; must be initialized

21 21 Names for Variables We’ll sometimes use fairly generic names (like varA) in our initial examples; when you’re writing a real macro, use meaningful names for your variablesWe’ll sometimes use fairly generic names (like varA) in our initial examples; when you’re writing a real macro, use meaningful names for your variables You cannot use a key word as a variable name, as keywords are reservedYou cannot use a key word as a variable name, as keywords are reserved The name must start with a letter and is made up of letters, digits and underscores; examples:The name must start with a letter and is made up of letters, digits and underscores; examples: My_value min3 It can be up to 255 characters in lengthIt can be up to 255 characters in length

22 22 Names for Variables In our class, we will start all variable names with a lower case letterIn our class, we will start all variable names with a lower case letter An example of a long descriptive name would be primaryInterestRate or primary_interest_rateAn example of a long descriptive name would be primaryInterestRate or primary_interest_rate VBA does not distinguish by case, so varA and vara are the same name to VBAVBA does not distinguish by case, so varA and vara are the same name to VBA Again: VBA variable names are not case sensitive!!Again: VBA variable names are not case sensitive!!

23 23 A Final Caution Things are not always exactly the same in VBA as in Excel. Each has their own built-in functions, precedence rulesThings are not always exactly the same in VBA as in Excel. Each has their own built-in functions, precedence rules For example: Excel cells are named by alphabetic columns followed by numeric lines, e.g. B4 --meaning column 2, row 4For example: Excel cells are named by alphabetic columns followed by numeric lines, e.g. B4 --meaning column 2, row 4 VBA Cells() indices are both numeric and list the row number first, such as Cells( 4, 5 ) --meaning row 4, column 5VBA Cells() indices are both numeric and list the row number first, such as Cells( 4, 5 ) --meaning row 4, column 5


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