Chapter COS120 3.4 Strings
Data Types: The programmer must specify the type of data that will be stored in a variable. Each data type is stored using a unique format and has available to it a certain set of operators and built-in functions. Data Type Integer Double Description Whole numbers Larger floating point numbers Range Approx. +/- 2 billion 14 digits of precision
Some math functions include: CInt (string) CDbl (string) Int (double) Math.Sqrt (number) Math.Round (number,n) return the integer value equivalent to the string of digits return the double value equivalent to the string of digits return the whole integer after truncating the decimal portion Ex. Int (4.5) 4 return the square root of the number Ex. Math.Sqrt (9) 3 return the value of the number rounded to the specified number digits to the right of the decimal point. Ex. Math.Round (2.317, 1) 2.3
Note: Math is an object that has associated function (methods). Each of the functions is invoked by writing the name of the object, period ( . ), and the name of the function with its parentheses and arguments (ex. Math.Sqrt (9.4) ) Data Type String Description Group of characters
A character is stored in one byte (8 bits) via its ASCII (ANSI code). For example the code for letter A is: 0100 0001 Most ASCII charts do not list the bit sequence but instead convert the code to an integer and report the integer value. Mathematically, the number 0100 0001 in base 2 is equivalent to 65 in base 10. 0100 0001 = 1 * 26 + 1 * 20 = 64 + 1 = 65 The Text property of a Text Box is data type String. Recall the CInt or CDbl functions were used to convert strings of digits into numbers for math processing. Dim x As Double x = CDbl (txtHeight.Text) It is considered proper programming technique to do this manual conversion upon data entry although the computer will do it for you.
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The Items in a List Box are also String data type. Although there is a CStr function that converts a number to a string, it is often omitted upon display. Dim x As Double = 5.5 lstOut.Items.Add (CStr(x)) Dim x As Double = 5.5 lstOut.Items.Add (x) Some ASCII codes are listed below and the full chart is in the back of your text. A B a b 1 2 65 66 97 98 49 50 _ space 169 176 188 32
Chr (number) Asc (string)
return the ASCII character with the number code Ex. "45" & Chr(176) & " Fahrenheit" 45 Fahrenheit return the ASCII code of the first character in the string Ex. Asc ("Apple") 65
String Properties and Methods: Each string also has a number of (data properties attributes) and methods available to it. These are useful when strings need to be manipulated by pulling them apart or searching for substrings. Dim s As String Dim t As String s = "Larry Smith" t = " Hello there " With lstOut.Items .Add (s.Length) .Add (t.Length) .Add (s.ToUpper) .Add (s.ToLower) .Add (t.Trim) End With 11 17 LARRY SMITH larry smith Hello there
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` two blanks are part of the string
` display the number of characters in s ` display the number of characters in t ` display s in upper case ` display s in lower case ` display t with leading and trailing blanks removed.
Each character in a string has a position (0, 1, 2, etc.) A substring is a portion of a string that begins at a particular location. In string s above, substring "Larry" begins at position 0 and has 5 characters. Substring "Smith" begins at position 6 and has 5 characters. Dim s As String = "Larry Smith" With lstOut.Items .Add (s.IndexOf ("L")) .Add (s.IndexOf ("Smith")) .Add (s.IndexOf ("xxx")) .Add (s.SubString (0, 5)) .Add (s.SubString (6, 5)) .Add (s.SubString (2)) End With 0 6 -1 Larry Smith rry Smith ` returns the start position of "L" in s (0) ` displays start position of "Smith" in s (6) ` since "xxx" is not found in s, display -1 ` display substring of s with 5 characters ` starting at position 0 ("Larry") ` display substring of s with 5 characters ` starting at position 6 ("Smith") ` display substring of s starting at position ` 2 and continuing until the end ("rry Smith")
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The program below illustrates common string processing. The user enters his or her full name in the form: first, blank, and last (Ex. "Joan Myers" or "Peter Mason"). The string is separated into the two pieces for processing.
Private Sub btnCompute_Click( . . . Dim full As String Dim i As Integer full = txtFullName.Text i = full.IndexOf(" ") ' user full name (first last) ' position of blank ' get position of blank
' get first name from position 0 to index of blank position lblFirst.Text = full.Substring(0, i) ' get last name from position one more than blank to end of string lblLast.Text = full.Substring(i + 1) End Sub Full: i: 3 first: last: Full: i: 6 first: last: "Amy Folsom" full.Substring (0, 3) full.Substring (4) "Robert Cyr" full.Substring (0, 6) full.Substring (7) "Robert" "Cyr" "Amy" "Folsom"
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