NOTES: String Functions (module 12)

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1 Computer Science 110 NAME: NOTES: String Functions (module 12) String Functions In the previous module, we had our first look at the String data type. We looked at declaring and initializing strings, how to concatenate, or join, strings and the null string. As you have learned, strings hold character data. There are times when the programmer does not care what is in a string the string is just data that is passed from one part of the program to another. Other times, it may be necessary to perform such tasks as verifying the contents of a string, modifying the string or retrieving specific information from a string. In this module, we will look at some of the Visual Basic built-in string functions that allow you to manipulate strings to obtain data from them. String Conversion Functions String conversion functions include functions for converting the case of letters in a string. These functions include: LCase: Converts letters to lower case UCase: Converts letters to upper case StrConv: Converts according to a Visual Basic predefined constant The LCase Function The LCase function uses the following syntax: LCase returns the string argument (in parentheses) as an all lower case string, as shown in the following example: strnurseryrhyme = "Three Blind Mice" lbldisplaynurseryrhyme.caption = LCase(strNurseryRhyme) The output that would be displayed in the label would be three blind mice. Notice the characters in the string are all converted to lower case. The UCase Function The UCase function uses the following syntax: UCase returns the string argument as an all upper case string, as demonstrated in the following example: strnurseryrhyme = "Three Blind Mice" lbldisplaynurseryrhyme.caption = UCase(strNurseryRhyme) The output that would be displayed in the label would be THREE BLIND MICE,with the characters all converted to upper case. The StrConv Function The syntax for the StrConv function is: The conversion argument can be: vbuppercase: Converts the string to upper case, similar to UCase vblowercase: Converts the string to lower case, similar to LCase vbpropercase: Converts the first letter in each word of the string to upper case and converts all other letters to lower case The string will be returned according to which of these conversion constants is used. The following example shows the StrConv function: strnurseryrhyme = "ThrEE blind mice" lbldisplaynurseryrhyme.caption = StrConv(strNurseryRhyme, vbpropercase) The output that would be displayed in the label is Three Blind Mice, with the first letter of each word changed to upper case.

2 Manipulating Strings Visual Basic has several built-in functions for manipulating strings. Many of these functions are useful for parsing, or analyzing, a string. These functions can be used for extracting smaller sections, called substrings, from a string. For example, there are functions that allow you to locate and extract one character, one word or a phrase from a string. The Left Function The syntax for the Left function is: Left returns the length, or number of characters, starting from the left of the string. Let's look at an example: strname = "Janice" lbldisplayname.caption = Left(strName, 3) The label would display the characters Jan, the three characters from the left in the Janice string. The Right Function The syntax for the Right function is: Right returns a length of characters starting from the right side of the string, as in the next example: strname = "Janice" lbldisplayname.caption = Right(strName, 4) In this example, the label would display the characters nice, the four characters from the right side of the Janice string. The Mid Function The Mid function returns characters from any part of the string. The syntax for the Mid function is: Length is the number of characters to return, as in the example: strname = "Janice" lbldisplayname.caption = Mid(strName,2,4) In this example, the code would return the characters anic, beginning with the second character and returning four characters from the Janice string.

3 The Mid Statement The Mid statement is similar to the Mid function. The Mid statement is used to replace a specified number of characters in a string. The syntax for the Mid statement is shown in the graphic on the right: The following code demonstrates how the Mid statement is used: strname = "April" Mid(strName,2,3) = "Janice" lbldisplayname.caption = strname The strname variable is assigned the value April as it first appears in the code. Starting with the second letter, p, in April, the three characters pri in April are replaced with Jan in Janice. The label will show AJanl. Note that when the Mid statement is used, it appears on the left side of the assignment statement. The Mid function appears on the right side of the assignment statement. For example: Mid statement: Mid(strName,2,3) = "Janice" Mid function: lbldisplayname.caption = Mid(strName,2,4) The Len Function The Len function returns a count of the number of characters in a string. It, too, is a Visual Basic built-in function. The syntax for the Len function is: Len returns the number of characters in the string, as shown in this example: strnurseryrhyme = "Three Blind Mice" lbldisplaynurseryrhyme.caption = Len(strNurseryRhyme) The label will display the return value of 16. Why 16 and not 14? Remember, a space is a character, so there are 16 characters in Three Blind Mice. The InStr Function The InStr function is a built-in Visual Basic function that returns the starting position of a substring (a portion of a string). The syntax for the InStr function is: The following code demonstrates InStr: Dim strmonth As String, strsubstring As String Dim intstart As Integer strmonth = "January" strsubstring = "ar" intstart = 3 lbldisplayposition.caption = InStr(intStart, strmonth, strsubstring) The label will display the return value 5, as the substring begins at position 5. The search began at position 3. In this example: Dim strmonth As String, strsubstring As String strmonth = "January" strsubstring = "ar" lbldisplayposition.caption = InStr(intStart, strmonth, strsubstring) The label will display the value 5. A start position was not indicated, so searching began at position one, by default.

4 In this final example: Dim strmonth As String, strsubstring As String Dim intstart As Integer strmonth = "January" strsubstring = "an" intstart = 4 lbldisplayposition.caption = InStr(intStart, strmonth, strsubstring) The label will display 0 because the substring is never found. The start position was position 4 and the substring appears before position 4. If a substring is not found, Visual Basic returns zero (0), by default. Generating Strings In this lesson, we will look at Visual Basic built-in string functions that allow us to generate strings. These functions include: String Space & operator vbtab vbcrlf The String Function The String function is a Visual Basic built-in function that returns a string of repeating characters. The syntax for the String function is: The following code example demonstrates the use of the String function: Dim strhorizontalrule As String strhorizontalrule = String(20, "*") lbldisplaystring.caption = strhorizontalrule This code will display the following in the label: ******************** The Space Function The Space function is the Visual Basic built-in function that returns a string of spaces. The syntax for the Space function is: The following code demonstrates the Space function: Dim strblanks As String strblanks = Space(20) lbldisplaystring.caption = strblanks The label, according to the above code, would display (20 spaces).

5 String Concatenation The & operator is used to concatenate or join two or more strings into one string. The syntax for use of the & operator to concatenate strings is: The following is an example of string concatenation and the use of the & operator: Dim strfirstword As String, strsecondword As String Dim strthirdword As String Dim strphrase As String strfirstword = "tic" strsecondword = "tac" strthirdword = "toe" strphrase = strfirstword & strsecondword & strthirdword lblshowphrase.caption = strphrase 'Adds one space between words strphrase = strfirstword & Space(1) & strsecondword & Space(1) & strthirdword lblshowphrase.caption = strphrase The statement: strphrase = strfirstword & strsecondword & strthirdword will output tictactoe in the label. A space is then inserted between tic and tac and between tac and toe with the statement: strphrase = strfirstword & Space(1) & strsecondword & Space(1) & strthirdword to change the output to tic tac toe. The vbtab and vbcrlf Functions Two constants that are useful with strings are vbtab and vbcrlf. vbtab: Represents the Tab character Can be used to place eight spaces in a string vbcrlf: Represents a carriage return/linefeed combination Used to move text to the next line The following example: MsgBox "One step" & vbtab & "forward," & vbcrlf & "two steps back." will show the following string in a message box: One step forward, two steps back. Comparing Strings Recall from earlier units that computers can only store numerical data stored in memory as bytes. Each byte is made up of eight bits. Each bit is either on or off. When a bit is on, it is said to store a one (1), and when it is off, it stores a zero (0). The 1s and 0s stored by a computer represent the binary number system. Also recall that when a character, such as a letter of the alphabet, needs to be stored in memory, it is stored as a number. The ASCII code represents characters as numbers. Strings (which are strings of characters) can be compared, for example, to determine if one string is the same as another string or to see if one string comes alphabetically before or after another. If we use relational operators (=, >, <, <=, >=, <>) for making these comparisons, we would use the internal binary representation of the characters to determine the relationships. For example, the comparison "B" < "a" would be True because an uppercase B is represented in ASCII by the number 66 and the lowercase representation for a is 97. Visual Basic has built-in functions that allow us to perform textual comparisons on strings. A textual comparison is a comparison that does not distinguish between uppercase and lowercase letters. Two ways to perform textual comparisons are to use the: 1. StrComp function with the vbtextcompare constant 2. Like operator

6 The StrComp Function The syntax for the StrComp function is: Example #1 Dim strfirstword As String, strsecondword As String strfirstword = "olives" strsecondword = "artichokes" 'The following will display: First word comes after second word If StrComp(strFirstWord, strsecondword, vbtextcompare) = 0 Then lblmessage.caption = "Both words are the same" ElseIf StrComp(strFirstWord, strsecondword, vbtextcompare) = -1 Then lblmessage.caption = "First word comes before second word" Else lblmessage.caption = "First word comes after second word" End If Example #2 Dim strfirstword As String, strsecondword As String strfirstword = "artichokes" strsecondword = "artichokes" 'The following will display: Both words are the same If StrComp(strFirstWord, strsecondword, vbtextcompare) = 0 Then lblmessage.caption = "Both words are the same" ElseIf StrComp(strFirstWord, strsecondword, vbtextcompare) = -1 Then lblmessage.caption = "First word comes before second word" Else lblmessage.caption = "First word comes after second word" End If The Like Operator The Like operator can also be used to perform textual comparisons on strings. Like is usually used to perform pattern matching. For example, you may wish to look for strings that end with the pattern "*ion." In this example, the * is a wildcard that replaces characters and ion is the pattern you are looking for. The syntax for comparisons using the Like operator is shown.

7 Pattern matching allows for the use of wildcard characters, character lists and character ranges to compare strings (as shown in the table below). Wildcard How to use Example pattern? Used in place of any single character * Used in place of many characters # Used in place of any single number [ ] Used to enclose a list of characters - Used to indicate a range of characters in a character list, Used to separate characters in a character list strpattern = "?un" strpattern = "Letter to *" strpattern = "Case ##8#" strpattern = "[a,b,c,d,e]" strpattern = "[a - e]" strpattern = "[a,b,c]" Code Examples Dim strword As String, strpattern As String strword = "Run" strpattern = "?un" strword = "Run" strpattern = "?um" lblmessage.caption = strword Like strpattern 'Displays False strword = "Letter to Jordan" strpattern = "Letter to *" strword = "Case 9876" strpattern = "Case 987#" strword = "Case 9876" strpattern = "Case ##6#" lblmessage.caption = strword Like strpattern 'Displays False strword = "c" strpattern = "[a,b,c,d,e]" strword = "b" strpattern = "[a-f]"

String Functions on Excel Macros

String Functions on Excel Macros The word "string" is used to described the combination of one or more characters in an orderly manner. In excel vba, variables can be declared as String or the Variant