String Comparisons
Just like any other variable type, String variables can be used within comparisons such as If- Then, Repeat-Until, and While-Wend . In fact, it's an essential element of any programming language.
Equal (=) or Not Equal (<>) comparisons are the only type that apply to Strings, because one String can only ever be equal or not equal to another String. It would be unusual (unless your using the C language) to compare if one String was greater or less than another.
So a valid comparison could look something like the lines of code below: -
If String1 = String2 Then
HrsoutLn "Equal"
Else
HrsoutLn "Not Equal"
EndIfBut as you've found out if you read the Creating Strings section, there is more than one type of String in a PIC24® and dsPIC33® microcontroller. There is a RAM String variable, a code memory string, and a quoted character string.
Note that pointers to String variables are not allowed in comparisons, and a syntax error will be produced if attempted.
Starting with the simplest of string comparisons, where one string variable is compared to another string variable. The line of code would look similar to either of the two lines above.
Example 1
' Simple string variable comparison
Device = 24FJ64GA002
' Select the device to compile for
Declare Xtal = 16
' Tell the compiler the device will be running at 16MHzDeclare Hserial1_Baud = 9600
Declare Hrsout1_Pin = PORTB.14Dim MyString1 as String * 20
' Create a String of 20 characters
Dim MyString2 as String * 20
' Create another String
PPS_Output(cOut_Pin_RP14, cOut_Fn_U1TX) ' Make Pin RB14 U1TX
MyString1 = "EGGS"
' Pre-load String MyString1 with the text EGGS
MyString2 = "BACON"
' Load String MyString2 with the text BACON
If MyString1 = MyString2 Then ' Is MyString1 equal to MyString2?
HrsoutLn "Equal"
' Yes. So display Equal on a serial terminal
Else
' Otherwise
HrsoutLn "Not Equal"
' Display Not Equal on a serial terminal
EndIf
MyString2 = "EGGS"
' Now make the strings the same as each other
If MyString1 = MyString2 Then ' Is MyString1 equal to MyString2?
HrsoutLn "Equal"
' Yes. So display Equal on a serial terminal
Else
' Otherwise
HrsoutLn "Not Equal"
' Display Not Equal on a serial terminal
EndIfThe example above will display "Not Equal" on the serial terminal because MyString1 contains the text "EGGS" while MyString2 contains the text "BACON", so they are clearly not equal. The line underneath will show "Equal" because MyString2 is then loaded with the text "EGGS" which is the same as MyString1, therefore the comparison is equal.
A similar example to the previous one uses a quoted character string instead of one of the String variables.
Example 2
' String variable to Quoted character string comparison
Device = 24FJ64GA002
' Select the device to compile for
Declare Xtal = 16
' Tell the compiler the device will be running at 16MHzDeclare Hserial1_Baud = 9600
Declare Hrsout1_Pin = PORTB.14
Dim MyString as String * 20
' Create a String of 20 characters
PPS_Output(cOut_Pin_RP14, cOut_Fn_U1TX) ' Make Pin RB14 U1TX
MyString = "EGGS"
' Pre-load String MyString1 with the text EGGS
If MyString = "BACON" Then
' Is MyString equal to "BACON"?
HrsoutLn "Equal"
' Yes. So display Equal on a serial terminal
Else
' Otherwise…
HrsoutLn "Not Equal"
' Display Not Equal on a serial terminal
EndIf
If MyString = "EGGS" Then
' Is MyString equal to "EGGS"?
HrsoutLn "Equal"
' Yes. So display Equal on a serial terminal
Else
' Otherwise…
HrsoutLn "Not Equal"
' Display Not Equal on a serial terminal
EndIfThe example above produces exactly the same results as example1 because the first comparison is clearly not equal, while the second comparison is equal.
Example 3
' Use a string comparison in a Repeat-Until loop
Device = 24FJ64GA002
' Select the device to compile for
Declare Xtal = 16
' Tell the compiler the device will be running at 16MHz
Declare Hserial1_Baud = 9600
Declare Hrsout1_Pin = PORTB.14
Dim SourceString as String * 20 ' Create a String
Dim DestString as String * 20
' Create another String
Dim Charpos as Byte
' Character position within the strings
PPS_Output(cOut_Pin_RP14, cOut_Fn_U1TX) ' Make Pin RB14 U1TX
Clear DestString
' Fill DestString with nulls
SourceString = "Hello"
' Load String SourceString with the text Hello
Repeat
' Create a loop
'
' Copy SourceString into DestString one character at a time
'
DestString[Charpos] = SourceString[Charpos]
Inc Charpos
' Move to the next character in the strings
'
' Stop when DestString is equal to the text "Hello"
'
Until DestString = "Hello"
HrsoutLn DestString
' Display DestString on a serial terminalExample 4
' Compare a string variable to a string held in flash memory
Device = 24FJ64GA002
' Select the device to compile for
Declare Xtal = 16
' Tell the compiler the device will be running at 16MHz
Declare Hserial1_Baud = 9600
Declare Hrsout1_Pin = PORTB.14Dim MyString as String * 20
' Create a String of 20 characters
Dim CodeString as Code = "EGGS", 0 ' Create a flash memory string
PPS_Output(cOut_Pin_RP14, cOut_Fn_U1TX) ' Make Pin RB14 U1TX
MyString = "BACON"
' Pre-load String MyString with the text BACON
If CodeString= "BACON" Then
' Is CodeString equal to "BACON" ?
HrsoutLn " Equal "
' Yes. So display equal on a serial terminal D
Else
' Otherwise…
HrsoutLn "Not Equal"
' Display not equal on a serial terminal
EndIf
MyString = "EGGS"
' Pre-load String MyString with the text EGGS
If MyString = CodeString Then
' Is MyString equal to CodeString?
HrsoutLn " Equal "
' Yes. So display equal on a serial terminal
Else
' Otherwise…
HrsoutLn "Not Equal "
' Display not equal on a serial terminal
EndIfExample 5
' String comparisons using Select-Case
Device = 24FJ64GA002
' Select the device to compile for
Declare Xtal = 16
' Tell the compiler the device will be running at 16MHz
Declare Hserial1_Baud = 9600
Declare Hrsout1_Pin = PORTB.14Dim MyString as String * 20
' Create a String for 20 characters
PPS_Output(cOut_Pin_RP14, cOut_Fn_U1TX) ' Make Pin RB14 U1TX
MyString = "EGGS"
' Pre-load String MyString with the text EGGS
Select MyString
' Start comparing the string
Case "EGGS"
' Is MyString equal to EGGS?
HrsoutLn "Found EGGS"
Case "BACON"
' Is MyString equal to BACON?
HrsoutLn "Found BACON"
Case "COFFEE"
' Is MyString equal to COFFEE?
HrsoutLn "Found COFFEE"
Case Else
' Otherwise…
HrsoutLn "No Match"
' Displaying no match
EndSelect