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A TLex
object may be constructed as an empty object, from another TLex,
or from an existing string as below. The code fragments are taken from an example that implements a simple Reverse Polish
Notation calculation engine.
TInt RPNCalc(const TDesC& aCommand, TReal& aReturnValue)
{
...
TLex input (aCommand) ;
...
Code can then proceed to move through the TLex
data to:
mark positions for rolling back to later
delimit the start of lexical tokens
delete parts of the string held by the TLex
object.
...
input.Mark() ; // Remember where we are.
input.SkipCharacters() ; // Move to end of character token.
...
_LIT(KTextMemset,"MEMSET");
if ( input.TokenLength() != 0 ) // if valid potential token
{
TPtrC token = input.MarkedToken() ; // then extract token
if ( token.CompareF(KTextMemset == 0) // and test.
{
...
}
...
}
...
Analysis can also be done by character using functions that move through the TLex
data, extracting, returning and jumping specified character lengths. For example:
...
// ensure we are looking at a digit or sign
if (!(input.Peek().IsDigit() || (input.Peek() == '.') ) )
{
return KErrNotFound ;
}
...
// deal with sign
if (input.Peek() == '+')
{
input.Inc();
}
Additionally, numeric conversion functions permit a variety of numeric formats to be extracted from the TLex
data, with provision for conversion using the most common number systems (radixes).
...
if (input.Val(extractUint) == KErrNone)
{
stack.Push(TReal(extractUint));
}
else if (input.Val(extractReal) == KErrNone)
{
stack.Push(extractReal);
}
...
where stack
, is an instance of a class implementing a stack.
This converts an real number into a string, which is then assigned
to a TLex
.
TBuf<0x100> convertRealToString;
// want a TLex from a value
if (convertRealToString.Num(value,format) < KErrNone )
{
...
}
else
{
convertRealToString.ZeroTerminate();
TLex string(convertRealToString) ;
}
This takes a descriptor as a function parameter, and copies it to a
TLex
.
TInt RPNCalc(const TDesC& iCommand, TReal& returnValue)
{
TLex input (iCommand) ;
}
This shows a code flow decision made according to next character to be
read from the TLex
:
if (!(input.Peek()).IsDigit()) // found non-digit after decimal point if
This shows the use of the Inc()
function to move past a
character that has been peeked:
if (input.Peek() == '-')
{
input.Inc() ; // move past minus sign & flag
negative = ETrue ;
}
This shows the use of UnGet()
to restore the previously
"got" character.
....
if (input.Offset() > 0) // if not at start of line
{
input.UnGet() ; // restore 'got' character
}
If the previous character is before the start of the string, then the
function raises a USER 59 panic for the TLex8
variant and a USER 64 panic for
the TLex16
variant.
This shows how to allow part of a TLex
can be parsed again:
if (!(input.Peek()).IsDigit())
{
// found non-digit after decimal point. Error, so rewind
input.UnGetToMark(startMark);
}
This parses a TLex
for the next token:
input.Mark() ; // remember where we are
input.SkipCharacters() ; // move to end of character token
if ( input.TokenLength() != 0 ) // if valid potential token
...
This shows how TokenLength()
is used to return the
difference between the position of the next character and the extraction mark.
This gives a check as to whether the token length is valid. An invalid token
length implies an invalid token.
if ( input.TokenLength() != 0 ) // if valid token length
...
This shows how to return the offset of the next character position from the start of the string.
if (input.Offset() > 0) // if not at start of line
{
input.UnGet() ; // restore 'got' character
...
}
This example shows how to return the offset of the next character position from the start of the string.
if (input.Val(extractUint) == KErrNone)
{
stack.Push(TReal(extractUint)) ;
}
else
{
if (input.Val(extractReal) == KErrNone)
{
stack.Push(extractReal) ;
}
}
This extracts an unknown number type. Tries an integer first and then, if this fails, tries a real:
if (input.Val(extractUint) == KErrNone)
stack.Push(TReal(extractUint)) ;
else if (input.Val(extractReal) == KErrNone)
stack.Push(extractReal) ;