guys, can u help me make a lexical analyzer that outputs invalid/valid only?
it would really help if you guys help me with this. this is my midterm project. thanks!!
techie2008 0 Newbie Poster
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Jump to PostProbably. Read the sticky posts...
Jump to PostThat would depend on the complexity of the "language" you were attempting to parse?
A C expression would probably be do-able.
A C program would not.
Jump to PostGiven an input of say
10*(x + 2)Something which prints
Found "10"
Found "*"
Found "("
Found "x"
Found "+"
Found "2"
Found ")"
would be a useful thing to accomplish.When that works reliably, then think about adding handling for parentheses and operator precedence.
We're here …
All 9 Replies
WaltP 2,905 Posting Sage w/ dash of thyme Team Colleague
Probably. Read the sticky posts...
Salem 5,265 Posting Sage
That would depend on the complexity of the "language" you were attempting to parse?
A C expression would probably be do-able.
A C program would not.
techie2008 0 Newbie Poster
yes! it could be a C expression.. thanks in advance..
ArkM 1,090 Postaholic
Well, let's remember C language (it's almost so called expression language) lexical grammar...
Here it is expression part:
C language expression tokens::
Identifiers:: all but sizeof
<letter|_>[<letter>|_|<digit>]...
----------------------------------Operators and punctuators:: one of
[ ] ( )
:
? .
+ - * / % ^ & | ~
! = < > += -= *= /= %=
^= &= |= << >> >>= <<= == !=
<= >= && || ++ -- , ->
sizeof
-----------------Literals:: one of
integer-literal
character-literal
floating-literal
string-literal
-----------------Whitespaces:: one of
blanks
horizontal tab
vertical tab
newlines
formfeed
comment
-------
Add some punctuators (~5-6) and recognize some identifiers as keywords - and you have full C language scanner...
techie2008 0 Newbie Poster
ahhhmmm..... would it be possible if you give me the source code?? i really can't get it.. i'm really stupid when it comes to this subject
ArkM 1,090 Postaholic
This very minute? It's impossible. The scanner for C expressions is a program with ~1000 lines of source code (of course, it's a rough estimate). Probably, it's possible to write more compact (and fast) scanner with special methods (based on automata theory, for example).
Better start from a simple grammar. For example, consider simplest arithmetical expressions:
Tokens:: one of
identifier
decimal-integer-literal
Operators:: one of
+ - / *
Punctuators:: one of
( )
Try to implement a scanner for this lexical grammar.
Input: source text string (or char array)
Output: next token code
Salem 5,265 Posting Sage
Given an input of say 10*(x + 2) Something which prints
Found "10"
Found "*"
Found "("
Found "x"
Found "+"
Found "2"
Found ")"
would be a useful thing to accomplish.
When that works reliably, then think about adding handling for parentheses and operator precedence.
We're here to "help", not "give".
ArkM 1,090 Postaholic
Follow Salem's advice!
I have wrote a scanner for simple C expressions (~300 LOCs) but I will post the source (may be ;)) only after you will try to do it yourself.
What's a bliss to write parsers for automat grammatics!..
Some remark: fortunately, a scanner does not know operator precedence (let syntax parser knows).
techie2008 0 Newbie Poster
thanks for your help guys.. i have gotten through the basics, a friend helped me with it...
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