best way to parsing this line

[mickey]

One more question please: If you see my first post, I have "char* delim" as parameter; so I can read file that contains ',' as separator and other separator at choice. But I can choose as delim ' '; in that case inside your switch (that won't compile) what'll happen? In general ' ' and '\t' can be in the file because the file constructor (the user) can be a bit distracted and insert space at choice (and my program will accept if even if not full correct); but how can I do if user decide (and it can) choose ' ' as delimiters (eg. 200 3 4 5 3 instead of 200,3,4,5,3) ? (i have no idea how to treat this case).

Thanks in advance.

[wysota]

But I can choose as delim ' '; in that case inside your switch (that won't compile) what'll happen?

If you want tokens to be configurable either don't use switch and use the equality operator instead or in each case check if a particular symbol is a token you expect depending on the configuration.

In general ' ' and '\t' can be in the file because the file constructor (the user) can be a bit distracted and insert space at choice (and my program will accept if even if not full correct);

The lexer ignores or spaces and tabs - it treats the [\t ]+ regular expression as a token separator (just like for example C syntax does). \n should probably also be in the group but I guess it depends on your needs.

but how can I do if user decide (and it can) choose ' ' as delimiters (eg. 200 3 4 5 3 instead of 200,3,4,5,3) ? (i have no idea how to treat this case).

See above.

BTW. Have you seen bison/flex and QLALR?

[mickey]

Hello,
I don't understand if I don't understand you or viceversa (I refere to the case of ' ' as delimiter); However, I modified your initial switch to cope this case; is it right or there is a clear way to do this?; (the problem with previous switch and delimiter ' ' was that one time lexer see a blank it return immediately Separator as token (but I could not be right because afte,r it could find other blank or '\n' or '\t'; maybe the code explain better what I mean))....

switch(ch){
				 case ' ' : case '\t': 
					 if (delim == ' ') {
						 switch (fileStream.peek()) {
						 case delim: break;
						 case '\n': break;
						 case '\t': break;
						 default: return Separator;
						 };
					 }
 
					 else  break; // white space - ignore
				 case '\n': line++; return EndOfLine;
				case ',' : 
					 if (delim == ',') 
						 return Separator; 
					 else return Error;
				 case  -1 : return EndOfStream;
				 default  : return Error;
		  };

switch(ch){
				 case ' ' : case '\t': 
					 if (delim == ' ') {
						 switch (fileStream.peek()) {
						 case delim: break;
						 case '\n': break;
						 case '\t': break;
						 default: return Separator;
						 };
					 }
					
					 else  break; // white space - ignore
				 case '\n': line++; return EndOfLine;
				case ',' : 
					 if (delim == ',') 
						 return Separator; 
					 else return Error;
				 case  -1 : return EndOfStream;
				 default  : return Error;
		  };

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Second:
enum State { NumberOrEnd, Number };
State token = Number;
cout << token; // I'd like print the string "Number" and not '1'; is it possibile?

[wysota]

This case code of yours doesn't look good... If you want a whitespace as a separator simply change your grammar, you don't have to change the lexer except maybe removing the part that handles commas. You don't need to report a separator, just return numbers.

Here is EBNF for comma separated lines

CommaSeparatedLine ::== Number, { Separator, Number } ;

CommaSeparatedLine ::== Number, { Separator, Number } ;

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And here is one for space separated lines:

SpaceSeparatedLine ::== Number, { Number } ;

SpaceSeparatedLine ::== Number, { Number } ;

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In the second case you simply don't handle commas. As the lexer returns the value of Number and not single digits it should be easy.

Oh, and about your question about printing enum names. It's possible in Qt when you register an enum in a metaobject using Q_ENUMS.

[mickey]

my parser have to accept files that contain as separator (only) "delim"; delim is a console parameter; so at times it could be ','; at other times could be blank; if user doesn't specify anything from console, by default the (only) accepted separator will be ','

[wysota]

How about you just use strchr() and atof() and forget about the parser?

[mickey]

At the moment it enjoy me. What do u think about this mixed if-switch (is it accetable?Or is it better a pure if-chains?). thanks.

if (token >=0) { //in this case token could be more digits than one
   ..............
}
else {
  switch (token) {
     case Special
     case EndOfLine:
     case EndOfStream:
     case ........
  };
}

if (token >=0) { //in this case token could be more digits than one
   ..............
}
else {
  switch (token) {
     case Special
     case EndOfLine:
     case EndOfStream:
     case ........
  };
}

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[wysota]

Switch and if are equivalent, so that doesn't really matter...

[mickey]

? But with if in the worst case if have to do 'n' compare where 'n' is the number of if in chains. switch is implemented in a totally different way; with this last I have to do only one compare. Isn't it? Should be quicker......

[wysota]

switch is implemented in a totally different way; with this last I have to do only one compare. Isn't it?

Not really. There is no machine instruction that would allow to make an arbitrary number of comparisons. Compile to assembly (gcc -S) and see for yourself.

[mickey]

Hello,
I'm still using the parser and assuming it a good way, I have this question: I have in many part of my program to know how many lines have the file and how many elements have the lines (10, 20, 20 40 -> this line has 4 elements). My program has to check if all line have the same number of element, so I thought to do this check inside the parser; if they haven't the same number of element the parse generates Error; and I thought to count the lines of file in the parser too. All of these soil the parser with additional if and variables; isn't better put the counting of lines out parser (in the constructor class for examples). I don't like the parser become so large.....any suggests, please?

[wysota]

Let me put it this way - the parser is something that deals with grammar (syntax), not logic (semantics). I suggest you parse your file into some kind of list or tree or whatever else you want and then perform semantic checks or actions on the resulting structure. Of course you can combine the two operations to save time when you can deduce while parsing that the result will be useless (like when the number of items in each line differs). If you wrap that all in a class with different methods performing different tasks, the code should be simple and easily readable.