- Write A Lex Program For Basic Desktop Calculator Using Yacc
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Implementation of Calculator using LEX and YACC. Lex and yacc help you write programs that transform structured input. System software lab(lex and yacc, unix. Title:Write a program to implement calculator using LEX and YACC Also See: System Programming Programs and Assignments Click here to download the LEX and YACC Program Files.
NextPreviousContentsC parser lex yacc free download. SCons SCons is a software construction tool that is a superior alternative to the classic 'Make' build too. The libraries are written in C and currently use a language (lex/yacc). Lex, yacc, assembly, BASIC, LOGO etc. In Hindi, Bangla, Gujrati, Assamese, Nepali and other Indian languages. Downloads: 3 This. Pre-Configured EditPlus IDE: The package also contains EditPlus IDE which contains pre-defined Blank templates for the Lex/Yacc/C/C++/Java Files, thus each time you want to type a program you can simply use the New Lex / New Yacc template, and the basic code will be inserted thus saving your time and efforts to type:P.
3. Lex
The program Lex generates a so called `Lexer'. This is a function that takesa stream of characters as its input, and whenever it sees a group ofcharacters that match a key, takes a certain action. A very simple example:
The first section, in between the %{ and %} pair is included directly in theoutput program. We need this, because we use printf later on, which isdefined in stdio.h.
Sections are separated using '%%', so the first line of the second sectionstarts with the 'stop' key. Whenever the 'stop' key is encountered in theinput, the rest of the line (a printf() call) is executed.
Besides 'stop', we've also defined 'start', which otherwise does mostly thesame.
We terminate the code section with '%%' again.
To compile Example 1, do this:
NOTE: If you are using flex, instead of lex, you may have to change '-ll' to '-lfl' in the compilation scripts. RedHat 6.x and SuSE need this, even whenyou invoke 'flex' as 'lex'!This will generate the file 'example1'. If you run it, it waits for you totype some input. Whenever you type something that is not matched by any ofthe defined keys (ie, 'stop' and 'start') it's output again. If you enter 'stop' it will output 'Stop command received';
Terminate with a EOF (^D).
You may wonder how the program runs, as we didn't define a main() function.This function is defined for you in libl (liblex) which we compiled in withthe -ll command.
3.1 Regular expressions in matches
This example wasn't very useful in itself, and our next one won't be either.It will however show how to use regular expressions in Lex, which aremassively useful later on.
Example 2:
This Lex file describes two kinds of matches (tokens): WORDs and NUMBERs.Regular expressions can be pretty daunting but with only a little work it iseasy to understand them. Let's examine the NUMBER match:
[0123456789]+
This says: a sequence of one or more characters from the group 0123456789.We could also have written it shorter as:
[0-9]+
Now, the WORD match is somewhat more involved:
[a-zA-Z][a-zA-Z0-9]*
The first part matches 1 and only 1 character that is between 'a' and 'z',or between 'A' and 'Z'. In other words, a letter. This initial letter thenneeds to be followed by zero or more characters which are either a letter ora digit. Why use an asterisk here? The '+' signifies 1 or more matches, buta WORD might very well consist of only one character, which we've alreadymatched. So the second part may have zero matches, so we write a '*'.
This way, we've mimicked the behaviour of many programming languages whichdemand that a variable name *must* start with a letter, but can containdigits afterwards. In other words, 'temperature1' is a valid name, but '1temperature' is not.
Try compiling Example 2, lust like Example 1, and feed it some text. Here isa sample session:
You may also be wondering where all this whitespace is coming from in theoutput. The reason is simple: it was in the input, and we don't match on itanywhere, so it gets output again.
The Flex manpage documents its regular expressions in detail. Many peoplefeel that the perl regular expression manpage (perlre) is also very useful,although Flex does not implement everything perl does.
Make sure that you do not create zero length matches like '[0-9]*' - yourlexer might get confused and start matching empty strings repeatedly.
3.2 A more complicated example for a C like syntax
Let's say we want to parse a file that looks like this:
We clearly see a number of categories (tokens) in this file:
- WORDs, like 'zone' and 'type'
- FILENAMEs, like '/etc/bind/db.root'
- QUOTEs, like those surrounding the filename
- OBRACEs, {
- EBRACEs, }
- SEMICOLONs, ;
Write A Lex Program For Basic Desktop Calculator Using Yacc
The corresponding Lex file is Example 3:
When we feed our file to the program this Lex file generates (usingexample3.compile), we get:
When compared with the configuration file mentioned above, it is clear thatwe have neatly 'Tokenized' it. Each part of the configuration file has beenmatched, and converted into a token.
And this is exactly what we need to put YACC to good use.
3.3 What we've seen
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We've seen that Lex is able to read arbitrary input, and determine what eachpart of the input is. This is called 'Tokenizing'.
NextPreviousContentsNextPreviousContents3. Lex
The program Lex generates a so called `Lexer'. This is a function that takesa stream of characters as its input, and whenever it sees a group ofcharacters that match a key, takes a certain action. A very simple example:
The first section, in between the %{ and %} pair is included directly in theoutput program. We need this, because we use printf later on, which isdefined in stdio.h.
Sections are separated using '%%', so the first line of the second sectionstarts with the 'stop' key. Whenever the 'stop' key is encountered in theinput, the rest of the line (a printf() call) is executed.
Besides 'stop', we've also defined 'start', which otherwise does mostly thesame.
We terminate the code section with '%%' again.
To compile Example 1, do this:
NOTE: If you are using flex, instead of lex, you may have to change '-ll' to '-lfl' in the compilation scripts. RedHat 6.x and SuSE need this, even whenyou invoke 'flex' as 'lex'!This will generate the file 'example1'. If you run it, it waits for you totype some input. Whenever you type something that is not matched by any ofthe defined keys (ie, 'stop' and 'start') it's output again. If you enter 'stop' it will output 'Stop command received';
Terminate with a EOF (^D).
You may wonder how the program runs, as we didn't define a main() function.This function is defined for you in libl (liblex) which we compiled in withthe -ll command.
3.1 Regular expressions in matches
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This example wasn't very useful in itself, and our next one won't be either.It will however show how to use regular expressions in Lex, which aremassively useful later on.
Example 2:
This Lex file describes two kinds of matches (tokens): WORDs and NUMBERs.Regular expressions can be pretty daunting but with only a little work it iseasy to understand them. Let's examine the NUMBER match:
[0123456789]+
This says: a sequence of one or more characters from the group 0123456789.We could also have written it shorter as:
[0-9]+
Now, the WORD match is somewhat more involved:
[a-zA-Z][a-zA-Z0-9]*
The first part matches 1 and only 1 character that is between 'a' and 'z',or between 'A' and 'Z'. In other words, a letter. This initial letter thenneeds to be followed by zero or more characters which are either a letter ora digit. Why use an asterisk here? The '+' signifies 1 or more matches, buta WORD might very well consist of only one character, which we've alreadymatched. So the second part may have zero matches, so we write a '*'.
This way, we've mimicked the behaviour of many programming languages whichdemand that a variable name *must* start with a letter, but can containdigits afterwards. In other words, 'temperature1' is a valid name, but '1temperature' is not.
Try compiling Example 2, lust like Example 1, and feed it some text. Here isa sample session:
You may also be wondering where all this whitespace is coming from in theoutput. The reason is simple: it was in the input, and we don't match on itanywhere, so it gets output again.
The Flex manpage documents its regular expressions in detail. Many peoplefeel that the perl regular expression manpage (perlre) is also very useful,although Flex does not implement everything perl does.
Make sure that you do not create zero length matches like '[0-9]*' - yourlexer might get confused and start matching empty strings repeatedly.
3.2 A more complicated example for a C like syntax
Let's say we want to parse a file that looks like this:
We clearly see a number of categories (tokens) in this file:
- WORDs, like 'zone' and 'type'
- FILENAMEs, like '/etc/bind/db.root'
- QUOTEs, like those surrounding the filename
- OBRACEs, {
- EBRACEs, }
- SEMICOLONs, ;
The corresponding Lex file is Example 3:
When we feed our file to the program this Lex file generates (usingexample3.compile), we get:
When compared with the configuration file mentioned above, it is clear thatwe have neatly 'Tokenized' it. Each part of the configuration file has beenmatched, and converted into a token.
And this is exactly what we need to put YACC to good use.
3.3 What we've seen
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We've seen that Lex is able to read arbitrary input, and determine what eachpart of the input is. This is called 'Tokenizing'.
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