Mercurial > ~dholland > hg > ag > index.cgi
view doc/misc/html/examples/fc.html @ 7:57b2cc9b87f7
Use memcpy instead of strncpy when we know the length anyway.
Modern gcc seems to think it knows how to detect misuse of strncpy,
but it's wrong (in fact: very, very wrong) and the path of least
resistance is to not try to fight with it.
| author | David A. Holland |
|---|---|
| date | Mon, 30 May 2022 23:47:52 -0400 |
| parents | 13d2b8934445 |
| children |
line wrap: on
line source
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> <HTML> <HEAD> <TITLE> Fahrenheit-Celsius Converter</TITLE> </HEAD> <BODY BGCOLOR="#ffffff" BACKGROUND="tilbl6h.gif" TEXT="#000000" LINK="#0033CC" VLINK="#CC0033" ALINK="#CC0099"> <P> <IMG ALIGN="right" SRC="../images/agrsl6c.gif" ALT="AnaGram" WIDTH=124 HEIGHT=30 > <BR CLEAR="all"> Back to <A HREF="../index.html">Index</A> <P> <IMG ALIGN="bottom" SRC="../images/rbline6j.gif" ALT="----------------------" WIDTH=1010 HEIGHT=2 > <P> <H1> Fahrenheit-Celsius Converter</H1> <IMG ALIGN="bottom" SRC="../images/rbline6j.gif" ALT="----------------------" WIDTH=1010 HEIGHT=2 > <P> <H2>Introduction</H2> <P> Conversion of temperatures from Fahrenheit to Celsius is a traditional starting point for learning how to use a programming language. This directory contains a graded sequence of Fahrenheit to Celsius conversion programs, starting with a very simple case and working up to one of some complexity. This sequence of programs illustrates an important aspect of syntax directed programming: In contrast to conventional programming methods it is quite easy to begin with a simple case and then extend it to more complex situations. <P> All of these programs accept input from <tt>stdin</tt> and write output to <tt>stdout</tt>. These programs are somewhat exceptional, since, except for FC5, they do not have any embedded C and therefore do not require explicit definition of a main program. <P> <tt>fc1</tt> is the first and simplest of the Fahrenheit to Celsius conversion programs. It expects the user to type a positive integer value, assumed to be a Fahrenheit temperature, which it converts to Celsius. It then exits. <P> <tt>fc2</tt>, the next example, is a somewhat more interesting Fahrenheit-Celsius converter. This time the input stream may contain any number of temperatures, either Fahrenheit or Celsius, each terminated by a newline character. The program will continue until it encounters an end of file. If it encounters a syntax error in the input, it will skip to the next newline character and continue. <tt>fc2</tt> has been set up to illustrate the usage of the File Trace feature of AnaGram. <P> <tt>fc3</tt> adds two new features to <tt>fc2</tt>: It uses floating point arithmetic, so that it can deal with non-integral values, and it allows optional white space in the input, except within numbers. In addition, it changes the output format, so that results are printed in degrees Kelvin, as well as in Fahrenheit and Celsius. <font size=-1>(Yes, we know that in Newspeak the official usage is "Kelvins", not "degrees Kelvin". Shush.)</font> <P> <tt>fc4</tt> illustrates a shift-reduce conflict which arose when modifying the <tt>fc3</tt> grammar to allow input in degrees Kelvin. You should probably skip this example until you encounter a shift-reduce conflict in one of your own grammars. <tt>fc4a</tt> and <tt>fc4b</tt> are two different resolutions of the conflict. <P> <tt>fc5</tt> illustrates the use of an event driven parser. The actual grammar is the same as <tt>fc4b</tt>. The only difference is in the method of providing input to the parser. <P> <H2>FC1</H2> <tt>fc1</tt> is the first and simplest of the Fahrenheit to Celsius conversion programs. It expects the user to type an integer value, assumed to be a Fahrenheit temperature, which it converts to Celsius. It then exits. <P> The following features of AnaGram are introduced in <tt>fc1</tt>: <UL> <LI> recursive definition of tokens </LI> <LI> definition of a set as a range of characters</LI> <LI> token type declaration </LI> <LI> default token type </LI> <LI> passing token values to reduction procedures </LI> <LI> long and short form reduction procedures </LI> </UL> <P> <tt>fc1</tt> defines two nonterminal tokens, "grammar", which describes the entire input stream, and "integer", which describes a simple unsigned integer value. "grammar", defined by one production, describes the input as consisting of an "integer" followed by a newline character. There is a following reduction procedure to print out both the input and converted values. <P> "integer" is recursively defined by two productions. The first production says that an "integer" may be represented by a single decimal digit. '0-9' represents the set of ascii characters on the range '0' through '9'. The token can be matched by any character from this set. The second production contains the recursion. It says that the combination of any "integer" followed by another decimal digit is also an integer. Note that the left side is the same for these two productions and it need not be repeated. <P> Note the type cast preceding "integer". This type cast defines the data type of the semantic value of "integer" to be int. When the parser stores a token value for "integer" on its value stack or retrieves a value from the stack, the type of the data transmitted will be int. <P> Since there is no type cast for the token "grammar", the data type for the semantic value of "grammar" is given by the "default token type" configuration parameter, which defaults to void. <P> The semantic values of the tokens in a grammar rule may be passed to the associated reduction procedure. The name of the variable in the reduction procedure is simply appended to the token name or expression in the rule with a colon as a separator. <P> All three reduction procedures in <tt>fc1</tt> operate on the semantic values of tokens on the parser stack as parameters. In the first reduction procedure, the variable <tt>f</tt> represents the value of the integer typed by the user. It is taken as a Fahrenheit temperature and converted to Celsius by the reduction procedure. This reduction procedure uses the long form, consisting of an equal sign followed by a block of C code. <P> The reduction procedures for the two productions for "integer" convert the integer from ascii form as typed by the user to binary form. The first reduction procedure calculates the value of a single digit integer. The second reduction procedure calculates the value for an integer with more than one digit. Notice that these reduction procedures both use the short form: a C expression terminated by a semicolon. The value of the expression is saved as the semantic value of the reduced token. <P> <H3>Testing FC1</H3> Run AnaGram and build a parser, <tt>fc1.c</tt>. Compile it and link it with your C compiler. Run <tt>fc1</tt> from the command line. Type an integer and press Enter. <tt>fc1</tt> will print out Fahrenheit and Celsius temperatures. <P> <H2>FC2</H2> The next example of a syntax file, <tt>fc2.syn</tt>, is a somewhat more interesting Fahrenheit-Celsius converter. This time the input stream may contain any number of temperatures. The program will continue until it encounters an end of file. If it encounters a syntax error in the input, it will skip to the next newline character and continue. <tt>fc2</tt> has been set up to illustrate the File Trace feature of AnaGram. <P> The following features of AnaGram are introduced in <tt>fc2</tt>: <UL> <LI> configuration section </LI> <LI> character set defined by set union </LI> <LI> end of file token </LI> <LI> virtual productions </LI> <LI> use of '?' to define a virtual production </LI> <LI> error token resynchronization </LI> <LI> File Trace </LI> </UL> <P> <tt>fc2</tt> allows the temperature values to be signed integers which may be either Fahrenheit or Celsius, as determined by a following 'f' or 'c'. Each temperature value to be converted must be followed by a newline. Spaces in the input are not allowed. <P> To facilitate testing, a configuration section has been added at the beginning of the file to set two configuration parameters, discussed below. The lines specifying the parameters have been labeled C1 and C2 at the right margin to make them easy to refer to in this documentation. Similarly, productions have been labeled P1, P2, etc. <P> After the configuration section, an end of file token, eof, is defined. Remember that when using stream I/O, the end of file is signalled by a -1. <P> The first production, P1, describes the entire input file as an optional sequence of temperatures followed by an end of file. <P> The expression <CODE>[temperature, '\n']...</CODE> is a "virtual production". The square brackets indicate the rule inside the brackets is optional. The ellipsis (<CODE>...</CODE>) indicates that the rule may be repeated an arbitrary number of times. <P> Productions P2 and P3 define "temperature" in the normal case. Production P4 controls error recovery, described below.In AnaGram, <CODE>'f' + 'F'</CODE> represents the set of characters containing both upper and lower case 'f'. (The plus sign is the union operator of set theory.) Either an upper or lower case 'f' in the input will match this set. <CODE>'c' + 'C'</CODE> is similarly interpreted. Thus a temperature consists of a number followed by an 'f' or 'c' which can be either upper or lower case. The reduction procedures, of course, are different for 'f' and 'c'. <P> Productions P5 and P6 define "number" as consisting of an integer with an optional sign. <CODE>'+'?</CODE> is a virtual production. The question mark indicates that the preceding element, in this case the plus sign, is optional. These productions allow you to write numbers in the form 17, +17, or -17. <P> Productions P7 and P8 define "integer" exactly as in <tt>fc1</tt>. <P> <H3> Recovering and Continuing after a Syntax Error </H3> The production P4 controls error recovery. "error" is a special token in an AnaGram grammar, which can be matched by any sequence of input which contains a syntax error. If your grammar has an error token, when your parser encounters a syntax error it looks to see if there is an error token to match with the syntax error. If "error" is not admissible in the current state, it discards the previous token on the input stack and looks again. It continues until it gets back to a state where "error" is acceptable input or the stack is empty. If the stack is empty, it terminates the parse. Otherwise, it then looks to see if the next input token is admissible. If so, the parse continues. If not, the token is discarded and the parser reads input until it finds an acceptable token or the end of file. In this example, the parser will read characters until it finds a newline character At this point the parse will continue as though nothing had happened. This process is called "error token resynchronization". It is one of several ways to continue after a parser detects a syntax error in its input stream. <P> <H3> Configuration Parameters </H3> Two configuration parameters have been set in the configuration section of <tt>fc2</tt> to facilitate testing using the File Trace. The first, test file mask, limits the choice of test files to be used with the File Trace option to files with the extension <tt>.fc2</tt>. The second, traditional engine, turns off certain optimizations AnaGram normally builds into its parsers. When the traditional engine switch is set, the parsers AnaGram builds use only the four traditional parser actions: shift, reduce, error, and accept. Otherwise, AnaGram parsers use a number of compound actions in order to reduce the size of the parsing tables and increase the speed of the parser. In this case, the traditional engine switch has been turned on in order to make the behavior of the parser as seen with the File Trace correspond to textbook behavior. <P> <H3> Testing FC2 </H3> Test <tt>fc2</tt> just as you did <tt>fc1</tt>: Run AnaGram and build a parser, <tt>fc2.c</tt>. Compile it and link it with your C compiler. Run <tt>fc2</tt> from the command line. Type an integer, with or without a sign, the letter 'f' or 'c', and press Enter. <tt>fc2</tt> will print out Fahrenheit and Celsius temperatures. Repeat until you are satisfied the program works. Try making a few deliberate typos to test the error token resynchronization. Note that the parser automatically provides error diagnostics. These diagnostics are created by the default <tt>SYNTAX_ERROR</tt> macro. <P> Type ^Z and Enter (Windows) or ^D (Unix) to generate an end of file and end the program. (Of course, you could also use ^C or ^Brk.) <P> Alternatively, you may use the test file, <tt>test.fc2</tt>, which has been provided for use with the File Trace (see below). Simply run <tt>fc2</tt> with input redirection to take input from <tt>test.fc2</tt>. At the command prompt, type: <PRE> fc2 < test.fc2 </PRE> <P> <H2> File Trace </H2> The File Trace feature of AnaGram allows you to test a grammar without actually building a parser. This enables you to completely decouple the debugging of the grammar from the debugging of the reduction procedures. You can try out test files before you have written anything more than the grammar. This allows for very early testing in your projects. <P> File Trace allows you to see in fine detail just exactly how your parser will analyze an input file. A File Trace consists of a window with various panes so you can see what is going on, and an interpretive parser which works in the background. <P> You can select File Trace from the Action menu once you have analyzed your grammar. For the <tt>fc2</tt> example, you will be offered a choice of test files with extension <tt>.fc2</tt>. A good choice would be <tt>test1.fc2</tt>. The File Trace window will show a Parser Stack pane to your left, a Test File pane, which shows you the input file you are parsing, to your right, and a Rule Stack pane across the bottom of the window. <P> The way the File Trace parser works is this: Initially none of the test file has been parsed. If you double-click with the left mouse button at a point in the Test File pane, the parser parses to that point. The unparsed part of the file will be colored differently from the parsed part (in the default color scheme, parsed characters have a lighter background). To back up the parse to a previous location, double-click at that spot, or single-click and press Enter or the Synch Parse button at the bottom of the File Trace window. To check a file for syntax errors, all you need to do is to click the Parse File button. If there is a syntax error, the parse will not advance beyond the error point. Normally, however, you will probably want to proceed more deliberately, moving the cursor one character at a time. <P> If you wish to see even finer detail, you may make the parser work in single step mode. by clicking on Single Step or pressing Enter. Each time you click on the Single Step button or press Enter, the parser will perform one parser action. Note that in its normal configuration, AnaGram produces parsers that use a number of parser actions more complex than the traditional shift and reduce actions. <P> The Parser Stack pane shows the levels of the parser stack, the state numbers on the stack and the tokens that have been recognized so far. As you advance through the test file, you can see by looking at the Parser Stack pane how the parser stack changes as characters are shifted in and reductions occur. The Rule Stack pane is an alternate view of the parser stack showing the grammar rules in play at any moment. Notice how the syntax file window is synched with the Rule Stack. <P> If the parse position is not located at the blinking cursor, the Single Step button will be changed to read "Synch Parse". Clicking on the button will move the parse position to the cursor. <P> If you now click on tokens at various levels in the Token Stack, the Test File characters corresponding to these tokens will be highlighted. You can restart the parse at any level by double-clicking just preceding the highlight. The Rule Stack is also synched with the Token Stack and Test File panes. <P> You may interrupt the File Trace at any time to inspect any other window without interfering with the File Trace. Whenever you come back to it, you can proceed as though nothing has happened. <P> If you have a long file, a complex grammar, or a (very) slow computer, it can sometimes take a while for the parse to catch up with the cursor. If you have a long test file and press Parse File to move the cursor to the end of the file, the parser has a lot of computation to do to catch up. <P> For further details about the File Trace, please refer to the AnaGram User's Guide and the on-line documentation. <P> <BR> <H2> FC3 </H2> <tt>fc3</tt> adds two new features to <tt>fc2</tt>: It uses floating point arithmetic, so that it can deal with non-integral values, and it allows optional white space, including comments, in the input. In addition, it changes the output format, so that results are printed in degrees Kelvin, as well as in Fahrenheit and Celsius. <P> The following features of AnaGram are introduced in <tt>fc3</tt>: <UL> <LI> Setting the default token type </LI> <LI> The "disregard" statement </LI> <LI> The "lexeme" statement </LI> <LI> Right recursion </LI> <LI> Default value of a reduction token </LI> </UL> <P> <H3> Floating Point Arithmetic </H3> To deal with floating point arithmetic, a number of new productions have been added and two productions have been changed. Productions P5 and P6 have been changed to define a "number" in terms of an "unsigned number" instead of an "integer". Productions P6a, P6b, and P6c define "unsigned number" in terms of its integer part and its fraction part. Productions P9 and P10 define the fraction part of the number. Note that "fraction" is described using right recursion rather than left recursion. This makes the reduction procedures neater. Since reduction does not occur until a rule is complete, note that with right recursion each new digit causes the stack depth to increase by one. You can observe this with the File Trace. <P> In order to replace integer arithmetic with floating point arithmetic, statement C3 was added to the configuration section of the grammar. It declares the default token type, the type assigned to nonterminal tokens absent a specific declaration, to be "double". Since we are not interested in values for "grammar" and "temperature", they have been explicitly cast to "void". <P> Note that production P6a does not have a reduction procedure. If a rule has no reduction procedure the value of the reduction token defaults to the value of the first element in the rule. In this case, the value of "unsigned number", in the absence of a reduction procedure, is taken to be the value of "integer". <P> <H3> Skipping White Space </H3> Two statements, C4 and C5, are used to skip over uninteresting white space in the input to <tt>fc3</tt>. The "disregard" statement instructs AnaGram to rewrite your grammar in a standard way so that your parser will skip over any instance of the "white space" token that occurs between lexemes, or lexical units, in the input to your parser. Of course, you can use any token name you wish in a "disregard" statement. You can even have multiple "disregard" statements and all of the tokens you specify will be disregarded. <P> The "lexeme" statement is used to declare that certain nonterminal tokens are each to be considered as indivisible lexical units, or lexemes, from the point of view of lexical analysis, so that the "disregard" statement is inoperative inside the nonterminal tokens listed. In this case, statement C5 simply guarantees that white space will not be allowed inside a number. All terminal tokens are automatically lexemes. <!-- when not part of a larger lexeme. --> <P> <!-- It would be nice to rewrite this so the example expansion is vaguely close to syntactically legal. --> To make your parser skip white space, AnaGram renames and redefines the lexemes in your grammar and defines a number of new tokens. For example, if '+' is a lexeme in your grammar and your grammar is to disregard "white space", AnaGram will rename the plus sign token as '+'%. It then introduces a new production in your grammar as follows: <PRE> '+' -> '+'%, white space?... </PRE> This means that a plus sign followed by some white space will now be treated the same, syntactically, as a plus sign alone. The percent sign (a degree sign in AnaGram 1.x) is used to indicate the original, or "pure", definition of the token. <P> Productions P11 and P12 together define what is meant by white space in this grammar. P11 defines white space to include blanks and tab characters. P12 includes C style comments (not nested). <P> The white space defined in P11 and P12 does not include newline characters. There is a good reason for this. The grammar uses newline characters as delimiters, marking the end of a "temperature". In order to allow blank lines, production P1 was modified to make the temperature optional. <P> To allow "//" style comments, a new token, "end of line", defined by production P13, was added. In production P1, '\n' was then replaced by "end of line". <P> <H3> Testing FC3 </H3> The "test file mask" was changed to use files with the extension ".fc3" in the File Trace. TEST.FC3 can be used as input for the File Trace. <P> Build and compile <tt>fc3</tt> in the same way as previous versions. When you run it, try using numbers with decimal fractions. Try typing blanks in various places to see how the parser deals with them. Try redirecting input from <tt>test.fc3</tt>. <P> <H2> FC4 </H2> <tt>fc4</tt> illustrates a shift-reduce conflict in a grammar. You should probably skip this example until you encounter a shift-reduce conflict in one of your own grammars. In the meantime, skip ahead to <tt>fc5</tt>. <P> The following features of AnaGram are introduced in <tt>fc4</tt>: <UL> <LI> Conflicts window </LI> <LI> Auxiliary Windows menu </LI> <LI> State Definition window </LI> <LI> Expansion Chain window </LI> </UL> <P> In <tt>fc3</tt>, the output was changed to provide results in degrees Kelvin, as well as in Fahrenheit and Celsius. In <tt>fc4</tt>, a production, P3a, is added to accept input in degrees Kelvin. Since there is no such thing as a negative temperature on the Kelvin scale, it seemed appropriate to require that the temperature be an unsigned number. This, however, caused a conflict, i.e. an ambiguity, in <tt>fc4</tt>. In fact, it caused four conflicts to be diagnosed, all of which have a common source. <P> <H3> Finding a Conflict </H3> If you run AnaGram and analyze <tt>fc4</tt> you will find that AnaGram finds conflicts in the grammar. To determine the nature of the conflicts, you should first open the Conflicts window. The Conflicts window is available via the Browse Menu or a Control Panel button. <P> The first thing you see is that there are conflicts in states S005 and S025. In each state, one conflict occurs because a decimal point can either be shifted, in accordance with rule R021, or it can reduce rule R014, an empty rule, or null production. The other conflict occurs because a decimal digit can either be shifted, in accordance with rule R022, or it can reduce rule R014, the same rule that gives trouble with the decimal point. <P> The first step in understanding the conflict is to see rules R014, R021, and R022 in context. The Conflicts window is synchronized with the syntax file window. Arrange these windows so you can see them both at once. Then, in the Conflicts window, move the cursor bar up and down. In the syntax file window, the cursor will move between productions for number, unsigned number and integer. <P> Although it is possible to recognize rules R021 and R022 in the grammar, there is no explicit null production in the grammar. To find out for sure what rule R014 is, pop up the Rule Table (listed in the Browse menu) and look for R014. It turns out that R014 is the null production that corresponds to an optional plus sign, written '+'? in the grammar. <P> To get a better idea of what is going on, it is worthwhile to find out what the parser is expecting to see in states S005 or S025. To find out, click the right mouse button in the Conflicts window on any line describing a state S005 conflict to pop up the Auxiliary Windows menu. Select State Definition to find out what state S005 is all about. It seems that state S005 occurs when the parser has skipped over the initial white space and is about to begin dealing with the actual input. But, looking at this, it is still not clear why a decimal digit or a decimal point is ambiguous. The Expansion Chain windows for rules R021, R022 and R014 will show how these rules are derived from the characteristic rule for state S005. <P> Return to the Conflicts window. Click the right mouse button on rule R021, the rule that expects the decimal digit, to pop up the Auxiliary Windows menu. Select Expansion Chain. The Expansion Chain window shows how rule R021 derives from the characteristic rule for the state. Each line in this window is a grammar rule produced by the marked token in the rule on the previous line. <P> Now return to the Conflicts window, and get the Expansion Chain window for rule R014. Rearrange the windows on the screen so you can compare the Expansion Chain windows for rules R014 and R021. Note that rule R014 derives from the production <PRE> temperature -> number, 'c' + 'C' </PRE> and rule R021 derives from the production <PRE> temperature -> unsigned number, 'k' + 'K' </PRE> <P> It is now possible to see the nature of the conflict. On the one hand, if the input is a supposed to be a Kelvin temperature, the parser can go right ahead accumulating a number. On the other hand, if the input is supposed to be a Celsius number, the parser has to first acknowledge the optional plus sign. <P> It is the nature of LALR parsers that they can keep track of many threads of possible parses simultaneously as long as they don't have to do any reductions. When they come to the end of a rule, however, they are forced to decide whether the rule has been successfully matched. In this case, the parser is at the end of a null production which arises from the virtual production <CODE> '+'? </CODE> in P6 and is forced to decide whether this null production has been matched. The conflict diagnostics discussed above say that if the next token is a digit or a decimal point, the parser cannot decide between several possibilities. That is, it cannot decide whether or not it has seen an elided plus sign. In effect the parser is being required, because of the null production, to make a premature decision as to whether a Kelvin or Celsius temperature is present in the input. <P> The conflicts can be eliminated by rewriting the grammar so the parser will not come to the end of a rule and be forced to choose among the several threads of the parse until it encounters the determining letter 'f', 'c', or 'k'. <P> Two ways to remove the conflict are illustrated. The first is found in FC4A. In this grammar, production P6 has been replaced with two productions, P6x and P6y. This is a standard method of rewriting a grammar to eliminate null productions. If you analyze <tt>fc4a</tt>, you will find that it no longer has a conflict, so this solves the problem. <P> However, if you look at the <tt>fc4a</tt> grammar closely, you will notice that +23.7K is not acceptable input, although common sense suggests that you ought to be able to use a plus sign on Kelvin temperatures. <tt>fc4b</tt> shows another way to fix the grammar which deals with this quibble. In this grammar, production P3a has been modified to allow an optional plus sign before a Kelvin temperature. If you analyze <tt>fc4b</tt>, you will find that this change also solves the problem. <P> In both these instances, the technique used to resolve the conflicts was to rewrite the grammar so that there are no differences between constructs upstream of the point where they diverge. Another way to put it is this: In the original grammar Kelvin temperatures were distinguished from Fahrenheit and Celsius not only by the 'K' suffix, but also by the optional plus sign. The essence of both fixes to the problem is to remove this distinction. <P> Other AnaGram windows available from the Auxiliary Windows popup menu for the Conflicts window such as Rule Derivation, Token Derivation, Conflict Trace and Problem States are helpful in tracking down conflicts. See the help messages and your AnaGram User's Guide for further details. <P> <H3>Testing FC4A and FC4B </H3> Build and compile <tt>fc4a</tt> and <tt>fc4b</tt> in the same way you built previous versions. When you run the parsers, try using Kelvin temperatures with and without leading plus signs. <P> The "test file mask" was changed to use files with the extension ".fc4" in the File Trace. <tt>test.fc4</tt> can be used as input either for the parsers themselves or for the File Trace. <P> <H2> FC5 </H2> <tt>fc5</tt> illustrates the use of an event driven parser. The grammar is essentially the same as for <tt>fc4b</tt>. The primary difference is in the method of providing input to the parser. The following features of AnaGram are introduced in <tt>fc5</tt>: <UL> <LI> event driven parser </LI> <LI> embedded C </LI> <LI> main program </LI> <LI> initializing and calling the parser </LI> </UL> Statement C6 in the configuration section causes AnaGram to build an event driven parser. An event driven parser is first explicitly initialized and then called once for each input unit. <P> A small main program has been included in a block of embedded C at the end of the file. This program calls the initializer and then reads characters from <CODE>stdin</CODE> and passes them on to the parser. Previous <tt>fc</tt> programs did not include a main program, but relied on AnaGram to create one. However, AnaGram does not automatically create a main program for parsers which are event driven, use pointer input, or have any embedded C. Therefore a main program is necessary in this syntax file. <P> Note that the default names for the initializer and parser are <CODE>init_fc5</CODE> and <CODE>fc5</CODE> respectively. Only event driven parsers require that the user explicitly call the initializer function. <P> In addition, a global constant, "zero", was defined in the embedded C to provide the value of absolute zero, and the reduction procedures were modified to refer to "zero" instead of the explicit value. <P> This illustrates an important point about the C parser file that AnaGram builds: All blocks of embedded C precede the reduction procedures, so that the reduction procedures can access all variables and definitions included in embedded C, no matter where they are located in the file. <P> <H3>Testing FC5 </H3> Since <tt>fc5</tt> uses the same grammar as <tt>fc4</tt>, you can use the same test files for <tt>fc5</tt> as for <tt>fc4</tt>. </P> <BR> <IMG ALIGN="bottom" SRC="../images/rbline6j.gif" ALT="----------------------" WIDTH=1010 HEIGHT=2 > <P> <IMG ALIGN="right" SRC="../images/pslrb6d.gif" ALT="Parsifal Software" WIDTH=181 HEIGHT=25> <BR CLEAR="right"> <P> Back to <A HREF="../index.html">Index</A> <P> <ADDRESS><FONT SIZE="-1"> AnaGram parser generator - examples<BR> Fahrenheit-Celsius Converter<BR> Copyright © 1993-1999, Parsifal Software. <BR> All Rights Reserved.<BR> </FONT></ADDRESS> </BODY> </HTML>