interpreter.cpp 6.49 KB
Newer Older
Ronald Charles Moore's avatar
Ronald Charles Moore committed
1
2
3
4
// This code made available to his students by 
// Prof. Ronald Moore  
//     https://fbi.h-da.de/personen/ronald-moore/  
//     mailto:ronald.moore@h-da.de
Ronald Charles Moore's avatar
Ronald Charles Moore committed
5
6
7
8
9
10
11
12
13
14
15
16
17
18
// with no warranties whatsoever!


#include <cassert>
#include <cctype> // for isspace
#include <cstdlib> // for strtod
#include <iostream>
#include <fstream>
#include <string>
// include <vector>

// ===================
// LEXICAL ANALYSIS
// The following are taken to be tokens:
Ronald Charles Moore's avatar
Ronald Charles Moore committed
19
20
21
22
23
24
25
26
27
28
// left and right parenthesis, the plus and minus characters,
// as well as asterisk and forward slash -- and numbers.
// In the script, substraction and division are not supported,
// but it seems like time to add them.

// Preliminaries and Utilities
// ============================

// Utility Types
typedef double numberType; // feel fee to change this to something else like int or float or bigint....
Ronald Charles Moore's avatar
Ronald Charles Moore committed
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
typedef enum Token { 
	tok_number = 'n',
	tok_lparen = '(',
	tok_rparen = ')',
	tok_plus   = '+',
	tok_minus  = '-',
	tok_times  = '*',
	tok_div    = '/',
	tok_eof    = 'E',
	bad_tok    = 'X'
} Token;

// global variables -- sue me if you don't like that!
static std::istream *input = &(std::cin); // until proven otherwise
static std::string	currentLine( "" );
static int	currentLineNumber = -1;
static int	currentColumnNumber = 0;
Ronald Charles Moore's avatar
Ronald Charles Moore committed
46

Ronald Charles Moore's avatar
Ronald Charles Moore committed
47
48
static Token next_token; // again with the global variables... 
static numberType currentNumber; // = zero....
Ronald Charles Moore's avatar
Ronald Charles Moore committed
49

50
51
52
53
54
55
56
57
58
59
60
static void printErrorMsg( const std::string Error )
{
	std::cout << "ERROR on line " << currentLineNumber
	          << ", column " << currentColumnNumber << " : " 
	          << Error << std::endl;
	std::cout << currentLine;
	for ( int col = 0; col < currentColumnNumber-1; col++ )
		std::cout << '-';
	std::cout << '^' << std::endl;
} // end printErrorMsg

Ronald Charles Moore's avatar
Ronald Charles Moore committed
61
62
// The Lexer
// ==========
Ronald Charles Moore's avatar
Ronald Charles Moore committed
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
static bool skippedWhiteSpace( ) { // return true if not at EOF, i.e. if skipped
	while ( true ) {
		int currentLineLength = currentLine.length();
		while ( currentColumnNumber < currentLineLength )
			if ( isspace( currentLine[ currentColumnNumber ] ) ) 
				currentColumnNumber++;
			else // if NOT isspace() 
				return true;
				
		// if we're here, we're at the end of a line.
		std::getline( *input, currentLine ); 
		currentLineNumber++;
		currentColumnNumber = 0;
		if ( ! *input ) // EOF!!
			return false;
		// else, repeat! 
		// Which is the same as 
		// return skippedWhiteSpace()  -- i.e. tail recursion.
	};
};		
Ronald Charles Moore's avatar
Ronald Charles Moore committed
83

Ronald Charles Moore's avatar
Ronald Charles Moore committed
84
85
86
87
static Token gettok( ) {
	assert( input ); // we assume nullptr != input
	if ( ! *input ) return bad_tok;
	// else, we can read from input
Ronald Charles Moore's avatar
Ronald Charles Moore committed
88
	
Ronald Charles Moore's avatar
Ronald Charles Moore committed
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
	// Skip white space, going to next line as necessary
	if ( ! skippedWhiteSpace( ) ) return tok_eof;
	        
	// We're have visible text in front of us.
	char currentChar = currentLine[ currentColumnNumber ];
	currentColumnNumber++; // usually, but see num...
	switch (  currentChar ) {
		case '(' : return tok_lparen;
		case ')' : return tok_rparen;
		case '+' : return tok_plus;
		case '-' : return tok_minus;
		case '*' : return tok_times;
		case '/' : return tok_div;
		default : 
			// either we have a number in front of us, or we don't
			assert( 0 < currentColumnNumber );
			char *alpha = &(currentLine[ currentColumnNumber-1 ]);
			// minus one because we incremented it before the switch
			char *omega = nullptr; // until we call strtod...
			double tmpValue = strtod( alpha, &omega );
			if ( alpha == omega ) {
				return bad_tok; // !!!
			};
			// else if strtod found a real number (or at least a double)
			currentNumber = tmpValue; // let C++ do the converison
			currentColumnNumber += (omega - alpha) -1;
			// minus one because we incremented it before the switch
			return tok_number;
	
	}; // end switch
	assert( false ); // we should never get here!
	return bad_tok;
} // end gettok
	
// PARSING!!!
// ===========
// 
// The grammar we are going to parse here is:
// Grammar:
// 	E	→ T E´
// 	E´	→ + T E´  | - T E´ | ε
// 	T	→ F T´
// 	T´	→ * F T´  |  / F T´  |  ε
// 	F	→ ( E ) | num
// Note that the recursive descent function for (e.g.) E´ 
// is nameded "E2ndHalf"-

136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
// Forward Declarations
static numberType E();
static numberType E2ndHalf();
static numberType T();
static numberType T2ndHalf();
static numberType F();

// 	E	→ T E´
numberType E() { return T() + E2ndHalf(); }

// 	T	→ F T´
numberType T() { return F() * T2ndHalf(); }

// 	E´	→ + T E´  | - T E´ | ε
numberType E2ndHalf() {
    switch ( next_token ) {
		case tok_plus : 
			next_token = gettok(); // eat +
            return T() + E2ndHalf();

		case tok_minus : 
			next_token = gettok(); // eat -
			return (-1.0 * T()) + E2ndHalf();
Ronald Charles Moore's avatar
Ronald Charles Moore committed
159

160
161
162
163
164
165
166
		default : 
			return 0.0;
    }; 
} // end E2ndHalf

// 	T´	→ * F T´  |  / F T´  |  ε
numberType  T2ndHalf() { 
167
    numberType tmp; // Compiler forces me to put this here :-(
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
    switch ( next_token ) {
		case tok_times : 
			next_token = gettok(); // eat *
			return F() * T2ndHalf();
    case tok_div : 
		next_token = gettok(); // eat /
		tmp = F();
		if ( 0.0 != tmp ) 
			return (1.0/tmp) * T2ndHalf();
		// else if T() returned zero
		printErrorMsg( "Division by zero!" );
		// fall through to default return one
        
    default : 
		return 1.0;
    }; 
} // end T2ndHalf

// 	F	→ ( E ) | num
numberType  F() {
	numberType result = 0; 
    switch ( next_token ) {
		case tok_lparen : 
			next_token = gettok(); // eat lparen
            result = E();
            if ( tok_rparen == next_token ) {
				next_token = gettok(); // eat rparen
                return result;
			}; 
			// else if rparen not found
            printErrorMsg( "Expected Right Parenthesis" );
            return 0.0;

		case tok_number : 
			result = currentNumber; // side-effect of last gettok() 
            next_token = gettok(); // eat id
            return result;

		default : 
			printErrorMsg( "Expected Left Parenthesis or number" );
            return 0.0;
    }; 
}
Ronald Charles Moore's avatar
Ronald Charles Moore committed
211
212


Ronald Charles Moore's avatar
Ronald Charles Moore committed
213
214
215
216
217
// main (!)
// =========

int main( int argc, char **argv ) {

Ronald Charles Moore's avatar
Ronald Charles Moore committed
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
	if (2 != argc) {
		std::cerr << "Usage: " << argv[0] << " <fileName>.\n" 
		          << "You provided " << argc-1 << " arguments, we take exactly one (only).\n";
		return( -1 );
	};
	// else if 1 == argc ....
	std::string	fileName( argv[1] );
	if ( "-" != fileName ) {
		static std::ifstream ifs( fileName );
		input = &ifs;
	}

	// Prime the pump!
	next_token = gettok( );
	
	// get tokens and dump them... 
	while ( tok_eof != next_token ) {
235
236
237
238
		std::cout << currentLineNumber << ":" 
		          << currentLine << std::endl;
		numberType value = E( );
		std::cout << "INTERPRETER: " << value << std::endl;
Ronald Charles Moore's avatar
Ronald Charles Moore committed
239
	};
240
	std::cout << "End Of File!" << std::endl;
Ronald Charles Moore's avatar
Ronald Charles Moore committed
241
	
Ronald Charles Moore's avatar
Ronald Charles Moore committed
242
243
	return 0; // Alles klar!!!
}