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Interpreter:
- Interpreter executes each statement of a program at a time.
- It calls recursive functions for “statement”, “expression”, “factor”, “term” and “Identifier” to execute a statement.
- It allows arithmetic operations addition, subtraction, multiplication, division and exponentiation.
/***********************************************************
* This program extends the interpreter with exponentiation *
* process. *
***********************************************************/
Explanation of Solution
//interpreter.h
//Include header files
#ifndef INTERPRETER
#define INTERPRETER
#include <iostream>
#include <list>
#include <algorithm> // find
using namespace std;
//Definition of class IdNode
class IdNode
{
//Declare public methods
public:
//Declare Parameterized constructor
IdNode(char *s = "", double e = 0)
{
//Initialize variables
id = strdup(s);
value = e;
}
//Function to overload operator ==
bool operator== (const IdNode& node) const
{
/*Return true if both string are same, otherwise return false*/
return strcmp(id,node.id)==0;
}
//Declare private variables and overloading function
private:
//Declare variables
char *id;
double value;
//Declare class object
friend class Statement;
//Prototype for overloading function
friend ostream& operator<<(ostream&, const IdNode&);
};
//Definition of class Statement
class Statement
{
//Declare public functions
public :
//Constructor
Statement(){}
//Declare function getStatement()
void getStatement();
//Declare private variables
private:
//Declare variables
list<IdNode>idList;
char ch;
//Function prototypes
double factor();
double term();
double expression();
double exponent();
void readId(char*);
//Declare function to print error message
void issueError(char* s)
{
cerr << s << endl; exit(1);
}
//Function prototypes
double findValue(char*);
void processNode(char*, double);
friend ostream& operator<< (ostream&, const Statement&);
};
#endif
//interpreter.cpp
//Include header files
#include <cctype>
#include <stdio.h>
#include <string.h>
#include <string>
#include <list>
#include "interpreter.h"
//Function to find value of identifier
double Statement::findValue(char *id)
{
//Create object of class IdNode for id
IdNode tmp(id);
/*If identifier is present in IdNode list, return position */
list<IdNode>::iterator i = find(idList.begin(),idList.end(),tmp);
//If position is not end of list
if (i != idList.end())
//Return value of identifier
return i->value;
//Otherwise
else
//Print error message
issueError("Unknown variable");
// This statement will never be reached;
return 0;
}
//Function to process node
void Statement::processNode(char* id,double e)
{
/*Create object of class for IdNode for statement*/
IdNode tmp(id,e);
//If identifier is present in IdNode list find position of identifier
list<IdNode>::iterator i = find(idList.begin(),idList.end(),tmp);
//If position of identifier is not end of IdNode list
if (i != idList.end())
//Set value of identifier as value of expression
i->value = e;
//Otherwise
else
//Insert New value into list
idList.push_front(tmp);
}
/* readId() reads strings of letters and digits that start with a letter, and stores them in array passed to it as an actual parameter.*/
/* Examples of identifiers are: var1, x, pqr123xyz, aName, etc.*/
//Function to read identifier
void Statement::readId(char *id)
{
//Initialize variable i as 0
int i = 0;
//If ch is space
if (isspace(ch))
//Skip balks and read next character
cin >> ch;
//If character is alphabet
if (isalpha(ch))
{
//Read character till read a non-alphanumeric
while (isalnum(ch))
{
//Copy each character to identifier
id[i++] = ch;
//Read next character
cin.get(ch);
}
//Set last character as end of string
id[i] = '\0';
}
//Otherwise
else
//Print error message
issueError("Identifier expected");
}
//Function to find exponent
double Statement::exponent()
{
// <exponent> ::= <factor> ^ <exponent>,
double f = factor();
//If ch is operatotr ^
if (ch == '^')
//Compute power and Return it
return pow(f,exponent());
//Otherwise
else
//Return f
return f;
}
//Function factor()
double Statement::factor()
{
//Declare variables
double var, minus = 1.0;
static char id[200];
//Read character
cin >> ch;
//If ch is operator + or -
while (ch == '+' || ch == '-')
{
// take all '+'s and '-'s.
//If ch is -
if (ch == '-')
/*Set value as negative by multiplying with -1*/
minus *= -1.0;
//Read next character
cin >> ch;
}
//If ch is digit or operator dot
if (isdigit(ch) || ch == '.')
{ // Factor can be a number
//Read digits of number
cin.putback(ch);
cin >> var >> ch;
}
//If ch is paranthesis (
else if (ch == '(')
{
// or a parenthesized expression,
//Get next expression after (
var = expression();
//If character is )
if (ch == ')')
//Read next character
cin >> ch;
//Otherwise
else
//Print error message
issueError("Right paren left out");
}
//Otherwise
else
{
//Read identifier
readId(id);
//If chracter is space
if (isspace(ch))
//Read next character
cin >> ch;
//Get value of identifier
var = findValue(id);
}
//Return value
return minus * var;
}
//Function for term()
double Statement::term()
{
//Call function exponent()
double f = exponent();
//Do calculation for operators * and /
while (true)
{
//Switch statement
switch (ch)
{
//If ch is *, multiply recursively
case '*' : f *= exponent(); break;
//If ch is *, divide recursively
case '/' : f /= exponent(); break;
//Defaultly return f
default : return f;
}
}
}
//Function for expression()
double Statement::expression()
{
//Call function term()
double t = term();
//Do calculation for operators + and -
while (true)
{
//Switch statement
switch (ch)
{
//If ch is +, add recursively
case '+' : t += term(); break;
//If ch is -, subtract recursively
case '-' : t -= term(); break;
// Return f
default : return t;
}
}
}
//Function to read statement
void Statement::getStatement()
{
//Declare variables
char id[20], command[20];
double e;
//Prompt and read statement
cout << "Enter a statement: ";
cin >> ch;
//Read identifier
readId(id);
//Copy identifier to string command
strupr(strcpy(command,id));
//If command is STATUS
if (strcmp(command,"STATUS") == 0)
//Print current values of all variables
cout << *this;
//If command is PRINT
else if (strcmp(command,"PRINT") == 0)
{
//Read identifier from user
readId(id);
//Get value of identifier and print it
cout << id << " = " << findValue(id) << endl;
}
//If command is END
else if (strcmp(command,"END") == 0)
//Retun from program
exit(0);
//Otherwise
else
{
//If ch is space
if (isspace(ch))
//Read next character
cin >> ch;
//If ch is =
if (ch == '=')
{
//Read expression
e = expression();
//If ch is not ;
if (ch != ';')
//print error message
issueError("There are some extras in the statement");
//Otherwise process statement
else processNode(id,e);
}
//Otherwise print error message
else issueError("'=' is missing");
}
}
/*Function to overload operator << to print values all identifiers in list*/
ostream& operator<< (ostream& out, const Statement& s)
{
//Initialize iterator
list<IdNode>::const_iterator i = s.idList.begin();
//For each identifier
for ( ; i != s.idList.end(); i++)
//Print value of identifier
out << *i;
//Print new line
out << endl;
return out;
}
/*Function to overload operator << to print value od identifier*/
ostream& operator<< (ostream& out, const IdNode& r)
{
//Print value of identifier
out << r.id << " = " << r.value << endl;
return out;
}
//useInterpreter.cpp
//Include header files
#include "interpreter.h"
using namespace std;
//Program begins with main()
int main()
{
//Declare object of class Statement
Statement statement;
//Prompt message
cout << "The program processes statements of the following format:\n"<< "\t<id> = <expr>;\n\tprint <id>\n\tstatus\n\tend\n\n";
// This infinite loop is broken by exit(1)
while (true)
// in getStatement() or upon finding an
statement.getStatement();
//Return 0
return 0;
}
Explanation:
- Define function “exponent()” as member of class “Statement”.
- In function “exponent()”,
- It calls function “factor()”
- If next character is “^” compute exponentiation by calling function itself recursively.
- Return result.
- Function “term()” calls function “exponent()” instead of “factor()”.
Output:
The program processes statements of the following format:
<id> = <expr>;
print <id>
status
end
Enter a statement: var1=2+(2^3);
Enter a statement: var2=var1-2;
Enter a statement: var3=var2*2;
Enter a statement: status
var3 = 16
var2 = 8
var1 = 10
Enter a statement: var3=var3-4;
Enter a statement: print var3
var3 = 12
Enter a statement: end
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Chapter 5 Solutions
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