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/ / Find the next token, skipping blanks, and return it / / Print error message if input is unrecognized
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template <class NumericType> Token<NumericType> Tokenizer<NumericType>::getToken()
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char ch; NumericType thevalue;
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/ / Skip blanks while( inget( ch
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ch ! = ' \ n l&& ch ! = '\0' )
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' ' return EXP; ' / ' : return DIV;
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default : inputback( ch ) ; if ( ! ( in >> thevalue
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cerr < < "Parse error" << endl; return EOL;
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return Token<NumericType>( VALUE, thevalue
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Figure 1216 The getToken routine for returning the next token in the input stream
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getvalue function could be made more robust by signaling an error if theType is not VALUE) The Tokenizer class has one member function Figure 1216 shows the getToken routine First we skip past any
blanks, and when the loop at line 10 ends, we have gone past any blanks If we have not reached the end of line, we check to see whether we match any of the one-character operators If so, we return the appropriate token (a
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1 / / Public routine that performs the evaluation 2 / / ~xaminesthe postfix machine to see if a single result 3 / / is left and if so, returns it; otherwise prints error 4 template <class NumericType> 5 NumericType Evaluator<NumericType>::getValue( ) 6 { 7 Tokenizer<NumericType> tok( str ) ; 8 Token<NumericType> lastToken; 9 10 do 11 { 12 lastToken = tokgetToken( ) ; 13 processToken( 1astToken ) ; 14 } while( lastTokengetType( ) ! = EOL ) ; 15 if ( postFixStackempty ( ) ) 16 17 I 18 cerr < < "Missing operand!" < < endl; 19 return 0; 20 1 21 22 NumericType theResult = postFixStackback( 1 ; 23 postFixStackpop-back( ) ; 24 if( !postFixStackempty( ) ) 25 cerr < < "Warning: missing operators!" < < endl; 26 return theResult; 27 28 1
Figure 1217 The getvalue routine for reading and processing tokens and then returning the item at the top of the stack
Token object is constructed by using an implicit type conversion by virtue of a one-parameter constructor) Otherwise, we reach the default case in the switch statement We expect that what remains is an operand, so we unread ch,use operator>> to get the value, and then return a Token object by expKcitly constructing a Token object based on the value read Note that for the putback to work we must use getThat is why we do not C++ note: get must simply use operator>> (in place of lines 10-13) to skip implicitly past the be used so that blanks gutback works We can now discuss the member functions of the Evaluator class The only publicly visible member function is getvalueShown in Figure 1217, getvalue repeatedly reads a token and processes it until the end of line is detected At that point the item at the top of the stack is the answer
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1 / / top and pop the postfix machine stack; return the result 2 / / If the stack is empty, print an error message 3 template <class NumericType> 4 NumericType Evaluator<NumericType>::getTop( ) 5 I 6 if ( postFixStackempty ( ) 1 7 { 8 cerr << "Missing operand" << endl; 9 return 0; 10 1 11 12 NumericType tmp = postFixStackback( ) ; 13 postFixStackpop-back( ) ; a 14 return tmp; 15 1
Figure 1218 The getTop routine for getting the top item in the postfix stack and removing it
A precedence table is used to decide what is removed from the operator stack Leftassociative operators have the operator stack precedence set at 1 higher than the input symbol precedence Rightassociative operators go the other way
Figures 1 218 and 1219 show the routines used to implement the postfix machine The getTop routine returns and removes the top item in the postfix stack The binaryop routine applies topop (which is expected to be the top item in the operator stack) to the top two items on the postfix stack and replaces them with the result It also pops the operator stack (at line 33) signifying that processing for topop is complete The pow routine is presumed to exist for NumericType objects; we can either use the math library routine or adapt the one previously shown in Figure 814 Figure 1220 declares a precedence table, which stores the operator precedences and is used to decide what is removed from the operator stack The operators are listed in the same order as the enumeration type TokenType Because enumeration types are assigned consecutive indices beginning with zero, they can be used to index an array (The array initialization syntax used here was described in Section 126) We want to assign a number to each level of precedence The higher the number, the higher is the precedence We could assign the additive operators precedence 1, multiplicative operators precedence 3, exponentiation precedence 5, and parentheses precedence 99 However, we also need to take into account associativity To do so, we assign each operator a number that represents its precedence when it is an input symbol and a second number that represents its precedence when it is on the operator stack A left-associative operator has the operator stack precedence set at 1 higher than the input symbol precedence, and a right-associative operator goes the other way Thus the precedence of the + operator on the stack is 2