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private String convertToPostfix(String infix) { /* Task:
private String convertToPostfix(String infix)
{
/*
Task: Convert an infix expression to a postfix expression
Use Stack<Character> S to hold operator characters
Valid characters are '(',')','+','-','*','/', predefined variable names
Use a StringBuffer PE to hold postfix expression
Need to consider : ‘(‘ & ‘)’, precedence of operators and left to right association
Step 1. Process each character ch in infix expression from left to right
switch(ch)
{
case operand : append to PE; break;
case ‘(‘ : S.push(ch); break;
case ‘)’ : repeat // loop until “(“
{
symbol = S.pop()
if symbol != ‘(‘ then append to PE
else exit loop
}
break;
case operator : get TopSymbol from S if S is not empty
while (!S.isEmpty()) and (TopSymbol != ‘(‘) and
(precedence(ch) <= precedence(TopSymbol))
{
symbol = S.pop()
append symbol to PE
get TopSymbol from S if S is not empty
}
S.push(ch)
break;
} // end switch
Step 2. After scanning the whole infix expression. Append remaining operators in S into PE
while (Stack != empty)
{
symbol = S.pop();
append symbol to PE
}
Return PE.toString() // convert StringBuffer to String
Example : (a*b+c) â€" (d-e*f) == ab*c+def*--
Char Stack PE
( (
a ( a
* (* a
b (* ab
+ (+ ab*
c (+ ab*c
) ab*c+
- - ab*c+
( -( ab*c+
d -( ab*c+d
- -(- ab*c+d
e -(- ab*c+de
* -(-* ab*c+de
f -(-* ab*c+def
) - ab*c+def*-
ab*c+def*--
*/
return null; //change it
} // end convertToPostfix
/** Evaluates a postfix expression.
Must only use variable names as defined in variable table
@param postfix : A valid postfix expression.
@return The double result of the postfix expression. */
private double evaluatePostfix(String postfix)
{
/*
Task: Evaluate a postfix expression
Use a Stack<Double> S to hold operands
Process each character ch in postfix expression from left to right
if a character is an operand : push into S
if a character is an operator :
pop two operands from S
evaluate the result (need to consider +,-,*,/)
push the result back to S
Final result is in S
Hint: Use getVariableValue(X) to get value of variable X
Use checkValidVariable(X) to check if X is a variable
Use checkValidOperator(X) to check if X is an operator
Example : Let A=2, B=3, C=4, D=5.
Evaluate postfix expr “ABC+*D-“
234+*5- = 2 * (3+4) â€" 5 = 9
Char Stack
2 2
3 2,3
4 2,3,4
+ 2,7 // 3 + 4
* 14 // 2 * 7
5 14,5
- 9 // 14 - 5
Result = 9
