Description
You are to write a program name calc.java that evaluates an infix expression entered by the user. The expression may contain the following tokens:
(1) Integer constants (a series of decimal digits).
(2) x (representing a value to be supplied later).
(3) Binary operators (+, -, *, / and %).
(4) Parentheses
Spaces between tokens are allowed but not required. The program will convert the expression to postfix (RPN) form and display the converted expression. The program will repeatedly prompt the user for the value of x, displaying the value of the expression each time. When the user enters the letter q instead of a number, the program terminates.
The following example illustrates the behavior of the program (user input is in bold and red):
Porgram output is in bold and green.
Enter infix expression: (x + 1) * (x – 2) / 4
Converted expression: x 1 + x 2 – * 4 /
Enter value of x: 5
Answer to expression: 4
Enter value of x: 7
Answer to expression: 10
Enter value of x: q
If the infix expression contains an error of any kind, the program must display the message Error in expression (with an optional explanation) and then terminate. The following examples illustrate various types of errors:
Enter infix expression: 1 2 +
Error in expression!! No operator between operands. Also last token must be an operand.
Enter infix expression: 10.4
Error in expression!! Cannot accept floating point numbers.
Enter infix expression: 1 ( + 2)
Error in expression!! No operator between operand and left parentheses.
Enter infix expression: 5 – (x – 2))
Error in expression!! No matching left parentheses for a right parentheses.
Enter infix expression: 1 ** 2
Error in expression!! The * operator cannot be preceded by a * operator.
The output of your program must match the format illustrated in this example.
Here are some other additional requirements for this program:
(1) You must use stack objects during the translation from infix to postfix and during the evaluation of the postfix expression.
(2) Operators must have the correct precedence and associativity.
What to turn in:
- All of the .java and the .class/jar files at the D2L website in the Dropbox folder for A5 no later than 11:00 p.m. on the respective due date.
- A print out of each of .java files to be given to the TA in LAB during next week. NOTE: Print out what you have already up the LAB time even if you are not finished.
Hints:
1. Do the program in stages. First, get the infix to postfix conversion working for binary operators. Next, implement the evaluation of the postfix expression. Finally, add code to check the infix expression for errors. Use the examples from the hand out as a starting point for the program, but keep in mind that this code does not handle associativity properly.
2. To detect errors in the infix expression, you will need to check for several situations:
A binary operator is preceded by an operator or an operand is preceded by an
operand.
An illegal character is encountered (such as a period).
The last token in the expression is not an operand or a right parentheses
There is no left parentheses anywhere in the stack when a right parentheses is
encountered.
The stack contains a left parenthesis when the expression ends.
3. Use a string to store the postfix expression. Use a stack of characters
during the translation from infix to postfix.
4. Use the Scanner class to read the input string and extract each token. Careful when you are reading an unknown (eg. x) from the input expression.
