Chapter 13, Problem 21PE

Program Plan Intro

Expression Evaluator

Program plan:

Filename: “Evaluator.java”

This code snippet creates a class “Evaluator”. In the code,

• Define a class “Evaluator”.
  • Declare the class member “tokenizer”.
  • Define a constructor “Evaluator()”.
    • Define “tokenizer”.
  • Define the method “getExpressionValue()”.
    • Set “value”.
    • Declare “done”.
    • While true,
      • Get the token to “next”.
      • If “next” equal to “+” or “-”,
        • Call “nextToken()” to get the token.
        • Define “value2”.
        • If “next” equal to “+”,
        • • Evaluate “value”.
        • Else,
        • • Evaluate “value”.
      • Else,
        • Set “done” equal to “true”.
        • • • Return “value”.
  • Define the method “getTermValue()”.
    • Set “value”.
    • Declare “done”.
    • While true,
      • Get the token to “next”.
      • If “next” equal to “*” or “/” or “%”,
        • Call “nextToken()” to get the token.
        • Define “value2”.
        • If “next” equal to “*”,
        • • Evaluate “value”.
        • Else if “next” equal to “/”,
        • • Evaluate “value”.
        • Else,
        • • Set the “value”.
      • Else,
        • Set “done” equal to “true”.
        • • • Return “value”.
  • Define the method “getExponentiationValue()”.
    • Set “value”.
    • Declare “done”.
    • While true,
      • Get the token to “next”.
      • If “next” equal to “^”.
        • Call “nextToken()” to get the token.
        • Define “value2”.
        • Set the “value”.
      • Else,
        • Set “done” equal to “true”.
        • • • Return “value”.
  • Define the method “getFactorValue()”.
    • Set “value”.
    • Set the value of “next”.
    • If “next” equal to “^”.
      • Call “nextToken()” to get the token.
      • Define “value”.
      • Call “nextToken()”
        • • • Else,
              • Set “value”.
            • Return “value”.

Filename: “ExpressionTokenizer.java”

This code snippet creates a class “ExpressionTokenizer”. In the code,

• Define a class “ExpressionTokenizer”.
  • Declare a string “input”.
  • Declare an integer “start”.
  • Declare an integer “end”.
  • Define a constructor “ExpressionTokenizer()”.
    • Set the value of class members “input”, “start” and “end”.
    • Call “nextToken()”.
  • Define the method “peekToken()”.
    • If the value of “start” is greater than or equal to length of “input”.
      • Return “null”.
        • Else,
        • • Return substring of “input”.
  • Define the method “nextToken()”.
    • Set the value of “start” equal to “end”.
    • While “true”,
      • Increment the value of “start” by 1.
        • If the condition is true,
        • • Return the value of “r”.
        • If the condition is true,
        • • Set “end” equal to “start+1”.
          • While “true”,
            • Increment the “end” by 1.
        • Else,
        • • Set “end” equal to “start+1”.
        • Return the value of “r”.

Filename: “expressionCalculator.java”

This code snippet creates a class “expressionCalculator”. In the code,

• Define a class “expressionCalculator”.
  • Define the “main” method.
    • Define the object of the class “Scanner”.
    • Prompt the user to enter the expression.
    • Scan for the “input”.
    • Define the object of class “Evaluator”.
    • Set the “value”.
    • Print new line.
    • Print the value of “input” and “value”.