Question

Complete the following java program in which infix expressions should be converted to postfix expressions

/** * An algebraic expression class that has operations such as conversion from infix

* expression to postfix expression,

* and evaluation of a postfix expression

*/

public class Expression {

/** * The infix of this expression

*/

private String infix;

/** * Constructs an expression using an infix.

*/

public Expression(String infix){

this.infix = infix;

}

/** * Converts an infix expression into a postfix expression.

* Tokens of the postfix expression are stored in order in an

* array list. The array list is returned.

* @return An array list of tokens of the postfix expression */

public ArrayList infixToPostfix(){

Stack<String> stack = new Stack<String>();

ArrayList postFixString = new ArrayList();

…

}

public static void main(String[] args) {

//Test method infixToPostfix using the following infix expressions. A + B * C + D, (A + B) * (C + D), A * B + C * D, and A + B + C + D } }

// End of Expression

Answer 1

ANSWER :

Hello, below is the code for the class above.

I have added additional methods for utility, like calculating precedence of the operators and for printing the arraylist which contains the postfix expressions.

Code :

import java.util.ArrayList;
import java.util.Stack;

/** * An algebraic expression class that has operations such as conversion from infix
* expression to postfix expression,
* and evaluation of a postfix expression
*/
public class Expression {

/** * The infix of this expression
*/
private String infix;

/** * Constructs an expression using an infix.
*/
public Expression(String infix){
this.infix = infix;
}

/** * Converts an infix expression into a postfix expression.
* Tokens of the postfix expression are stored in order in an
* array list. The array list is returned.
* @return An array list of tokens of the postfix expression */
public ArrayList infixToPostfix(){

Stack<String> stack = new Stack<>();
ArrayList postFixString = new ArrayList();
for(int i = 0; i<infix.length();i++){
char ch = infix.charAt(i);

// If the character is an empty space, skip the iteration.
if(ch==' ')
continue;

// If the character is an operand, add it to arraylist.
if((ch>=65 && ch<=90)||(ch>=97 && ch<=122))
postFixString.add(ch);

// If the character is an '(', push it to the stack.
else if(ch == '(')
stack.push(ch+"");

//If the character is an ')', pop and output from the stack until an '(' is encountered.
else if(ch == ')')
{
while (!stack.isEmpty() && !stack.peek().equals("("))
postFixString.add(stack.pop());

if (!stack.isEmpty() && !stack.peek().equals("(")){
System.out.println("Invalid Expression");
System.exit(0);
} else
stack.pop();
}
// the character is an operator
else {
while (!stack.isEmpty() && precedence(ch)<=precedence(stack.peek().charAt(0))){
if(stack.peek() == "(" ){
System.out.println("Invalid Expression");
System.exit(0);
}
postFixString.add(stack.pop());
}
stack.push(ch+"");
}
}
//return the converted post fix expression after conversion.
return postFixString;
}

//Method to determine the precedence while calculating the precedence of operators
public int precedence(char ch){
switch(ch){
case '^': return 3;
case '/':
case '*': return 2;
case '+':
case '-': return 1;
}
return -1;
}

//Method to print the post fix expression
public static void printPostfix(ArrayList arrayList){
for(Object object : arrayList)
System.out.print(object.toString()+" ");
System.out.println();
}

public static void main(String[] args) {
Expression expression = new Expression("A + B * C + D");
printPostfix(expression.infixToPostfix());

expression = new Expression("(A + B) * (C + D)");
printPostfix(expression.infixToPostfix());

expression = new Expression("A * B + C * D");
printPostfix(expression.infixToPostfix());

expression = new Expression("A * B + C * D");
printPostfix(expression.infixToPostfix());
}
}
//End of expression

Sample Output :​​​​​​PLEASE LEAVE A THUMBS UP IF THIS ANSWER HELPS YOU.

​​​​​THANK YOU:)