Prefix Calculator An arithmetic expression is in prefix form when operators are
ID: 3712951 • Letter: P
Question
Prefix Calculator An arithmetic expression is in prefix form when operators are written before their operands. Here are some examples of prefix expressions and the values they evaluate to: Expression Value 12 12 +2 51 53 .5 7 35 +164+31 80 An expression (such as 12) that begins with an integer is a prefix expression that evaluates to itself. Otherwise, an expression is a prefix expression if it begins with an operator and is followed by two prefix expressions. In this latter case, the value of the expression is recursively computed from the values of its constituent prefix sub-expressions. Write a program that allows the user to enter prefix expressions in a text field. The program reads the expression, evaluates it, and displays the value in a suitable GUI component. Assume that the user enters expressions that use only positive integers and the two operators+and *. it.Explanation / Answer
package prefixcalculator;
import java.util.Scanner;
public void start(Stage primaryStage) throws Exception
{
import java.util.Scanner;
import javafx.application.Application; import javafx.event.ActionEvent; import javafx.event.EventHandler; import javafx.scene.Scene; import javafx.scene.control.Button; import javafx.scene.control.Label; import javafx.scene.control.TextField; import javafx.scene.layout.HBox; import javafx.scene.layout.VBox; import javafx.stage.Stage; public class PrefixCalc extends Application{ //private TextField displayField; private TextField resultField; private TextField cmdField; @Overridepublic void start(Stage primaryStage) throws Exception
{
PrefixCalc PrefixCalc(); //setting up stuff for Prefix Exp Label prefixExp = new Label("Prefix Expression"); //result output set up Label result = new Label("Result Output: "); resultField = new TextField(); resultField.setEditable(false); //result hbox HBox results = new HBox(); results.getChildren().addAll(result, resultField); //input line set up cmdField = new TextField(); Label cmdLabel = new Label("Enter Your Expression: "); //hbox for input line HBox cmd = new HBox(); cmd.getChildren().addAll(cmdLabel, cmdField); //button to submit and calculate input Button submit = new Button("Calculate"); //main vbox for all hboxes VBox main = new VBox(20); main.getChildren().addAll( results, cmd, submit); EventHandler<ActionEvent> handle = new CmdTextListener(); submit.setOnAction(handle); //majic for stage ;) primaryStage.setScene(new Scene(main)); primaryStage.setTitle("Prefix Calculator"); primaryStage.show(); } //reads prefix input private class CmdTextListener implements EventHandler<ActionEvent> { @Override public void handle(ActionEvent event) { String pExpr = cmdField.getText(); Node exprTree = buildExpr(new Scanner(pExpr)); int value = eval(exprTree); //displayField.setText(pExpr); resultField.setText(String.valueOf(value)); } //reads tree and reutrns val private int eval(Node tree) { if (tree.left == null && tree.right == null) return Integer.parseInt(tree.value); else { int leftOp = eval(tree.left); int rightOp = eval(tree.right); if (tree.value.equals("*")) return leftOp * rightOp; if (tree.value.equals("+")) return leftOp + rightOp; } return 0; // Will never be reached } //is it a number starting? private boolean isNumber(String s) { return Character.isDigit(s.charAt(0)); } private class Node { String value; Node left, right; Node(String v, Node l, Node r) { value = v; left = l; right = r; } } //builds and reads tree Node buildExpr(Scanner sc) { String s = sc.next(); if (isNumber(s)) return new Node(s, null, null); else { // s is an operator, so build a non-leaf tree Node leftOp = buildExpr(sc); Node rightOp = buildExpr(sc); return new Node(s, leftOp, rightOp); } } } public static void main(String[] args){ launch(args); } }