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Create Java stacks

public static int evaluate(Scanner input)
{
if (input.hasNextInt())
{
return input.nextInt();
}
else
{
String operator = input.next();
int operand1 = evaluate(input);
int operand2 = evaluate(input);
return evaluate(operator, operand1, operand2);
}
}
// pre : operator is one of *, /, %, + or -
// post: returns the result of applying the given operator to
// the given operands
public static int evaluate(String operator, int operand1, int operand2)
{
if (operator.equals("*"))
{
return operand1 * operand2;
}
else if (operator.equals("/"))
{
return operand1 / operand2;
}
else if (operator.equals("%"))
{
return operand1 % operand2;
}
else if (operator.equals("+"))
{
return operand1 + operand2;
}
else if (operator.equals("-"))
{
return operand1 - operand2;
}
else
{
throw new RuntimeException("illegal operator " + operator);
}
}
I want to take this code and convert it to 2 stacks (one stack for the operators and the other stack for the operands) for using prefix expressions in a user-input GUI with an actionlistener. How can I write the code to convert this code to 2 stacks? By the way, this is homework and I understand that you're not allowed to give me the answers outright, so if you can provide me easy-to-understand pseudo code, that will be much appreciated. Thank you for your help.

Your pseudo is here:
public static int evaluate(Scanner input)
{
if (input.hasNextInt())
{
int stack_top_value=input.nextInt();
stack_for_operand.push(stack_top_value);
return stack_top_value;
}
else
{
String operator = input.next();
stack_for_operator.push(operator);
int operand1 = evaluate(input);
int operand2 = evaluate(input);
return evaluate(operator, operand1, operand2);
}
}

Prefix Expression evaluating incorrectly - Java

private class InputListener implements ActionListener
{
public void actionPerformed(ActionEvent e)
{
// Create an integer and character stack
Stack<Integer> operandStack = new Stack<Integer>();
Stack<Character> operatorStack = new Stack<Character>();
// User input in input text field
String input = inputTextField.getText();
// Create string tokenizer containing string input
StringTokenizer strToken = new StringTokenizer(input);
// Loop while there are tokens
while (strToken.hasMoreTokens())
{
String i = strToken.nextToken();
int operand;
char operator;
try
{
operand = Integer.parseInt(i);
operandStack.push(operand);
}
catch (NumberFormatException nfe)
{
operator = i.charAt(0);
operatorStack.push(operator);
}
}
// Loop until there is only one item left in the
// operandStack. This one item left is the result
while(operandStack.size() > 1)
{
// Perform the operations on the stack
// and push the result back onto the operandStack
operandStack.push(operate(operandStack.pop(),
operandStack.pop(), operatorStack.pop()));
}
// Display the result as a string in the result text field
resultTextField.setText(Integer.toString(operandStack.peek()));
}
// Sum and product computed
public int operate(Integer operand1, Integer operand2, char operator)
{
switch(operator)
{
case '*':
return operand2 * operand1;
case '/':
return operand2 / operand1;
case '%':
return operand2 % operand1;
case '+':
return operand2 + operand1;
case '-':
return operand2 - operand1;
default:
throw new IllegalStateException("Unknown operator " + operator + " ");
}
}
}
The prefix expression code provided is incorrectly evaluating expressions with more than one operator. The expression: * + 16 4 + 3 1 should evaluate to 80 = ((16 + 4) * (3 + 1)), but instead it evaluates to 128, which I think is evaluating as: ((16 + 4) * (3 + 1) + (16 * 3)). How do I edit the code to correct this problem? Thank you for your help.

In prefix evaluation,Important thing to remember is
operand 1= pop();
operand 2= pop();
and say the operator is -
The value that is pushed is operand 1 - operand 2 and not operand 2 - operand 1
But something else is causing * + 16 4 + 3 1 to evaluate to 128.
I used the following algo for evaluating in java and it works fine
1.Take the prefix expression as a string in a variable with characters separated by single space.
2.Traverse the string from index length-1 to 0
3.If its a number, perform push ,and if its a multidigit number,first obtain the full number and then push it
4.If its an operator then simply do the thing which i mentioned in beginning of the answer.
It is a simple code.
import java.io.*;
import java.util.*;
class PREFIXEVAL
{
public static void main(String []args)throws IOException
{
String p,n="";StringBuffer b;int i,op1,op2;char c;Stack<Integer> s=new Stack<Integer>();
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("enter the prefix expression separated by spaces");
p=br.readLine();
i=p.length()-1;
while(i>=0)
{
c=p.charAt(i);
if(c>=48&&c<=57)
n=n+c;
else if(c==' '&&!n.equals(""))
{/*handles both single and multidigit numbers*/
b=new StringBuffer(n);b.reverse();n=b.toString();
s.push(Integer.parseInt(n));n="";
}
else
{
if(c=='+')
{
op1=s.pop();
op2=s.pop();
s.push(op1+op2);
}
else if(c=='-')
{
op1=s.pop();
op2=s.pop();
s.push(op1-op2);
}
else if(c=='*')
{
op1=s.pop();
op2=s.pop();
s.push(op1*op2);
}
else if(c=='%')
{
op1=s.pop();
op2=s.pop();
s.push(op1%op2);
}
else if(c=='/')
{
op1=s.pop();
op2=s.pop();
s.push(op1/op2);
}
}
i--;
}
System.out.println("the prefix expression evaluates to "+s.peek());
}
}

Java geting operators from within a split for function purpose

Okay... I think I might have gone over my head with trying to simplify this code. I place operators such as ( *, +, /, -) in a split. Know i want to call them individually to do their perspective task in a if (operators.equals.(+)){
return num1 + num2. }
then for *, -, / perspectively
how can i do that correctly having using math in my earlier code:
String function = "[+\\-*/]+"; //this
String[] token = input.split(function);//and this
double num1 = Double.parseDouble(token[0]);
double num2 = Double.parseDouble(token[1]);
double answer;
String operator = input.toCharArray()[token[0].length()]+"";
if (operator.matches(function) && (token[0]+token[1]+operator).length()==input.length()) {
System.out.println("Operation is " + operator+ ", numbers are " + token[0] + " and " + token[1]);
} else {
System.out.println("Your entry of " + input + " is invalid");
}

You don't.
String.split only returns the parts of the String that were not matched. If you want to know the matched code, you need to use a more sophisticated regular expression, i.e. the Pattern and Matcher classes, or write your own String tokenization class yourself.
In this example Token is a class you make yourself):
public List<Token> generateTokenList(String input) {
List<Token> = new LinkedList<>();
for(char c : input.toCharArray()) {
if(Character.isDigit(c)) {
// handle digit case
} else if (c == '+') {
// handle plus
} else if (c == '-') {
// handle minus
} else {
/* you get the idea */
}
}
}
There are libraries that do this for you, such as ANTLR but this sounds like a school assignment so you probably have to do this the hard way.

Change your if body to something like
if (operator.matches(function) &&
(token[0] + token[1] + operator).length() == input.length())
{
double result = 0;
if (operator.equals("+")) {
result = num1 + num2;
} else if (operator.equals("-")) {
result = num1 - num2;
} else if (operator.equals("*")) {
result = num1 * num2;
} else if (operator.equals("/")) {
result = num1 / num2;
}
System.out.printf("%.2f %s %.2f = %.2f%n", num1, operator, num2,
result);
}
And your code works as expected.

Calculator using Switch in Java

I'm new to Java programming. I am trying to make a Fraction Calculator but when I try to run the program it gives me an error. The error is with the Switch statements but I don't know what happened.
public static void main(String[] args) {
Scanner console = new Scanner(System.in);
System.out.println("Welcome to My FracCalc");
boolean continueProcessing = true;
while (continueProcessing) {
System.out.println("Type an expression and press <enter>");
String Operand1 = console.next();
if (Operand1.equals("quit")) {
continueProcessing = false;
System.out.println("Good bye!");
break;
} else {
Operand1 = parseFullNumber(Operand1);
}
String Operator = console.next();
if (Operator.equals("quit")) {
continueProcessing = false;
System.out.println("Good bye!");
break;
} else if (Operator.equals("+") || Operator.equals("-") || Operator.equals("/") || Operator.equals("*")) {
} else {
throw new ArithmeticException();
}
String Operand2 = console.next();
if (Operand2.equals("quit")) {
continueProcessing = false;
System.out.println("Good bye!");
break;
} else {
Operand2 = parseFullNumber(Operand2);
}
System.out.println( Operand1 + " " + Operator + " " + Operand2);
//System.out.println("First Fraction is: " + Operand1);
//System.out.println("Operator is: " + Operator);
//System.out.println("Second Fraction is: " + Operand2);
float answer;
System.out.println(Operator);
switch (Operator) {
case "+":
answer = Operand1 + Operand2;
break;
case "-":
answer = Operand1 - Operand2;
break;
case "*":
answer = Operand1 * Operand2;
break;
case "/":
answer = Operand1 / Operand2;
break;
}
}
}
public static String parseFullNumber(String input) {
int wholeNumber = 0;
int numerator = 0;
int denominator = 0;
;
int underscoreId = input.indexOf('_');
int slashId = input.indexOf('/');
// Check for underscore "_" //
if (underscoreId > -1) {
wholeNumber = Integer.parseInt(input.substring(0, underscoreId));
numerator = Integer.parseInt(input.substring(underscoreId + 1, slashId));
denominator = Integer.parseInt(input.substring(slashId + 1, input.length()));
} else {
if (slashId > -1) {
// no underscore but there is a slash //
numerator = Integer.parseInt(input.substring(0, slashId));
denominator = Integer.parseInt(input.substring(slashId + 1, input.length()));
} else {
// there is no underscore or slash //
wholeNumber = Integer.parseInt(input);
}
}
return simplify(wholeNumber, numerator, denominator);
}
//simplifying fractions //
public static String simplify(int wholeNumber, int numerator, int denominator) {
// absolute values //
int absNumerator = Math.abs(numerator);
// factor if applicable //
if (absNumerator > 1) {
int commonFactor = 1;
for (int i = 2; i < Math.min(absNumerator, denominator); i++) {
if (numerator % i == 0 && denominator % i == 0) {
commonFactor = i;
}
}
numerator /= commonFactor;
denominator /= commonFactor;
}
// reduce if applicable //
if (absNumerator > denominator) {
int reduction = numerator / denominator;
if (wholeNumber >= 0) {
wholeNumber += reduction;
} else {
wholeNumber -= reduction;
}
numerator %= denominator;
}
// prints //
if (wholeNumber != 0) {
if (numerator != 0) {
return wholeNumber + "_" + numerator + "/" + denominator;
} else {
return String.valueOf(wholeNumber);
}
} else {
if (numerator != 0) {
return numerator + "/" + denominator;
} else {
return String.valueOf(0);
}
}
}
}
Here is the error i got:
Exception in thread "main" java.lang.Error:
Unresolved compilation problems:
Type mismatch:
cannot convert from String to float The operator - is undefined for the argument type(s) java.lang.String, java.lang.String
The operator * is undefined for the argument type(s) java.lang.String, java.lang.String
The operator / is undefined for the argument type(s) java.lang.String, java.lang.String
at FracCalcApp.main(FracCalcApp.java:53)

Operand1 and Operand2 are String(s). You need to parse them before you can perform arithmetic. Something like,
double answer;
System.out.println(Operator);
switch (Operator) {
case "+":
answer = Double.valueOf(Operand1) + Double.valueOf(Operand2);
break;
case "-":
answer = Double.valueOf(Operand1) - Double.valueOf(Operand2);
break;
case "*":
answer = Double.valueOf(Operand1) * Double.valueOf(Operand2);
break;
case "/":
answer = Double.valueOf(Operand1) / Double.valueOf(Operand2);
break;
}
Finally, by convention, Java variables should start with a lower case letter; operand1, operand2 and operator.

public static void main(String[] args){
Scanner sc = new Scanner(System.in);
System.out.println("Enter first number");
float a = sc.nextFloat();
System.out.println("Enter second number");
float b = sc.nextFloat();
System.out.println("choose your operation");
char operator = sc.next().charAt(0);
float answer;
switch (operator){
case '+' :
answer = a +b;
System.out.println("Answer:"+answer);
break;
case '-' :
answer = a-b;
System.out.println("Answer:"+answer);
break;
case '*' :
answer = a*b;
System.out.println("Answer:"+answer);
break;
default:
answer = a/b;
System.out.println("Answer:"+answer);
}
}

Postfix Evaluation using Stacks.

I'm going insane..I'm so close to getting this code to work the way I want to I just can't figure it out. I'm trying to solve a postfix equation for ex. 3 2 + , this equals 5. When I put for example
"3 2 +" in the mainmethod it works fine but as soon as I enter a 3rd digit like "3 2 + 2 *" (which equals 10) I get a arrayoutofboundserror relating back to number2 = s.pop() as you will see in the code below. Any help is greatly appreciated.
Heres the postfix meethod:
public int PostfixEvaluate(String e){
int number1;
int number2;
int result=0;
String[] tokens = e.split(" ");
for(int j = 0; j < tokens.length; j++){
String token = tokens[j];
if (!"+".equals(token) && !"*".equals(token) && !"-".equals(token) && !"/".equals(token)) {
s.push(Integer.parseInt(token));
} else {
String Operator = tokens[j];
number1 = s.pop();
number2 = s.pop();
if (Operator.equals("/")){
result = number1 / number2;}
else if(Operator.equals("*")){
result = number1 * number2;}
else if(Operator.equals("+")){
result = number1 + number2;}
else if(Operator.equals("-")){
result = number1 - number2;}
else System.out.println("Illeagal symbol");
}
s.push(result);
s.pop();
}
//s.pop();
System.out.println("Postfix Evauation = " + result);
return result;
}
public static void main(String[] args) {
Stacked st = new Stacked(100);
//String y = new String("((z * j)/(b * 8) ^2");
String x = new String("2 2 2 * +");
TestingClass clas = new TestingClass(st);
//clas.test(y);
clas.PostfixEvaluate(x);
}
}

/**
* Evaluate postfix arithmetic expression
*
* #example "1 12 23 + * 4 5 / -" => 34.2
* #author Yong Su
*/
import java.util.Stack;
class PostfixEvaluation {
public static void main(String[] args) {
String postfix = "1 12 23 + * 4 5 / -";
Double value = evaluate(postfix);
System.out.println(value);
}
/**
* Evaluate postfix expression
*
* #param postfix The postfix expression
*/
public static Double evaluate(String postfix) {
// Use a stack to track all the numbers and temporary results
Stack<Double> s = new Stack<Double>();
// Convert expression to char array
char[] chars = postfix.toCharArray();
// Cache the length of expression
int N = chars.length;
for (int i = 0; i < N; i++) {
char ch = chars[i];
if (isOperator(ch)) {
// Operator, simply pop out two numbers from stack and perfom operation
// Notice the order of operands
switch (ch) {
case '+': s.push(s.pop() + s.pop()); break;
case '*': s.push(s.pop() * s.pop()); break;
case '-': s.push(-s.pop() + s.pop()); break;
case '/': s.push(1 / s.pop() * s.pop()); break;
}
} else if(Character.isDigit(ch)) {
// Number, push to the stack
s.push(0.0);
while (Character.isDigit(chars[i]))
s.push(10.0 * s.pop() + (chars[i++] - '0'));
}
}
// The final result should be located in the bottom of stack
// Otherwise return 0.0
if (!s.isEmpty())
return s.pop();
else
return 0.0;
}
/**
* Check if the character is an operator
*/
private static boolean isOperator(char ch) {
return ch == '*' || ch == '/' || ch == '+' || ch == '-';
}
}

The number1 assignment should be after the number2 assignment. Remember that s.pop() will remove and return the number that is on the top.
number2 = s.pop();
number1 = s.pop();

Are you popping immediately after pushing?
s.push(result);
s.pop();

There is another logical error in this solution. You need to do:
number2 = s.pop();
number1 = s.pop();
your solution won't work if you had 32/ because you will evaluate it to 2/3.