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so I have a Stack that I built and I have a Machine to evalute expressions like, ( 9 + 0 ) and they can be more complex. I run it fun the commandline and then when I type the example ( 9 + 5 ) the program just sits there. I can get a new line but the expression doesn’t evaluate. So My question is what did I miss. I am sure there is something that I haven’t understood correctly and I was thinking that I am missing something about the Scanner or about arrays in Java in general.
Perhaps I was thinking last night that I should replace arrays with ArrayList. Does this make sense?
Here is the Fixed Capacity Stack
public class FCStack<Item> {
private Item[] a;
private int top; // pointer to top of Stack
private int capacity; // size of the Stack+1
public FCStack(int cap){
capacity = cap;
a = (Item[]) new Object[capacity];
top = 0;
}
public void push(Item i){ //will only push an Item to the Stack if there is room.
if (!isFull()) {
a[top++] = i;
}
}
public Item pop(){ //will only pop an Item from the stack if there is something to pop.
if (!isEmpty()) {
--top;
}
return a[top];
}
public boolean isFull(){ //returns true if is full
return top == capacity;
}
public boolean isEmpty(){ //returns true if is empty
return top == 0;
}
public int size(){ //returns the current size of the stack+1 or the array index
return top;
}
}
Here is the two stack Evaluator
import java.io.*;
import java.util.Scanner;
public class TwoStackMaths {
public static void main (String[] args) {
FCStack<String> ops = new FCStack<String>(10);
FCStack<Double> vals = new FCStack<Double>(10);
Scanner console = new Scanner(System.in);
while(console.hasNext()) {
String str = console.next();
if (str.equals("("))
;
else if (str.equals("+")) {
ops.push(str);
}
else if (str.equals("-")) {
ops.push(str);
}
else if (str.equals("*")) {
ops.push(str);
}
else if (str.equals("/")) {
ops.push(str);
}
else if (str.equals("^")) {
ops.push(str);
}
else if (str.equals(")")) {
String op = ops.pop();
double v = vals.pop();
if (op.equals("+")) {
v = vals.pop() + v;
}
else if (op.equals("-")) {
v = vals.pop() - v;
}
else if (op.equals("*")) {
v = vals.pop() * v;
}
else if (op.equals("/")) {
v = vals.pop() / v;
}
else if (op.equals("^")) {
v = Math.pow(v, vals.pop());
}
vals.push(v);
}
else {
vals.push(Double.parseDouble(str));
}
}
//console.close();
System.out.println(vals.pop());
}
}
Answer
Your code worked for me; I did change it to use an ArrayList, and I added a peek like this
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
public class FCStack<T> {
public static void main(String[] args) {
FCStack<String> ops = new FCStack<String>(10);
FCStack<Double> vals = new FCStack<Double>(10);
Scanner console = new Scanner(System.in);
try {
while (console.hasNext()) {
String str = console.next().trim();
if (str.equals(".")) {
System.out.println(vals.peek());
} else if (str.equals("(")) {
;
} else if (str.equals("+")) {
ops.push(str);
} else if (str.equals("-")) {
ops.push(str);
} else if (str.equals("*")) {
ops.push(str);
} else if (str.equals("/")) {
ops.push(str);
} else if (str.equals("^")) {
ops.push(str);
} else if (str.equals(")")) {
String op = ops.pop();
double v = vals.pop();
if (op.equals("+")) {
v = vals.pop() + v;
} else if (op.equals("-")) {
v = vals.pop() - v;
} else if (op.equals("*")) {
v = vals.pop() * v;
} else if (op.equals("/")) {
v = vals.pop() / v;
} else if (op.equals("^")) {
v = Math.pow(v, vals.pop());
}
vals.push(v);
} else {
vals.push(Double.parseDouble(str));
}
}
} finally {
console.close();
}
}
private List<T> a;
private int top; // pointer to top of FCStack
private int capacity; // size of the FCStack+1
public FCStack(int cap) {
capacity = cap;
a = new ArrayList<T>();
top = 0;
}
public void push(T i) { // will only push an Item to
// the FCStack if there is room.
if (!isFull()) {
a.add(i);
++top;
}
}
public T pop() { // will only pop an Item from the
// stack if there is something to pop.
if (!isEmpty()) {
return a.remove(--top);
}
return null;
}
public T peek() {
if (!isEmpty()) {
return a.get(top - 1);
}
return null;
}
public boolean isFull() { // returns true if is full
return top > capacity;
}
public boolean isEmpty() { // returns true if is empty
return top == 0;
}
public int size() { // returns the current size of the
// stack+1 or the array index
return top;
}
}
Tested like so
( 12.0 * 3.0 ) . 36.0
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