Question

Hi it's the java stack to convert from infix to postfix

If I want to fix this code to calculate each numbers not just displaying the numbers what should I do?

Would you help me to fix the result?

import java.util.Stack;
import java.util.Scanner;
class Main
{
//Function to return precedence
static int Prec(char ch)
{
switch (ch)
{
case '+':
case '-':
return 1;

case '*':
case '/':
return 2;

case '^':
return 3;
}
return -1;
}
static String infixToPostfix(String exp)
{
// initializing empty String for result
String result = new String("");
Stack<Character> stack = new Stack<>();
int i=0;
int len=exp.length();
for ( i = 0; i<len; ++i)
{
char c = exp.charAt(i);
if (Character.isLetterOrDigit(c))
result =result+ c;
else if (c == '(')
stack.push(c);
else if (c == ')')
{
while (!stack.isEmpty() && stack.peek() != '(')
result =result+ stack.pop();
if (!stack.isEmpty() && stack.peek() != '(')
return "Invalid Expression";   
else
stack.pop();
}
else
{
//for operators
while (!stack.isEmpty() && Prec(c) <= Prec(stack.peek()))
result =result+ stack.pop();
stack.push(c);
}
}
while (!stack.isEmpty())
result =result+ stack.pop();

return result;
}
public static void main(String[] args)
{
String exp = "(5+8)*(7-3)";
String exp2="6+7*(3+9)/2";
System.out.println("Infix "+exp);
System.out.println("Postfix "+infixToPostfix(exp));
System.out.println("Infix "+exp2);
System.out.println("Postfix "+infixToPostfix(exp2));
}
}

Answer 1

import java.util.Stack;
import java.util.Scanner;
class Main
{
//Function to return precedence
static int Prec(char ch)
{
switch (ch)
{
case '+':
case '-':
return 1;

case '*':
case '/':
return 2;

case '^':
return 3;
}
return -1;
}
// Method to evaluate value of a postfix expression
static int evaluatePostfix(String exp)

{

//create a stack

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

int val3,val4,val5,val6;

// Scan all characters one by one

for(int i=0;i<exp.length();i++)

{

char c=exp.charAt(i);

// If the scanned character is an operand (number here),

// push it to the stack.

if(Character.isDigit(c))

stack.push(c - '0');

// If the scanned character is an operator, pop two

// elements from stack apply the operator

else

{

int val1 = stack.pop();

int val2 = stack.pop();

switch(c)

{

case '+':

val3=val1+val2;

stack.push(val3);

pos(stack,i+1,val3,exp);

break;

case '-':

val4=val2-val1;

stack.push(val4);

pos(stack,i+1,val4,exp);

break;

case '/':

val5=val2/val1;

stack.push(val5);

pos(stack,i+1,val5,exp);

break;

case '*':

val6=val1*val2;

stack.push(val6);

pos(stack,i+1,val6,exp);

break;

}

}

}

return stack.pop();   

}

static int pos(Stack<Integer> stack,int m ,int val3,String exp){

int r=0;

int[] arr= new int[100];

System.out.printf("\n");

while(!stack.isEmpty())

{

arr[r]=stack.pop();

r++;

}

for(int j=r-1;j>=0;j--)

{

System.out.print(" "+arr[j]);

}

for(int k=r-1;k>=0;k--)

{

stack.push(arr[k]);

arr[k]=0;

}

//System.out.println(val3);

for(int l=m;l<exp.length();l++) {

char c=exp.charAt(l);

System.out.print(" "+c);

}
System.out.printf("\n");

return 0;

}
static String infixToPostfix(String exp)
{
// initializing empty String for result
String result = new String("");
Stack<Character> stack = new Stack<>();
int i=0;
int len=exp.length();
for ( i = 0; i<len; ++i)
{
char c = exp.charAt(i);
if (Character.isLetterOrDigit(c))
result =result+ c;
else if (c == '(')
stack.push(c);
else if (c == ')')
{
while (!stack.isEmpty() && stack.peek() != '(')
result =result+ stack.pop();
if (!stack.isEmpty() && stack.peek() != '(')
return "Invalid Expression";   
else
stack.pop();
}
else
{
//for operators
while (!stack.isEmpty() && Prec(c) <= Prec(stack.peek()))
result =result+ stack.pop();
stack.push(c);
}
}
while (!stack.isEmpty())
result =result+ stack.pop();

return result;
}
public static void main(String[] args)
{
String exp = "(5+8)*(7-3)";
String exp2="6+7*(3+9)/2";
System.out.println("Infix:"+exp);
System.out.println("Postfix expression:"+infixToPostfix(exp));
System.out.println("postfix evaluation: "+evaluatePostfix(infixToPostfix(exp)));
System.out.printf("\n");
System.out.println("Infix:"+exp2);
System.out.println("Postfix expression:"+infixToPostfix(exp2));
System.out.println("postfix evaluation: "+evaluatePostfix(infixToPostfix(exp2)));
}
}

OUTPUT:

Infix:(5+8)*(7-3)
Postfix expression:58+73-*

13 7 3 - *

13 4 *

52
postfix evaluation: 52

Infix:6+7*(3+9)/2
Postfix expression:6739+*2/+

6 7 12 * 2 / +

6 84 2 / +

6 42 +

48
postfix evaluation: 48