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POConverting expressions from Prefix to Postfix ( Facing Problems with Precedence)
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copied!<p>I have been attempting to write a code that converts Prefix expressions to Postfix expressions.</p> <p>So far here's what I have made (note that I am new and hence may not be efficient!) </p> <pre><code>/*************************************************** NOTE: This code fails to realise the precedence of parentheses and $ (exponential) expressions ****************************************************/ import java.util.Scanner; class Stack{ //Private Declarations private int top; private String a[] = new String [100]; //Public Declarations public Stack(){ top = -1; a[0] = "\0"; } public void push(String x){ if (top != 99){ a[++top] = x; } else{ System.out.println("Stack overflow"); } }; public String pop(){ if (top == -1){ System.out.println("\nStack empty!"); return "\n"; } else{ return a[top--]; } }; public int ret_top(){ return top; }; } public class Prefix2Postfix { public static void main(String[] args) { //Declaration Scanner in = new Scanner (System.in); Stack op = new Stack (); Stack sym = new Stack (); char ch; int i; String exp, str, str1, str2, str3; //Taking input from the user System.out.println("Enter the prefix expression : "); exp = in.next(); for (i=0; i<exp.length(); i++){ ch = exp.charAt(i); if((ch == '+') ||(ch == '-') ||(ch == '*') ||(ch == '/') ||(ch == '$')){ str = Character.toString(ch); op.push(str); } else{ str = Character.toString(ch); sym.push(str); if (sym.ret_top() == 1){ str1 = sym.pop(); str2 = sym.pop(); str3 = op.pop(); str1 = str2.concat(str1); str1 = str1.concat(str3); sym.push(str1); } } } //Output str = sym.pop(); System.out.println("After conversion to postfix" + ": " + str); in.close(); } } </code></pre> <p>As I have already commented in the code, my code fails to realize the precedence in the case of prefix expressions.</p> <p>For example:</p> <pre><code>Infix : a + (b*c) Prefix : +a*bc Postfix : abc*+ //Expected output Output I get : ab*c+ </code></pre> <p>Is something wrong in the logic I am using or is there something I need to add? Can you please suggest some method by means of which my code can work properly? If anybody can help I will be grateful.</p> <p>NB: Our professor has advised us to write our own stack class and code. Please also note that this is kind of a homework assignment.</p> <p>Thanks in advance.</p> <hr> <p><strong>Edit:</strong> In order to make sure that my STACK works fine I wrote the following piece of code and it works perfectly alright.</p> <pre><code>import java.util.Scanner; class STACK { //Private Declarations private int top; private int elem[] = new int [100]; //Public Declarations public STACK (){ top = -1; elem[0] = 0; } public void push(int x){ if (top != 99){ elem[++top] = x; } else{ System.out.println("Stack overflow"); } }; public int pop(){ if (top == -1){ System.out.println("Stack empty!"); return 0; } else{ return elem[top--]; } }; } public class StackPushPop { public static void main(String[] args) { STACK s = new STACK(); Scanner in = new Scanner (System.in); int choice, x; do{ System.out.println("Menu Options :"); System.out.println("1 -> Push an element"); System.out.println("2 -> Pop an element"); System.out.println("3 -> Empty complete stack"); System.out.println("Any other input for exit"); System.out.println("Your choice : "); choice = in.nextInt(); switch(choice){ case 1: System.out.println("\nEnter element : "); x = in.nextInt(); s.push(x); break; case 2: System.out.print("\nPopping element : "); x = s.pop(); if (x != 0){ System.out.println(x); } break; case 3: System.out.println("\nEmptying stack!"); x = 1; while (x!= 0){ x = s.pop(); if(x != 0){ System.out.print(x + " "); } } break; default: choice = 0; } }while (choice != 0); } } </code></pre> <hr> <p><strong>EDIT</strong> I finally managed to create a program that works perfectly fine.</p> <pre><code>import java.util.Scanner; class STACK{ private int top, MAX; private String a[] = new String [1000]; public STACK(){ top = -1; MAX = 1000; a[0] = ""; } public void push(String x){ if (top <= MAX-1){ a[++top] = x; } else{ System.out.println("Stack overflow"); } }; public String pop(){ if (top == -1){ System.out.println("\nStack empty!"); return "\n"; } else{ return a[top--]; } }; public int getTop(){ return top; }; } public class Prefix2Postfix_STACK{ static boolean isOperator (char ch){ switch (ch){ case '+': case '-': case '*': case '/': case '$': return true; default : return false; } } public static void main(String[] args) { //declarations Scanner in = new Scanner (System.in); String exp; int i; STACK s = new STACK (); String exp_str[] = new String[100]; String postfix_exp = "\n"; //input System.out.println("Enter prefix expression (No spaces or brackets) : "); exp = in.next(); //create a string array of all characters but in reverse for(i=0; i<=exp.length()-1; i++){ exp_str[exp.length()-1-i]=Character.toString(exp.charAt(i)); } //computing postfix: i=0; do{ if (!isOperator(exp_str[i].charAt(0))) s.push(exp_str[i]); else{ String str1 = s.pop(); String str2 = s.pop(); str1 = str1 + str2 + exp_str[i]; postfix_exp = str1; s.push(str1); } i++; }while(s.getTop()>=0 && i!=exp.length()); //Output System.out.println("After converting to postfix : " + postfix_exp); in.close(); } } </code></pre>

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