Postfix to infix

Question

Can someone please help. I am a little confused on how to convert an infix expression (ex. ab*cde/-+) to a postfix expression (ex. a*b+(c-d/e)). I don't expect you to write the code for me, although that would be nice, I just need to understand the logic behind this.
Thanks,
Tiffanee

Answer 1

Here is what I have. there is an error when reading in token by token. if I type as a postfix epr. 12+ it outputs
(0 + 12). For some reason 12 instead of 1 as the first token. I don't know whats wrong.
public class InfixParser {
static final int END = 0, VALUE = 1, LPAREN = 2, RPAREN = 3, EXP = 4, MULT = 5, DIV = 6, PLUS = 7, MINUS = 8;
private StringTokenizer str;
private String curValue;
private Stack infix;
private String infixValue; //the infix representation of a operation
public String getInfixValue()
return infixValue;
//constructor
public InfixParser(String s)
infix = new StackAr();
str = new StringTokenizer(s, "+-*/^() ", true);
infix.push(new Integer(END));
//converts from Postfix form to Infix form
public void toInfix () {
int newToken=0;
try {
do {
newToken = getToken(); //get the next token in the string
switch(newToken) {
//if the token is a variable or number, push onto infix stack
case VALUE:
System.out.println("curValue: "+curValue);
infix.push(curValue);
break;
case LPAREN:
String a="(-";
newToken = getToken();
//if the value inside the ( is positive, push onto stack
if (newToken != MINUS) {
a="("+curValue+")";
infix.push(a);
break;
//if the value is negative, push the negative onto stack
newToken = getToken();
a= a + curValue + ")";
infix.push(a);
break;
//if it is end of string, infixValue is the top of the stack
case END:
infixValue=(String)infix.top();
break;
case RPAREN:
break;
//if it is an operator, pop the top two objects from stack,
//put the toString form of the operator between them, and push
//back onto stack
case EXP:
case DIV:
case MULT:
case PLUS:
case MINUS:
Object var1=infix.topAndPop();
Object var2=infix.topAndPop();
infixValue="("+var2+" "+enumToString(newToken)+" "+var1+")";
infix.push(infixValue);
break;
default:
System.out.println("ERROR: Invalid Token" + newToken);
} while (newToken != END);
catch (Exception e)
System.out.println("ERROR: " + e);
//the string representation of the different operators
private String enumToString(int op) {
switch(op) {
case EXP: return "^";
case DIV: return "/";
case MULT:return "*";
case PLUS:
return "+";
case MINUS:
return "-";
case END:
return "";
default:
return "?";
//get the next token in the string
private int getToken() {
String s = "";
try
s= str.nextToken();
catch(java.util.NoSuchElementException e) {
return END;
if(s.equals(" ")) {
return getToken();
if(s.equals("^")) {
return EXP;
if(s.equals("/")) {
return DIV;
if(s.equals("*")) {
return MULT;
if(s.equals("(")) {
return LPAREN ;
if(s.equals(")")) {
return RPAREN;
if(s.equals("+")) {
return PLUS;
if(s.equals("-")) {
return MINUS ;
//if the token is none of the above, it must be a variable or number
//so the curVal becomes whatever the token is and VALUE is returned
curValue = s;
return VALUE;
//main method used to test InfixParser by itself
public static void main(String[] args) {
String str = "";
String quit = "q";
BufferedReader in = new BufferedReader (new
InputStreamReader(System.in));
try {
while (str.compareTo(quit) != 0) {
System.out.print("Enter Postfix expression or q to Exit: ");
if ((str = in.readLine()) == null) {
break;
System.out.print("Expression: " + str + "\n");
InfixParser par = new InfixParser(str);
par.toInfix();
System.out.print("InFix Expression: "+par.infixValue);
System.out.println();
catch (IOException e) {
System.out.println("ERROR" + e);