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String Index Exception

I was trying to convert an expression from infix form to postfix form.I used String a, p , s for stack, postfix result expression, input expression respectively.
Every time I am getting this error:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException:
String index out of range: -1 at
java.lang.String.charAt(String.java:658) at
javaapplication4.A.conversion(A.java:50) at
javaapplication4.A.main(A.java:83)
Please help me how can I solve it.
Here is my code:
import java.util.Scanner;
public class A {
String a="(", s = "", p = "";
int i, n = 1, top = 0, pp = 0;
void push(char ch) {
a = a + ch;
top = n;
n++;
}
void pop() {
n--;
top--;
}
int prio(char ch) {
int f = -1;
if (ch == '(') {
f = 0;
} else if (ch == '+' || ch == '-') {
f = 1;
} else if (ch == '*' || ch == '/' || ch == '%') {
f = 2;
} else if (ch == '^') {
f = 3;
}
return f;
}
void conversion() {
System.out.print("Enter infix form: ");
Scanner sd = new Scanner(System.in);
s = sd.nextLine();
//System.out.println(s);
int t, j, sz;
sz = s.length();
for (i = 0; i < sz; i++) {
if (s.charAt(i) >= '0' && s.charAt(i) <= '9') {
p = p + s.charAt(i);
pp++;
} else if (s.charAt(i) == '(') {
push('(');
} else if (s.charAt(i) == '-' || s.charAt(i) == '+' || s.charAt(i) == '*' || s.charAt(i) == '/' || s.charAt(i) == '%' || s.charAt(i) == '^') {
j = prio(s.charAt(i));
t = prio(a.charAt(top));
//System.out.println(t+" "+j);
while (j <= t) {
p = p + a.charAt(top);
pp++;
pop();
t = prio(a.charAt(top));
}
push(s.charAt(i));
} else if (s.charAt(i) == ')') {
while (a.charAt(top) != '(') {
p = p + a.charAt(top);
pp++;
pop();
}
pop();
}
}
while (a.charAt(top) != '(') {
p = p + a.charAt(top);
pp++;
pop();
}
pop();
}
void postfix() {
System.out.print("postfix form is: ");
System.out.println(p);
}
public static void main(String args[]) {
A h = new A();
h.conversion();
h.postfix();
//System.out.println(h.a);
//System.out.println(h.s);
}
}

Can you confirm if the error is here?
while (a.charAt(top) != '(')
Inside the loop you continuously pop(), which decrements from top, which has the risk of going negative if a match is never found. Even if the error is not here, it should check for that condition.

You may have made an extra pop() definition. Can you check this?

Changing case of words (all upper- and camel-case)

So, I'm a complete novice, and I've been having trouble with this for a while. I have to edit this file so that all the vowels and punctuation are gone, and so that the first and last sentence is capitalized, along with the first letter of every word. I've got the first half done, but the second part is giving me trouble.
Here's what I've got so far:
import java.io.*;
import java.io.File;
import java.io.FileNotFoundException;
public class SMcatcherInTheIO
{
public static void main (String [] args) throws Exception
{
File catcher= new File ("C:\\Users\\suvra\\Dropbox\\APCompSci 17-18\\AAA SM EXTRA CREDIT\\RyeCh1.txt");
BufferedReader br=new BufferedReader (new FileReader(catcher));
String rye;
while((rye=br.readLine()) !=null)
{
for (int i = 0; i < rye.length(); i++)
{
char c = rye.charAt(i);
if ((c == 'A') || (c == 'a') || (c == 'E') || (c == 'e') || (c == 'I') || (c == 'i') || (c == 'O') || (c == 'o') || (c == 'U') || (c == 'u')
|| (c == ',') || (c == '.') || (c == '?') || ( c == '!'))
{
String front = rye.substring(0, i);
String back = rye.substring(i + 1);
rye = front + "" + back;
}
}
System.out.println(rye);
}
br.close();
}}

Create two String objects :Last and First sentence. Than Loop trought you code and stop at the first point and add value to firstSentence. Do similar for the last sentence just stop at the point-1. After that:
String lastSentence=somethingalreadydoneabove;
String upperCase=lastSentence.toUpperCase();
Hope this would help you.
DK

How to use variables in stack?

I'm trying to write a calc program that finds the infix. In addition the user will input numbers for the x variable and the program will solve it. My program works but it only solves it the first time. The following times it gives the same answer as the first time.
import java.util.Scanner;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.LinkedList;
class Stack {
char a[] = new char[100];
int top = -1;
void push(char c) {
try {
a[++top] = c;
} catch (StringIndexOutOfBoundsException e) {
System.out.println("Stack full , no room to push , size=100");
System.exit(0);
}
}
char pop() {
return a[top--];
}
boolean isEmpty() {
return (top == -1) ? true : false;
}
char peek() {
return a[top];
}
}
public class intopost {
static Stack operators = new Stack();
public static void main(String argv[]) throws IOException {
String infix;
// create an input stream object
BufferedReader keyboard = new BufferedReader(new InputStreamReader(
System.in));
// get input from user
System.out.print("\nEnter the algebraic expression in infix: ");
infix = keyboard.readLine();
String postFx = toPostfix(infix);
// output as postfix
System.out.println("The expression in postfix is:" + postFx);
if (postFx.contains("x")) {
String line = "";
do {
System.out.println("Enter value of X : ");
line = keyboard.readLine();
if (!"q".equalsIgnoreCase(line)) {
postFx = postFx.replaceAll("x", line);
System.out.println("Answer to expression : "
+ EvaluateString.evaluate(postFx));
}
} while (!line.equals("q"));
} else {
System.out.println("Answer to expression : "
+ EvaluateString.evaluate(postFx));
}
}
private static String toPostfix(String infix)
// converts an infix expression to postfix
{
char symbol;
String postfix = "";
for (int i = 0; i < infix.length(); ++i)
// while there is input to be read
{
symbol = infix.charAt(i);
// if it's an operand, add it to the string
if (symbol != ' ') {
if (Character.isLetter(symbol) || Character.isDigit(symbol))
postfix = postfix + " " + symbol;
else if (symbol == '(')
// push (
{
operators.push(symbol);
} else if (symbol == ')')
// push everything back to (
{
while (operators.peek() != '(') {
postfix = postfix + " " + operators.pop();
}
operators.pop(); // remove '('
} else
// print operators occurring before it that have greater
// precedence
{
while (!operators.isEmpty() && !(operators.peek() == '(')
&& prec(symbol) <= prec(operators.peek()))
postfix = postfix + " " + operators.pop();
operators.push(symbol);
}
}
}
while (!operators.isEmpty())
postfix = postfix + " " + operators.pop();
return postfix.trim();
}
static int prec(char x) {
if (x == '+' || x == '-')
return 1;
if (x == '*' || x == '/' || x == '%')
return 2;
return 0;
}
}
class EvaluateString {
public static int evaluate(String expression) {
char[] tokens = expression.toCharArray();
// Stack for numbers: 'values'
LinkedList<Integer> values = new LinkedList<Integer>();
// Stack for Operators: 'ops'
LinkedList<Character> ops = new LinkedList<Character>();
for (int i = 0; i < tokens.length; i++) {
// Current token is a whitespace, skip it
if (tokens[i] == ' ')
continue;
// Current token is a number, push it to stack for numbers
if (tokens[i] >= '0' && tokens[i] <= '9') {
StringBuffer sbuf = new StringBuffer();
// There may be more than one digits in number
while (i < tokens.length && tokens[i] >= '0'
&& tokens[i] <= '9')
sbuf.append(tokens[i++]);
values.push(Integer.parseInt(sbuf.toString()));
}
// Current token is an opening brace, push it to 'ops'
else if (tokens[i] == '(')
ops.push(tokens[i]);
// Closing brace encountered, solve entire brace
else if (tokens[i] == ')') {
while (ops.peek() != '(')
values.push(applyOp(ops.pop(), values.pop(), values.pop()));
ops.pop();
}
// Current token is an operator.
else if (tokens[i] == '+' || tokens[i] == '-' || tokens[i] == '*'
|| tokens[i] == '/') {
// While top of 'ops' has same or greater precedence to current
// token, which is an operator. Apply operator on top of 'ops'
// to top two elements in values stack
while (!ops.isEmpty() && hasPrecedence(tokens[i], ops.peek()))
values.push(applyOp(ops.pop(), values.pop(), values.pop()));
// Push current token to 'ops'.
ops.push(tokens[i]);
}
}
// Entire expression has been parsed at this point, apply remaining
// ops to remaining values
while (!ops.isEmpty())
values.push(applyOp(ops.pop(), values.pop(), values.pop()));
// Top of 'values' contains result, return it
return values.pop();
}
// Returns true if 'op2' has higher or same precedence as 'op1',
// otherwise returns false.
public static boolean hasPrecedence(char op1, char op2) {
if (op2 == '(' || op2 == ')')
return false;
if ((op1 == '*' || op1 == '/') && (op2 == '+' || op2 == '-'))
return false;
else
return true;
}
// A utility method to apply an operator 'op' on operands 'a'
// and 'b'. Return the result.
public static int applyOp(char op, int b, int a) {
switch (op) {
case '+':
return a + b;
case '-':
return a - b;
case '*':
return a * b;
case '/':
if (b == 0)
throw new UnsupportedOperationException("Cannot divide by zero");
return a / b;
}
return 0;
}

There is a mistake inside while loop in main method. See snippet below.
postFx = postFx.replaceAll("x", line);
System.out.println("Answer to expression : "
+ EvaluateString.evaluate(postFx));
Here postFx = postFx.replaceAll("x", line); you lost reference to postfix form that contains variable x. Subsequent calls of replaceAll doesn't have any effect. So expression with first entered value is evaluated.
You can easily fix it by replacing code above with
System.out.println("Answer to expression : "
+ EvaluateString.evaluate(postFx.replaceAll("x", line)));

Final return statement, ignore

Im working on a token iterator (valid tokens, "true, false, "true", "&", "!", "(", "false", "^", "true", ")".
The code is working, my question is about return values. I often run into this problem, I have return statements, but the final return statement throws off my result by duplicating the last return statement.
I think I know for sure the error lays within my placement of { and } and while i've learned they aren't necessary, since there's so many nested if's i feel they are necessary.
This seems to be a common problem to me and others ive worked with, does anyone have an idea of how to prevent this problem from happening? Thanks!
My code outputs:
line: [ ! BAD (true ^ false) % truelybad]
next token: [!]
next token: [(]
next token: [true]
next token: [^]
next token: [false]
next token: [)]
next token: [)]
and should output
next token: [!]
next token: [(]
next token: [true]
next token: [^]
next token: [false]
next token: [)]
public class TokenIter implements Iterator<String> {
ArrayList<String> token = new ArrayList<String>();
static int count = 0;
// input line to be tokenized
private String line;
// the next Token, null if no next Token
private String nextToken;
// implement
public TokenIter(String line) {
this.line = line;
}
#Override
// implement
public boolean hasNext() {
// System.out.println(count);
return count < line.length();
}
#Override
// implement
public String next() {
while (hasNext()) {
char c = line.charAt(count);
if (c == '!' || c == '!' || c == '^' || c == '(' || c == ')') {
token.add(Character.toString(c));
count++;
nextToken = Character.toString(c);
return nextToken;
} else if (c == 't' || c == 'T') {
count++;
c = line.charAt(count);
if (c == 'r') {
count++;
c = line.charAt(count);
}
if (c == 'u') {
count++;
c = line.charAt(count);
}
if (c == 'e') {
count++;
c = line.charAt(count);
}if (c == ' ' || c == '!' || c == '!' || c == '^' || c == '(' || c == ')'){
token.add("true");
nextToken = "true";
//count++;
return nextToken;
}
} else if (c == 'f' || c == 'F') {
count++;
c = line.charAt(count);
if (c == 'a') {
count++;
c = line.charAt(count);
}
if (c == 'l') {
count++;
c = line.charAt(count);
}
if (c == 's') {
count++;
c = line.charAt(count);
}
if (c == 'e') {
count++;
c = line.charAt(count);
}
if (c == ' ' || c == '!' || c == '!' || c == '^' || c == '(' || c == ')'){
token.add("false");
nextToken = "false";
// count++;
return nextToken;
}
} else if (c == ' ') {
count++;
} else {
count++;
}
}
return nextToken;
}
#Override
// provided, do not change
public void remove() {
// TODO Auto-generated method stub
throw new UnsupportedOperationException();
}
// provided
public static void main(String[] args) {
String line;
// you can play with other inputs on the command line
if (args.length > 0)
line = args[0];
// or do the standard test
else
line = " ! BAD (true ^ false) % truelybad";
System.out.println("line: [" + line + "]");
TokenIter tokIt = new TokenIter(line);
while (tokIt.hasNext())
System.out.println("next token: [" + tokIt.next() + "]");
}
}

Problem with your code comes only when last digit is not a token.
Reason - You are checking hasNext() which is true it goes inside your code.You are not setting nextToken for this case so it uses your lask token and display it.
I updated your code to always return a value and check if value return is from token list then display otherwise ignore it.
public class test implements Iterator<String> {
static List<String> tokenList = Arrays.asList( "true", "&", "!", "(", "false", "^", "true", ")");
ArrayList<String> token = new ArrayList<String>();
static int count = 0;
// input line to be tokenized
private String line;
// the next Token, null if no next Token
private String nextToken;
// implement
public test(String line) {
this.line = line;
}
#Override
// implement
public boolean hasNext() {
// System.out.println(count);
return count < line.length();
}
#Override
// implement
public String next() {
while (hasNext()) {
char c = line.charAt(count);
if (c == '!' || c == '!' || c == '^' || c == '(' || c == ')') {
token.add(Character.toString(c));
count++;
nextToken = Character.toString(c);
return nextToken;
} else if (c == 't' || c == 'T') {
count++;
c = line.charAt(count);
if (c == 'r') {
count++;
c = line.charAt(count);
}
if (c == 'u') {
count++;
c = line.charAt(count);
}
if (c == 'e') {
count++;
c = line.charAt(count);
}if (c == ' ' || c == '!' || c == '!' || c == '^' || c == '(' || c == ')'){
token.add("true");
nextToken = "true";
//count++;
return nextToken;
}
} else if (c == 'f' || c == 'F') {
count++;
c = line.charAt(count);
if (c == 'a') {
count++;
c = line.charAt(count);
}
if (c == 'l') {
count++;
c = line.charAt(count);
}
if (c == 's') {
count++;
c = line.charAt(count);
}
if (c == 'e') {
count++;
c = line.charAt(count);
}
if (c == ' ' || c == '!' || c == '!' || c == '^' || c == '(' || c == ')'){
token.add("false");
nextToken = "false";
// count++;
return nextToken;
}
} else if (c == ' ') {
count++;
nextToken = null;
} else {
count++;
nextToken = null;
}
}
return nextToken;
}
#Override
// provided, do not change
public void remove() {
// TODO Auto-generated method stub
throw new UnsupportedOperationException();
}
// provided
public static void main(String[] args) {
String line;
// you can play with other inputs on the command line
if (args.length > 0)
line = args[0];
// or do the standard test
else
line = " ! BAD (true ^ false) % truelybad ";
System.out.println("line: [" + line + "]");
test tokIt = new test(line);
while (tokIt.hasNext()) {
String s = tokIt.next();
if (s != null && tokenList.contains(s))
System.out.println("next token: [" + s + "]");
}
}
}

The underlying problem here is that your hasNext() method returns true not if there is another token in the String, but if it hasn't finished parsing the String yet.
So what happens is if you put in the String " ! ! true lotsofcrap ", then calling next() will return "!", then "!", then "true", then after that has been returned, there are no more tokens in the String, yet hasNext() still returns true.
What you might consider doing is having hasNext() parse through the string, but instead of returning the next String, return true only if it finds another token ahead of the current position. Keep in mind that in hasNext(), you do not want to directly increment count. Instead, make a local variable int something = count; at the beginning of hasNext() and use that. If you fix that, then the rest of your code SHOULD work just fine.

Using replace to take a character and change it to another in java

I am trying to figure out how to remove certain characters to make it English after it being in l33t speak. For example, I 54w 3 5hip5, would translate to I saw 3 ships. I need the 3 to stay a 3 here but in, N3v3r f0rg37 y0|_|r t0w31, I would need the 3's to become e's. Here is my code as follows. All the characters translate over correctly, but I just can't figure out how to do the 3's to e's.
My question is, what is needed to be added to get the 3's to be e's at a certain time, and to have my 3's stay 3's another time. Just so that you know, is that we aren't allowed to use regex, arrays, or string builder for this.
Rules are that if the number is supposed to be a number that it stays a number when you translate it from l33t to English, if the l33t number is a letter than you replace the number and turn it into the letter that corresponds to it.
I also have a different block of code that already takes into consideration the 3 to e's, but instead adds two u's instead of one.
Here are the replacements for the letters, a = 4, b = 8, e = 3, l = 1, o = 0, s = 5, t = 7, u = |_|, z = 2.
I decided to go the route of mike's answer since I understand exactly what's going on.
Thanks to everyone for the help!

Input/Output examples
This following code translates
I 54w 3 5hip5
to
I saw 3 ships
and
3 5hip5 4r3 c0ming m3 w4y
to
3 ships are coming me way
Code
public static String translateToEnglish(String phrase) {
if (phrase == null)
return null;
boolean threeAtBeginning = false, threeAtEnd = fal;
if (phrase.charAt(0) == '3' && phrase.charAt(1) == ' ')
threeAtBeginning = true;
int length = phrase.length();
if (phrase.charAt(length - 1) == '3' && phrase.charAt(length - 2) == ' ')
threeAtEnd = true;
String finished = phrase.replace('4', 'a') .replace('1', 'l') .replace('2', 'z') .replace('5', 's') .replace('8', 'b') .replace('0', 'o') .replace('7', 't') .replace("|_|", "u") .replace("3", "e");
finished = finished.replace(" e ", " 3 ");
if (threeAtBeginning)
finished = '3' + finished.substring(1);
if (threeAtEnd)
finished = finished.substring(0, length - 1) + '3';
return finished;
}

This is clearly homework, and the restrictions are clearly intended to prevent any sane solution, but here's an O(n^2) solution that seems to avoid the restrictions:
public class RemoveL33t {
public static void main(String[] args) {
System.out.println(removeL33t("I 54w 3 5hip5"));
System.out.println(removeL33t("I 54w 33 5hip5"));
System.out.println(removeL33t("I 54w 45 5hip5"));
System.out.println(removeL33t("N3v3r f0rg37 y0|_|r t0w31"));
}
public static String removeL33t(String s) {
String result = "";
for (int pos = 0;;) {
// Find the beginning of the next word.
int whitespaceBegin = pos;
while (pos < s.length() && Character.isWhitespace(s.charAt(pos))) {
pos++;
}
// Add the whitespace to the result.
result += s.substring(whitespaceBegin, pos);
// If there is no next word, then we're done.
if (pos >= s.length()) {
return result;
}
// Find the end of the word. Determine if the word is entirely numbers.
int wordBegin = pos;
boolean nonNumber = false;
while (pos < s.length() && !Character.isWhitespace(s.charAt(pos))) {
nonNumber |= s.charAt(pos) < '0' || s.charAt(pos) > '9';
pos++;
}
// Append the word. Perform replacements if it contains a non-number.
if (nonNumber) {
result += s.substring(wordBegin, pos)
.replace('4', 'a')
.replace('8', 'b')
.replace('3', 'e')
.replace('1', 'l')
.replace('0', 'o')
.replace('5', 's')
.replace('7', 't')
.replace("|_|", "u")
.replace('2', 'z');
} else {
result += s.substring(wordBegin, pos);
}
}
}
}

I think this is it.
public static String translateToEnglish(String phrase) {
if (phrase == null) {
return null;
}
String finished = phrase.replace('4', 'a') .replace('1', 'l') .replace('2', 'z') .replace('5', 's') .replace('8', 'b') .replace('0', 'o') .replace('7', 't') .replace("|_|", "u") .replace("3", "e");
finished = finished.replace(" e ", " 3 ");
if(finished.startsWith("e ")){
finished = "3 " + finished.substring(2);
}
if(finished.endsWith(" e")){
finished = finished.substring(0, finished.length()-2) + " 3";
}
return finished;
}

I don't know if this is the answer, but is the best i could think of
public static void main (String[] args) throws java.lang.Exception
{
String c = "I 54w 45 5hip5";
for(String s: c.split(" ")){
try{
Integer.parseInt(s);
System.out.print(s + " ");
}
catch(NumberFormatException e){
s = s.replace('4', 'a').replace('1', 'l').replace('2', 'z').replace('5', 's').replace('8', 'b').replace('0', 'o').replace('7', 't').replace("|_|", "u").replace("3", "e");
System.out.print(s + " ");
}
}
}

This is for your "new" code that you decided to use, or this could just be an alternate solution. The input/output is identical to the samples I gave in my other answer:
public static String translateToEnglish(String phrase) {
if (phrase == null)
return null;
String finished = "";
for (int i = 0; i < phrase.length(); i++) {
char c = phrase.charAt(i);
if (c == '4')
finished += 'a';
else if (c == '3') {
if (i != phrase.length() - 1)
{
if (phrase.charAt(i + 1) == ' ') {
if (i == 0)
finished += c;
else
if (phrase.charAt(i - 1) == ' ')
finished += c;
else
finished += 'e';
}
else
finished += 'e';
}
else
{
if (phrase.charAt(i - 1) == ' ')
finished += c;
else
finished += 'e';
}
} else if (c == '1')
finished += 'l';
else if (c == '2')
finished += 'z';
else if (c == '5')
finished += 's';
else if (c == '7')
finished +='t';
else if (c == '8')
finished += 'b';
else if (c == '0')
finished += 'o';
else if (i + 2 < phrase.length() && phrase.charAt(i + 1) == '_' && phrase.charAt(i + 2) == '|') {
finished += 'u';
i += 2;
} else
finished += c;
}
return finished;
}