Styjun

I am setting up a method that turn a infix string into a postfix equation with a custom LinkStack.

I have tried to to check if the charAt(i) was null and a if statement to check if i is greater than exp.length() but neither worked.

public static String infixToPostfix(String exp) 
 
 // make variable
 String result = new String(""); 
 int temp = 0;
 LinkedStack stack = new LinkedStack();

 for (int i = 0; i<exp.length(); ++i) 
 
 char c = exp.charAt(i); 
 if(Character.isDigit(c)) 
 
 int n = 0; 

 //extract the characters and store it in num 
 while(Character.isDigit(c)) 
 
 n = n*10 + (int)(c-'0'); 
 i++;
 c = exp.charAt(i); //exception occurs
 System.out.println(n);
 
 i--; 

 //push the number in stack 
 stack.push(n); 
 //System.out.println(stack.size() + ", Stack size");
 

 // If ( push it to the stack. 
 if (c == '(') 
 stack.push(c); 

 // If ) pop and output from the stack 
 // until an '(' is encountered. 
 else if (c == ')') 
 
 while (!stack.isEmpty() && stack.peek() != '(') 
 result += stack.pop(); 

 if (!stack.isEmpty() && stack.peek() != '(') 
 return "Invalid Expression"; // invalid expression 
 else
 stack.pop(); 
 
 else // an operator is encountered 
 
 while (!stack.isEmpty() && pre(c) <= pre((char) stack.peek()))
 result += stack.pop(); 
 stack.push(c); 
 

 

 // pop all the operators from the stack 
 while (!stack.isEmpty()) 
 result += stack.pop(); 

 String temp2 = stack.print();
 System.out.println(temp2);
 return result; 
 

I expect the output to be 469 645 + if the input is 496+645 but the actual output is java.lang.StringIndexOutOfBoundsException: String index out of range: 7.

 while(Character.isDigit(c)) 
 
 n = n*10 + (int)(c-'0'); 
 i++;
 c = exp.charAt(i); //exception occurs
 System.out.println(n);
 

You aren't length checking here, so you readily parse right off the end of the string.

 while(i < exp.length() && Character.isDigit(c)) 
 
 n = n*10 + (int)(c-'0'); 
 if (++i < exp.length()) 
 c = exp.charAt(i); //exception occurs
 
 System.out.println(n);
 

Note: I'd cache the length because of how many times you use it, but that's not the cause of your problem.

Note, however, that this is cleaner code style:

public class Foo 
 public static void main(String[] args) 
 String myString = "12345";
 int index = 0;
 for (char c: myString.toCharArray()) 
 System.out.printf("Char at %d == %cn", index, c);
 ++index;
 
 

Notice the for-loop. I didn't do your calculations or break out or anything, but this is a cleaner way.

You can also do...

for (int index = 0; index < exp.length(); ++index) 
 char c = exp.charAt(index);
 if (!Character.isDigit(c)) 
 break;
 
 // Do other stuff here.

There are a variety of other ways to structure your code. Your while loop is awkward.