Loop is not resetting?

Question

Belowe is code that checks for matching parentheses to see if they are nested properly. It seems really simple, but I don't understand why i does not reset to 0 once a nested match is found. Any guidance would be much appreciated.

    String testString = "{}()[] ";
    char [] openParenthesis = {'(','{','['};
    char [] closeParenthesis = {')','}',']'};

    ArrayList<Character> characterList = new ArrayList<Character>();
    for(char c : testString.toCharArray())
    {
        characterList.add(c);
        System.out.println("This is what is added to the list Arraylist: " + c);
    }

    System.out.println();


for(int i = 0; i < characterList.size()-1; i++)
            {
                System.out.println("1st Loop: " +characterList.get(i));
                System.out.println("1st Loop: " + characterList.get(i + 1));
                System.out.println("1st Loop: " + i);
                System.out.println();

                for(int j = 0; j < openParenthesis.length; j++)
                {
                    if (characterList.get(i) == openParenthesis[j])
                    {
                        if(characterList.get(i + 1) == closeParenthesis[j])
                        {
                            System.out.println("Nested Match");
                            System.out.println(characterList.get(i));
                            System.out.println(characterList.get(i + 1));
                            System.out.println();
                            characterList.remove(i);
                            characterList.remove(i + 1);
                            i = 0;
                        }
                    }   
                }
        }

Answer 1

First off all you are removing the wrong spots in the ArrayList.

Because it is an ArrayList characterList.remove(i); will move everything over to the left one spot, so then the next like characterList.remove(i+1); removes the one to the right of where you want.

You also need to add a break in the openParenthesis loop so you can start the search over at the beginning of the array if you find a match.

You also need to change the i = 0 to i = -1 because it increments it to one BEFORE it starts the next iteration of the for loop.

Finally, you should be using .equals() instead of == because the ArrayList returns a Character object instead of a char .

Here is what I came up with:

for (int i = 0; i < characterList.size() - 1; i++) {
        System.out.println("1st Loop: " + characterList.get(i));
        System.out.println("1st Loop: " + characterList.get(i + 1));
        System.out.println("1st Loop: " + i);
        System.out.println();

        for (int j = 0; j < openParenthesis.length; j++) {
            if (characterList.get(i).equals(openParenthesis[j])) {
                if (characterList.get(i + 1).equals(closeParenthesis[j])) {
                    System.out.println("Nested Match");
                    System.out.println(characterList.get(i));
                    System.out.println(characterList.get(i + 1));
                    System.out.println();
                    characterList.remove(i);
                    characterList.remove(i);
                    i = -1;
                    break;
                }
            }
        }
    }

This code has fixed all the errors mentioned above and should run correctly.

Answer 2

Well it depends on what your testString is. I tested it with value foo() and loop did go inside if(characterList.get(i + 1) == closeParenthesis[j]) condition.

However there is a problem in your code where you have:

characterList.remove(i);
characterList.remove(i + 1);

Which will result in java.lang.IndexOutOfBoundsException since i+1 location has become invalid (out of range) after deleting ith element. It should be other way round:

characterList.remove(i + 1);
characterList.remove(i);

Also instead of i=0; you need to set it to: i = -1;

And more importantly break out of inner for loop by calling break.

So your code in the end should look like this:

i = -1;
break;

See working demo: http://ideone.com/DfGJ2m

Answer 3

Obviously because of the i++ in for loop i equal to 1 . if you really want to set 0 for i again. use i=-1 ;

Answer 4

Your for loop increments i by one AFTER the body of the loop is completed.

Thus, you set i=0 , you exit the body of the loop, i++ is called, and you enter the loop body once more with i ==1 .

Answer 5

我想您可能已经使用了堆栈数据结构，并且使用了类似于http://www.geeksforgeeks.org/check-for-balanced-parentheses-in-an-expression/的过程

Answer 6

When you store a character in a list, java autoboxes it to a Character. You cannot use the == operator to compare two Characters for equality, but must use the .equals() method, as this code demonstrates:

  Character aOne = new Character('a');
  Character aTwo = new Character('a');

  if(aOne == aTwo)
  {
     System.out.println("aOne and aTwo == operator says they are equal");
  }
  else
  {
     System.out.println("aOne and aTwo == operator says they are NOT equal");
  }

  if(aOne.equals(aTwo))
  {
     System.out.println("aOne and aTwo .equals operator says they are equal");
  }

Code prints out:

aOne and aTwo == operator says they are NOT equal
aOne and aTwo .equals operator says they are equal