Java deleteCharAt with ++ or --

Question

I have the following code:

public static void main(String[] args) {
    StringBuilder phoneNumber = new StringBuilder("828-707-5634");
    for(int i = 0; i < phoneNumber.length(); i++)
    {
      if (phoneNumber.charAt(i) == '-')
        phoneNumber.deleteCharAt(i--);
    }

    System.out.print(phoneNumber);
}

Indeed this removes the dash from the phone number, but intuitively I would have expected it to remove the character just before the dash (because of the --). How/why does this work? Additionally, I find that if I use ++ or omit the increment/decrement entirely the dashes are still removed.

you are changing the object during inspecting its content in the for loop.... — ΦXocę 웃 Пepeúpa ツ, Oct 31 '17 at 12:52
`i--` feeds the unchanged `i` to the method and afterwards decrements the value. It is the same as if you would call `deleteCharAt(i); i--;`. The `--i` operator does decrement it before feeding the method. — Zabuzard, Oct 31 '17 at 12:53
If you want it to delete de character before the dash, use ```--i``` — Artur Trapp, Oct 31 '17 at 12:53
To be fair, this is a "correct" solution to decrement the index since you update the String directly. But I prefer to simply read from the end, that way I don't have to bother with the new length. — AxelH, Oct 31 '17 at 13:08

Bathsheba · Accepted Answer · 2017-10-31T13:14:52.083

2

deleteCharAt receives the unincremented value of i, since the value of the expression i-- is the unincremented value. That's why your code works, even though you are relying on the increment expression of the for loop to compensate for a rather contrived and meaningless decrease in i. That all seems rather crude to me, and having i as -1 which will happen if the first character is a dash will raise eyebrows during debugging.

It's probably best to use the replace method in java.lang.String. See replace String with another in java

Else I'd prefer

for (int i = 0; i < phoneNumber.length(); /*intentionally empty*/){
    if (phoneNumber.charAt(i) == '-'){
        phoneNumber.deleteCharAt(i);
    } else {
        ++i;
    }
}

although this is still an O(N²) solution to an O(N) problem due to the repeated evaluation of length()! That can be solved by running the loop backwards, which will also reduce the number of element shuffles:

for (int i = -1 + phoneNumber.length(); i >= 0; /*intentionally empty*/){
    if (phoneNumber.charAt(i) == '-'){
        phoneNumber.deleteCharAt(i);
    } else {
        --i;
    }
}

edited Oct 31 '17 at 13:14

answered Oct 31 '17 at 12:58

Bathsheba

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"_Indeed this removes the dash from the phone number, but intuitively I would have expected it to remove the character just before the dash (because of the --)_ Lumbee " Seems your first statement answer OP question perfectly. – AxelH Oct 31 '17 at 13:03
1

Since you've added your prefered solution, for this kind of code I prefer with `for(int i = phoneNumber.length() -1; i >= 0; --i) ...` – AxelH Oct 31 '17 at 13:09
1

@AxelH: Yes that cuts down on the amount of shuffling. – Bathsheba Oct 31 '17 at 13:10
You shoud always decrement here, if not might end up with an `OutOfBoundsException` and we already checked the character that will come in that position PS :I like the `-1` before the length, simple but much more readable – AxelH Oct 31 '17 at 13:15

Java deleteCharAt with ++ or --

1 Answers1