将if / else块转换为三元运算符时的错误答案

Question

I am trying to convert my java code from if/else block to ternary operator.

What my code is doing is finding the position of an array at which all values of integers have occured already for example,

if array is {1,1,3,4,2,3,4,5,1,4}

the answer would be 7 because by the time my program iterated to 7th position, it already found all previous numbers less than 5.

if array is {3,1,1,2,1,3} the answer would be 3 because by the time my program iterated to 3 position of the array, it had already iterated all values that are less than 3 .

Now, my code is working fine when I am using if/else block.

this is the code that is working absolutely fine for me,

public static int solution1 (int arrayMaxValue, int[] myArray){

    boolean b[] = new boolean[arrayMaxValue + 1];
    for (int i = 0; i < myArray.length; i++) {
        if (!b[myArray[i]]){
            b[myArray[i]] = true;
            if (b[myArray[i]]==true)
                arrayMaxValue--;
        }
        if (arrayMaxValue==0) return i;
    }
    return -1;
}

but if i convert its if block to ternary operator ?

it gives the wrong output,

public static int solution2 (int arrayMaxValue, int[] myArray){
        boolean b[] = new boolean[arrayMaxValue + 1];
        for (int i = 0; i < myArray.length; i++) {


            b[myArray[i]] = (!b[myArray[i]]) ? true : false;
            if (b[myArray[i]]){
                arrayMaxValue--;
            }


            if (arrayMaxValue==0) return i;
        }
        return -1;
    }

This is how i checked it,

    int[] myArray = { 1,1,1,2,3,4,1 };
    // _ _ _ _ _
    int arrayMaxValue =4;

    System.out.println(solution1(arrayMaxValue, myArray));
    System.out.println("==============================");
    System.out.println(solution2(arrayMaxValue, myArray));

The output of the above array should be 5 because by the time the loop iterated to position 5, all values less than or equals to 4 were spotted, the first solution gives the right answer but the second doesn't.

here is the output from both solution for the above array,

5
==============================
4

what am I doing wrong in ternary operator ?

Answer 1

In your ternary operator version you are decrementing arrayMaxValue if b[myArray[i]] is true after the ternary operator is executed, and the ternary operator negates the value of b[myArray[i]] .

b[myArray[i]] = (!b[myArray[i]]) ? true : false;

is the same as

b[myArray[i]] = !b[myArray[i]];

In the original version you are decrementing arrayMaxValue only if b[myArray[i]] changes from false to true, and you never change b[myArray[i]] from true to false.

I don't think the ternary operator is useful in your code, since your condition does more than simply assigning a value to b[myArray[i]] based on a condition - it also has the side effect of decrementing arrayMaxValue . You can simplify your code this way :

if (!b[myArray[i]]) {
    b[myArray[i]] = true;
    arrayMaxValue--;         
}

In order for the ternary operator to assign the same value to b[myArray[i]] as your original code it should be :

b[myArray[i]] = (!b[myArray[i]]) ? true : b[myArray[i]];

(which is a complicated way to write b[myArray[i]] = true; )

but arrayMaxValue-- will be executed also when b[myArray[i]] was and stayed true, which is different from the logic of the original implementation.

Answer 2

This is not code I recommend. Your function is far cleaner and more readable using if/else than this way. I see this mainly as a demonstration of why one should usually ignore ?: .

However, taking it as a challenge to rewrite your function using only ?: ...

  public static int solution3(int arrayMaxValue, int[] myArray) {
    int result = -1;
    boolean b[] = new boolean[arrayMaxValue + 1];
    for (int i = 0; i < myArray.length && result == -1; i++) {
      arrayMaxValue = b[myArray[i]] ? arrayMaxValue : arrayMaxValue - 1;
      b[myArray[i]] = true;
      result = (arrayMaxValue == 0) ? i : result;
    }
    return result;
  }

The main thing to remember is that, if you don't want to change the left hand side of the assignment of a ?: result you can use its old value as one of the alternatives.

Answer 3

In the previous answer Eran states correctly that the ternary operator you used is just a complex way to write b[myArray[i]] = !b[myArray[i]]; In the comments to Eran's anwer you noted that your task was

to design this program using if/else and ternary operator both

It is surely possible to use the ternary operator. You just need to use the operator to assign some other value. Eg the value for arrayMaxValue :

for (int i = 0; i < myArray.length; i++) {
  arrayMaxValue -= b[myArray[i]] ? 0 : 1;
  b[myArray[i]] = true;
  if (arrayMaxValue==0) return i;
}

In this code you will use the ternary operator to decide whether you decrement the arrayMaxValue by 1 (if b[myArray[i]] is false ) or not (if b[myArray[i]] is true we will decrement it by 0).

PS Note that the b[myArray[i]] value is always set to true in each iteration of your cycle.