Pre/Post increment operators and arrays

Question

I am trying out the following Java snippet:

int[] testArray={10,20,30,40};
int i= 0;
testArray[i++]= testArray[i++]+1;

System.out.println("The value of i: "+i);

for(int i1=0;i1<testArray.length;i1++)
{
    System.out.println(testArray[i1]);
}

With i=0, the output values of the array are: 21, 20,30,40

I can't understand this output because the output should be: 10, 11, 30, 40

testArray[0]+1 would be 11 and it would be then assigned to testArray[1] but this is not the case. Can anybody explain the output?

Answer 1

In the following assignment:

testArray[i++] = testArray[i++] + 1;

First the value of i++ is evaluated, which comes out to be 0. Then the value of i is incremented. So, the value of i has become 1 before the RHS starts evaluating. So, basically, the above expression is equivalent to:

testArray[0] = testArray[1] + 1;

and after the above expression, the value of i would be 2. If you however change the expression to:

testArray[i] = testArray[i++] + 1;

.. you will get the expected result.

Answer 2

This:

int i= 0;
testArray[i++]= testArray[i++]+1;

is equivalent to:

int i= 0;
testArray[0]= testArray[1]+1;

which is equivalent to:

int i= 0;
testArray[0]= 20 + 1;

The post increment operator is increasing the value of the int causing it to pull the element in index 1 in the expression, that index is == 20. Obviously, the subsequent addition will make the value 21, which is then assigned to index 0 of the array.

To put it another way that relates to your description of the code. Your not using the index of the array that you assume you are.