Post increment operator not incrementing in for loop [duplicate]

Question

I'm doing some research about Java and find this very confusing:

for (int i = 0; i < 10; i = i++) {
  System.err.print("hoo... ");
}

This is never ending loop!

Anybody has good explanation why such thing happens?

Lapidate

Answer 1

for (int i = 0; i < 10; i = i++) {

The above loop is essentially the same as: -

for (int i = 0; i < 10; i = i) {

the 3^rd part of your for statement - i = i++, is evaluated as: -

int oldValue = i; 
i = i + 1;
i = oldValue;  // 3rd Step 

---

You need to remove the assignment from there, to make it work: -

for (int i = 0; i < 10; i++) {

---

(On OP request from Comments)

Behaviour of x = 1; x = x++ + x++; : -

As far as your issue as specified in the comment is concerned, the result of the following expression: -

x = 1; 
x = x++ + x++;

is obtained as follows: -

Let's mark different parts of the second statement: -

x = x++ + x++;
R    A     B

Now, first the RHS part (A + B) will be evaluated, and then the final result will be assignmed to x. So, let's move ahead.

First A is evaluated: -

old1 = x;  // `old1 becomes 1`
x = x + 1; // Increment `x`. `x becomes 2`
//x = old1; // This will not be done. As the value has not been assigned back yet.

Now, since the assignment of A to R is not done here, the 3rd step is not performed.

Now, move to B evaluation: -

old2 = x;  // old2 becomes 2. (Since `x` is 2, from the evaluation of `A`)
x = x + 1; // increment `x`. `x becomes 3`.
// x = old2; // This will again not be done here.

Now, to get the value of x++ + x++, we need to do the last assignment that we left in the evaluation of A and B, because now is the value being assigned in x. For that, we need to replace: -

A --> old1
B --> old2   // The last assignment of both the evaluation. (A and B)

/** See Break up `x = old1;` towards the end, to understand how it's equivalent to `A = old1; in case of `x = x++`, considering `x++ <==> A` in this case. **/

So, x = x++ + x++, becomes: -

x = old1 + old2;
  = 1 + 2;
  = 3;  // Hence the answer

---

Break up of 3rd part of x = x++, to see how it works in x = x++ + x++ case: -

Wonder why the replacement is done as A --> old1 and not x --> old1, as in case of x = x++.

Take a deep look at x = x++ part, specially the last assignment: -

x = oldValue;

if you consider x++ to be A here, then the above assignment can be broken into these steps: -

A = oldValue;
x = A;

Now, for the current problem, it is same as: -

A = old1;
B = old2;
x = A + B;

I hope that makes it clear.

Capriccio

This is the only answer that explained the difference between i = i++ and plain i++. – Faculty

Okey, this looks possible but I found somewhere an example of int x = 1; x = x++ and after that x is still 1 but here: x = x++ + x++ x won't be still 1 or 2 but 3 - how?? – Chun

Got curious about that and googled this, explaining why i=i++ does not work coderanch.com/how-to/java/PostIncrementOperatorAndAssignment – Archle

@emka86.. May be because x++ is evaluated to 1, but x++ + x++ is evaluated to 3. But still that's quite interesting. Will have a look at it in detail. – Capriccio

@emka86.. That really deserves a place for new question. You should ask it. that x = x++ + x++; one. For now, I'm trying to device the evaluation order. – Capriccio

@emka86.. Added some explanation about your doubt. You can have a look at the revised answer. – Capriccio

Answer 2

You're using post-increment: i = i++;, it means something like this:

temp = i;
i = i + 1;
i = temp;

because 15.14.2 Postfix Increment Operator ++:

The value of the postfix increment expression is the value of the variable before the new value is stored.

That is why you have the old value.

For-loop done right:

for (int i = 0; i < 10; i++) {
  System.err.print("hoo... ");
}

Chalone

Answer 3

because of i=i++

for (int i = 0; i < 10;  i++) {
  System.err.print("hoo... ");
}

Morrissette

Answer 4

i++ will report the value of i, and THEN increment. This also means that you don't need to set i equal to i++, just change to

for (int i = 0; i < 10; i++) {

Telescopic

Answer 5

The problem is in the statement i=i++, this statement does three operations in sequence to complete. these are two operations in i++ and one assignment( = ) operation. These are;

i++ does two operation before any operation

1. Returns i

2. Increments i

finally assignment operation

3. Assigns returned value

So i is incremented before assignment making it to have old value.

Lets see i=i++ in the first loop( where i = 0 ) with the three operations

1. returns 0 , it will not be assigned to i until the next operation is done
2. increments i ( i= 0 + 1 = 1 )
3. assigns returned value 0 to i, ( i = 0), finally i having its old value.

In i++ first i is returned, but immediately before any operation is done it is incremented(i becomes i+1 ), However we are yet left with assignment operation to complete so that the returned value will be assigned to i making it to have old value, thereby entering infinite loop.

so replace i=i++ with i++or i=i+1

Insubstantial