CODEWITHSUNDEEP
java
Yuri Ghensev
Yuri Ghensev

Reputation: 2545

Why does `for(;;i++)` works just like `for(;;++i)` in Java?

I mean:

for (int i = 0; i < 10; i++)
    System.out.println(i);

and

for (int i = 0; i < 10; ++i)
    System.out.println(i);

Will yield the same results. Is ++i not evaluated at all till the first loop completes?

Upvotes: 1

Views: 1818

Answers (7)

Wayne
Wayne

Reputation: 60414

Because your for loop is equivalent to this:

int i = 0;
while (i < 10) {
    System.out.println(i);
    i++;
}

The post- or pre-increment operation is evaluated at the end (and its return value thrown away), before the condition is checked on the next iteration.

They would be quite different if the return value were used. The following code is equivalent to the loop above:

int i = 0;
while (i < 10) {
    System.out.println(i);
    i = ++i; // pre
}

...but this code creates an infinite loop (i is never incremented):

int i = 0;
while (i < 10) {
    System.out.println(i);
    i = i++; // post
}

Warning: These last two examples are Java-specific. The behavior in C and C++ is undefined and the two may very well be equal on your compiler.

Upvotes: 5

Oleg Mikheev
Oleg Mikheev

Reputation: 17444

Because the language specification says so:

If the ForUpdate part is present, the expressions are evaluated in sequence from left to right; their values, if any, are discarded.

Upvotes: 0

BVSmallman
BVSmallman

Reputation: 601

Using i++ this means that i will be incremented immediately after use and ++i means i will be incremented just before use.

 i = 1;
 y = i++;

y = 1, i = 2

 i = 1;
 y = ++i;

y = 2, i = 2

In the case of a for loop, the increment operator is called entirely independent of when it is evaluated, so think of it as a line at the end of the loop by itself either being

 i++; OR ++i;

It is incremented and then evaluated against your center condition.

Upvotes: 1

Lycha
Lycha

Reputation: 10177

There is just a subtle difference in ++i and i++:

  • i++ first returns the value of i and then increments it by one.
  • ++i first increments value of i and then returns the value.

Since the return value is not used for anything, it doesn't matter in this case.

Upvotes: 2

Adam Zalcman
Adam Zalcman

Reputation: 27233

The ++i and i++differ in what they return (their side-effects are the same: increasing i by one). The loop for(start; condition; step) has three expressions in it, but it only looks at the value returned by the condition. While it does execute step every iteration it just drops the result, i.e. step's side-effect is all that matters.

Upvotes: 1

Mechkov
Mechkov

Reputation: 4324

You are correct. i++ and ++i yields the same results because it is evaluated in the end of the iteration and not used.

Upvotes: 1

K-ballo
K-ballo

Reputation: 81349

Whether you use i++ or ++i is the same, since the return value is unused. The expression is evaluated after going through the loop body, and before evaluating the continuation condition. Note that they are two separate expressions. That is, they are different from:

i++ < 10
++i < 10

which would yield different results for i = 9.

Upvotes: 1

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