Why doesn't changing the pre to the post increment at the iteration part of a for loop make a difference?

Question

Why does this

 int x = 2;
    for (int y =2; y>0;y--){
        System.out.println(x + \" \"+ y + \" \");
        x++;
    }

prints the s

Answer 1

There's no difference in terms of performance, if that's your concern. It can only be used wrongly (and thus sensitive to errors) when you use it during the increment.

Consider:

for (int i = 0; i < 3;)
   System.out.print(++i + ".."); //prints 1..2..3


for (int i = 0; i < 3;)
   System.out.print(i++ + ".."); //prints 0..1..2

or

for (int i = 0; i++ < 3;)
   System.out.print(i + ".."); //prints 1..2..3


for (int i = 0; ++i < 3;)
   System.out.print(i + ".."); //prints 1..2

Interesting detail is however that the normal idiom is to use i++ in the increment expression of the for statement and that the Java compiler will compile it as if ++i is used.

Answer 2

in your case, it's the same, no difference at all.

Answer 3

From the Java Language Specification chapter on for loops:

BasicForStatement:

    for ( ForInit ; Expression ; ForUpdate ) Statement

... if the ForUpdate part is present, the expressions are evaluated in sequence from left to right; their values, if any, are discarded. ... If the ForUpdate part is not present, no action is taken.

(highlight is mine).

Answer 4

About i++ (post-incrementation) vs. ++i (pre-incrementation) @me: "In both cases, the expression gets evaluated, and the result is used to check against the condition. In the pre-increment case, the increment expression increments the variable and returns the resulting value. For post-increment, the increment expression also increments the variable, but it returns the previous value. As a result, pre-increment compares against the incremented value, whereas post-increment compares against the original value; in both cases, the variable has been incremented when the condition is checked." – tdammers

Answer 5

The loop is equivalent to:

int x = 2;
{
   int y = 2;
   while (y > 0)
   {
      System.out.println(x + " "+ y + " ");
      x++;
      y--; // or --y;
   }
}

As you can see from reading that code, it doesn't matter whether you use the post or pre decrement operator in the third section of the for loop.

More generally, any for loop of the form:

for (ForInit ; Expression ; ForUpdate)
    forLoopBody();

is exactly equivalent to the while loop:

{
    ForInit;
    while (Expression) {
        forLoopBody();
        ForUpdate;
    }
}

The for loop is more compact, and thus easier to parse for such a common idiom.

Answer 6

The output is the same because the 'increment' item in the 'for (initial; comparison; increment)' doesn't use the result of the statement, it just relies on the side-effect of the statement, which in this case is incrementing 'i', which is the same in both cases.