two simple for statements...

**Spork Schivago** · 09-02-2015

I have a question. How come when I have a for statement and in the middle section, I increment a variable, the variable gets incremented before I go into the for loop? For example,

Code:

  for (i = 0; i < 10; ++i)
    printf("%d ", i);
  printf("\n");


  for (i = 0; i++ < 10;)
    printf("%d ", i);
  printf("\n");

The first for loop is a much more common example, and will print 0 through 9. After each time through the loop, the variable i gets incremented by one, as expected.

The second one puzzles me a bit though. I understand we don't need all three parameters with the for statement, however, when we execute it, it prints 1 - 10. i is getting incremented before we execute the body of the loop...

I believe I understand why the loop prints 10 and doesn't exit before then. Correct me if I'm wrong, but the < operator has higher precedence over the ++ operator, right? So first we check if i is greater than 10 and then we increment i, hence the reason we get 10 printed. I just don't understand why i gets incremented before the body gets executed. Is this the way it always is, or is this perhaps compiler specific? For example, with some compilers, it might get incremented after the body gets executed.

**FourAngels** · 09-02-2015

In the second for loop, it looks like when i is 9 it is tested and found true, it is less than 10, however after that test it is than incremented to 10 and printed in the printf statement because you used a post increment on the variable. Again, the i is tested, than it is incremented, than the loop is entered.

That is just my opinion.

**Spork Schivago** · 09-02-2015

Originally Posted by FourAngels

In the second for loop, it looks like when i is 9 it is tested and found true, it is less than 10, however after that test it is than incremented to 10 and printed in the printf statement because you used a post increment on the variable. Again, the i is tested, than it is incremented, than the loop is entered.

That is just my opinion.

You are correct there. I was incorrect with my statement. I tested and using a pre-increment on the variable (++i), it only goes to 9. I know if I use ++i or i++ in the first for loop, it doesn't make a difference. I incorrectly assumed it wouldn't make a difference in the second. Thank you for clearing that up for me.

However, I still wonder if I increment a variable in the middle parameter of the for statement, like I do in the second for example, will it always increment before entering the loop?

**FourAngels** · 09-02-2015

You would have to do the update in the for statements body if you wanted the update to happen in a specific place or else use another variable for the counter.

Code:

for (int i = 0, int y =0; i++ < 10;) {
   // do something here using y instead of i
   y = i;
}

It's my guess.

**laserlight** · 09-02-2015

You are mistaken: operator < has a lower precedence than post increment operator++

The thing to understand is that precedence determines grouping, not order of evaluation. Here, the increment happens in the loop condition, so of course it happens before control enters the loop body. Where pre vs post increment matters is with the comparison, hence post increment means that you're comparing with the value before increment.

**Spork Schivago** · 09-03-2015

Originally Posted by laserlight

You are mistaken: operator < has a lower precedence than post increment operator++

The thing to understand is that precedence determines grouping, not order of evaluation. Here, the increment happens in the loop condition, so of course it happens before control enters the loop body. Where pre vs post increment matters is with the comparison, hence post increment means that you're comparing with the value before increment.

Okay, that makes sense. I got it, thank you. I think I should try to find or create a list of the operators and their precedence until I can get them memorized. I think that would make things easier.

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