为什么这个“if”条件不会打破循环？

Question

I have a for loop with a conditional block that breaks the loop.

However, the loop never breaks.
This is my code:

for($counter=1;$counter>5 or $counter<=100;$counter*=2){

    echo $counter."<br>";
    if($counter==500){
      break;
    }
}

Answer 1

Because counter is never exactly 500. It will be 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, ...

You probably want

if($counter>=500){
  break;
}

but I can't say for sure because I have no idea what you are doing.

Answer 2

It looks like $counter will never be equal to 500 to satisfy the conditional if statement. You should change

if($counter==500){
  break;
}

to

if($counter>500){
  break;
}

Answer 3

Because your break logic is wrong:

loop calculation => next value

1 => 1
1*2 => 2
2*2 => 4
4*2 => 8
8*2 => 16
16*2 => 32
32*2 => 64
62*2 => 128
128*2 => 256
256*2 => 512
512*2 => 1024

To break your loop change your

if($counter==500){
  break;
}

to

if($counter>500){
  break;
}

Answer 4

Like others have said, $counter will never be exactly 500, as it goes through the powers of two (1, 2, 4, 8, 16, 32, 64, 128, 256, 512, etc).

The fix for this is simple: you use >500 instead of ==500

---

However, in addition, this code has general issues that I would like to point out to you, since it seems like you are learning to program.

Your loop condition is $counter > 5 or $counter <=100 . This covers all numbers, since all numbers are either more than 5 or less than (or equal to) 100.

This means that the loop will run forever as the check will always be true.

You then have a manual check after each loop that checks if $counter is greater than 500. This can be moved up to the condition with minor tinkering, as the loop checks the condition after every loop (same as your manual check).

The new condition becomes "the old condition AND NOT the manual condition" (boolean AND). Since the old condition is always true, the new condition becomes TRUE & !$counter > 500 , which is the same as $counter <= 500 .

---

I hope I've explained this well enough but if you have any confusion just ask me in the comments to this answer and I'll be happy to clear anything up.

Answer 5

If I understand your goal correctly, this should produce what you want.

for($counter = 8; $counter <= 500; $counter *= 2) {
    echo $counter."<br>";
}

This will echo 8, 16, 32, 64, 128, 256 and exit the loop because the next value, 512, will not be <= 500.

Another option, if you need to start at 1, would be:

for($counter = 1; $counter <= 500; $counter *= 2) {
  if($counter < 6 ) {
    continue; // this will jump back to the start of the for loop
  } else {
    echo $counter."<br>";
  }
}

In your original code: If you start with the $counter = 1, the values of 2, 4, 8, 16, 32, & 64 will all return true because of the <= 100 condition. If you include the $counter > 5 condition, every value beyond 5 will be true (8, 16, 32, ...to infinity). Since the counter doubles each time, it will never equal 500 (it will be 64, 128, 256, 512, 1024...) so the condition triggering the break will never happen.