Problem creating while Loop listing each occurance of a number in a string

**Dormilich** · Sep 23 '08, 12:48 PM

Problem creating while Loop listing each occurance of a number in a string

It's the last cycle. In the incrementation $posFirst (which is at that moment: false) is set to 1 (false +1 = 1), you could either break out (if (is_boolean($po sFirst)) break;) or use preg_match_all( ) with the PREG_OFFSET_CAP TURE flag instead of the while loop.

regards

**Alien** · Sep 23 '08, 12:56 PM

Originally posted by Dormilich

It's the last cycle. In the incrementation $posFirst (which is at that moment: false) is set to 1 (false +1 = 1)

I didn't get it. The increment in line 13 merely increments the value. So $posFirst was initialised to 0 then it became 1 after first run. Why is it false + 1 = 1?

**Atli** · Sep 23 '08, 01:00 PM

Yea. The problem is that if the strpos function doesn't find a match, it returns false. Which PHP will convert into 1 when you try to compare it with another integer.

If I were doing this, I would simply check if the return value of that function was false.
Like:
[code=php]
$position = -1;
while($position = strpos($heystac k, $needle, $position + 1))
{
echo "Found at $position ";
}
[/code]

**Dormilich** · Sep 23 '08, 01:05 PM

Problem creating while Loop listing each occurance of a number in a string

Originally posted by Atli

If I were doing this, I would simply check if the return value of that function was false.

definitely more elegant than my solution.... *sigh*

**Alien** · Sep 23 '08, 01:08 PM

Originally posted by Atli

Yea. The problem is that if the strpos function doesn't find a match, it returns false. Which PHP will convert into 1 when you try to compare it with another integer.

If I were doing this, I would simply check if the return value of that function was false.
Like:
[code=php]
$position = -1;
while($position = strpos($heystac k, $needle, $position + 1))
{
echo "Found at $position ";
}
[/code]

Ok so, you intially set the value of $position to -1.

In the first run you are searching from position 0 ($position + 1, so -1 +1 = 0) and it found a value and prints it.

I am kind of lost what happens in the second pass through the while loop. Because you are not using condition checking == just =. So how does the computer know when to break off?

I get how you say it returns a false like that php link in your post says
Returns the position as an integer. If needle is not found, strpos() will return boolean FALSE.

**Alien** · Sep 23 '08, 01:23 PM

Actually sorry guys ignore me.

I forgot that a variable in PHP can take different values like $var can be set to take string when you say

$var = "hello";

but later becomes a boolean if you say $var = FALSE or something like that. So after strpos() finishes searching the string it returns a boolean false. So the $position in the while loop of Atli's code becomes a boolean (false more specifically) and hence it breaks out.

I couldn't get how it was set to take string earlier and now taking boolean FALSE all of a sudden. Blame it on too much C coding.

Thanks for your help Atli & Dormilich.

**Marjeta** · Sep 23 '08, 01:24 PM

I thought this was an interesting problem, and I played with it some.

First I added an "echo" at the beginning of the loop:

Code:

while($posFirst <= $posLast) {
   echo $posFirst."-->";   //$posFirst value before the call to strpos()
   $posFirst = strpos($number, "1", $posFirst);
   echo $posFirst."<p>\n";   //$posFirst value AFTER the call
}

and got this output:
0-->01-->23-->56-->1-->23-->56-->1-->2
which displays in browser as:
0-->0
1-->2
3-->5
6-->
1-->2
3-->5
6-->
1-->2
So, calling strpos with 0 returns 0. Adding 1 gives you 1.
2nd time in the loop: calling strpos with 1 returns 2. Adding 1 gives you 3.
3rd time in the loop: calling strpos with 3 returns 5. Adding 1 gives you 6.
4th time in the loop. calling strpos with 6 returns false. Adding 1 gives you 1 !!!
5th time in the loop: calling strpos with 1 returns 2. Adding 1 gives you 3.
and so on...

I was wondering if ++ acts differently, and it does! Replacing the last line in the loop with:
$posFirst++;
gives the following output:
0-->0
1-->2
3-->5
6-->
-->0
1-->2
So, using ++ operator on false yields false, but adding 1 to false makes a 1.

**Atli** · Sep 23 '08, 01:36 PM

There is a bug in my previous code! (Hard to believe... I know :P)
But first, let me explain.

In a conditional statement, like a while or a if statement, PHP will look for a TRUE value, which is basically the same as comparing everything inside the while with TRUE.

So:
[code=php]
// This:
while(true)
// Is exactly like doing
while(true == true)

// And this
while(strpos($h , $n))
// Is exactly like
while(strpos($h , $n) == true)
[/code]
And, being both very smart and a bit annoying, PHP will consider any number above 0 and any non-empty string to be TRUE, but evaluating NULL, 0 and "" as FALSE.

So, strpost, returning a number when it finds a match and FALSE when it doesn't, will be considered TRUE when it finds a match...

With one exception (which is what I overlooked in my previous code).
When it finds a match at the first index (0), it returns 0, which will be evaluated as FALSE, ending the loop prematurely.

So to fix that, we just look for a boolean value rather than FALSE.
Like:
[code=php]
$position = -1;
while(!is_bool( $position = strpos($heystac k, $needle, $position + 1)))
{
echo "Found at $position ";
}
[/code]
Sorry about that. Was eating when I wrote my first post :P

**Alien** · Sep 23 '08, 01:43 PM

Originally posted by Atli

And, being both very smart and a bit annoying, PHP will consider any number above 0 and any non-empty string to be TRUE, but evaluating NULL, 0 and "" as FALSE.

Yeah it did terminate after printing zero but I thought there was something wrong with my syntax in while loop which is causing it to terminate at 0. Thanks for clarifying.

**Alien** · Sep 23 '08, 01:52 PM

that code works fine. Had a little trouble getting my head around that is_bool() becuase at first it looked like this:

is_bool(FALSE) is TRUE but digging a little deeper, is_bool looks at the type of variable not the content of the variable.

For a first timer dealing with is_bool() function it can be a little tricky! Kind of like a brain teaser.

**Dormilich** · Sep 23 '08, 01:58 PM

Problem creating while Loop listing each occurance of a number in a string

Originally posted by Alien

What happens when strpos returns a FALSE after finishing searching the string for $needle? Because logically when it $position will then be FALSE and

!is_bool(FALSE) is same as TRUE and hence it'll dig again into the loop.

er no, is_bool() checks for a boolean type, so both true and false return true as result, which is negated and therefore it breaks the loop.

regards

**Atli** · Sep 23 '08, 03:31 PM

Originally posted by Alien

that code works fine. Had a little trouble getting my head around that is_bool() becuase at first it looked like this:

is_bool(FALSE) is TRUE but digging a little deeper, is_bool looks at the type of variable not the content of the variable.

Yea I can see why it could be confusing at first :)
It's in a series of "is_*" functions that check for all sorts of variable types.

Can be a little difficult to use cause PHP is so good at converting the types for you.
Like using is_int() to check numbers from user input will not work, because user input is always a string. But using numbers from user input in math works fine because PHP just converts it when it needs to.

It's very smart, but potentially very annoying :)

Problem creating while Loop listing each occurance of a number in a string

Problem creating while Loop listing each occurance of a number in a string

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