PHP: for - Manual

for

thomas et luegger _dot_ de
17-Jul-2006 12:44


The previous post from Fatalerror is not correct. Variables enjoy no special treatment inside 'for' statements. At least I was not able to reproduce such a behavior. The code below shows, that the value of $a from loop 1 is still valid outside the scope of it's loop. It can be used without further initialisation for loop 2. So 'while' and 'for' loops are just a matter of taste.



<?php

echo ("In loop 1: ");

for ($a = 0; $a < 3; $a++) echo("$a,");

echo ("\r\nAfter loop 1:  a = $a");



echo ("\r\n\r\nIn loop 2: ");

for ( ; $a++ < 20; )

{

  echo ("$a,");

  if ($a==10) break;

}

echo ("\r\nAfter Loop 2: a = $a");

?>



output:



In loop 1: 0,1,2,

After loop 1: a = 3



In loop 2: 4,5,6,7,8,9,10,

After Loop 2: a = 10



Greetings, Tom

FatalError
28-Dec-2005 11:56


To add to my previous note, I found a big difference between the two statements. With a "while" statement, you can use $a (from my examples) outside the "while" statement. However, with the "for" statement, you can only use $a within the scope of that statement. In other words, you can't use $a after that "for" statement and expect it to still be equal to the same number it was before.

FatalError
27-Dec-2005 09:01


You can also use a "while" loop to get the same results as a "for" statement. These two examples are basically the same thing:



<?php



for ($a = 0; $a < $somevar; $a++) {

    /* Do something... */

}



?>



<?php



$a = 0;

while ($a < $somevar) {

    /* Do something... */

    $a++

}



?>

see_the_address at the_message dot text ;)
26-Nov-2005 06:29


Here a bit more flexible solution of a task "brutforce" by "jdtoth -at- gmail dot com", and also should be faster ;-) It uses recursion. But it requires quite much memory (especially when playing with $num_positions), I'm not sure it's OK...



If anybody has ideas about that - you are welcome: ultraBoy aat mail ddot ru



<?php

function get_result($len, &$alphabet) {

    if ($len == 1) {

        $result = array();

        while (list($row_key, $row) = each($alphabet)) {

            $result[] = array($row);

        };

    }

    else {

        $result = get_result($len - 1, $alphabet);

        reset($result);

        $new_result = array();

        while (list($row_key, $row) = each($result)) {

            reset($alphabet);

            while (list($l_key, $l) = each($alphabet)) {

                $new_row = $row;

                $new_row[] = $l;                

                $new_result[] = $new_row;

            };

        }

        $result = $new_result;

    }

    return $result;

}



// suspected combination numbers

$numbers = array(12,18,24,36);



// how many numbers are in one combunation

$num_positions = 3;



echo 'Total rows: '.pow(count($numbers), $num_positions).'<br />';

$result = get_result($num_positions, $numbers);



foreach ($result as $row_num => $row) {

    echo "$row_num: ";

    foreach ($row as $val) {

        echo $val.' ';            

    }

    echo '<br />';

}

?>

jdtoth -at- gmail dot com
25-Nov-2005 03:35


I was fooling around, trying to figure out the combination to a lock that I couldnt remember the combo to.  I did remember it involved a few numbers, so I wrote the following:



<?php

// ----------------------------------------------------------------------

// output all possible combinations for lock, based on supplied numbers

// ----------------------------------------------------------------------



// suspected combination numbers

$numbers = array (12,18,24,36);



for ($i=0; $i <= 10000; $i++)

{

    srand ((double) microtime() * 1000000);

    $random1 = rand(0,3);

    

    srand ((double) microtime() * 1000000);

    $random2 = rand(0,3);

    

    srand ((double) microtime() * 1000000);

    $random3 = rand(0,3);

    

    $results[] = $numbers[$random1].' - '.$numbers[$random2].' - '.$numbers[$random3];

}



// discard duplicates

$results = array_unique($results);



echo "** Unique Results ".count($results)." ** <br/><br/>";

foreach ($results as $output) echo $output . "<br />"; 



?>



What would be the proper way of writing this code?  Obviously my 'brute-force' method of running a random sequence an extreme amount of times does work and yields what i think is the correct number of combinations, but there must be a more efficient method?

Maciek
15-Nov-2005 11:12


in reply to mbvlist about his loop:



<?php



for($i=0; $i < $max; $i++ && print 'increment,')

{

  do_something();

  echo " i = $i\n";

}

echo "finally, i = $i\n";



?>



Your loop works as you observed, because php evaluates the following as expression:

($i++ && print 'increment)



When $i=0, $i++ returns false, so the second part of the expression (print 'increment') is not evaluated, since there is no output. Since $i > 0 returns true, your print will be evaluated and you will see output..

13-Nov-2005 03:23


if you wanted to make a next previous thing for mysql this would be it



$page = $_GET['page'];

// mysql query goes here

$limit = 30; //gets 30 of whatever

    if(!$page){ 

        $page = "1"; 

    } 

$limitvalue = $page * $limit - ($limit); //find the limit value

$query = mysql_query("SELECT * FROM `table`ORDER BY id DESC LIMIT $limitvalue, $limit");

$numrows = mysql_query("SELECT * FROM `forum_topics` WHERE `forumid` = '$fid'");

$numofpages = ceil($numrows / $limit);

while ($r = mysql_fetch_array($query)){

// your stuff goes here

}

$pageprev = $page-1;

$pagenext = $page+1;

if ($page != "1"){ 

echo "  <a href='linkhere.php?page=$pageprev'><< Previous</a> ";

}

for ($i = 1; $i <= $numofpages; $i++) {

if ($i == $page){

echo "<b>$i</b> ";

}

else {

echo "<a href='linkhere.php?page=$i'>$i</a> ";

}

}

if ($numrows > $limit){ 

if ($page != $numofpages){

echo "  <a href='linkhere.php?page=$pagenext'>Next >></a> ";

}

}



There is also a few other ways but this is how i do it ^^

mbvlist at fastmail dot fm
08-Nov-2005 12:37


While fuzzeling around with iterators (trying to find the next and previous item in a list), discovered something odd:



<?php



for($i=0; $i < $max; $i++ && print 'increment,') 

{

  do_something();

  echo " i = $i\n";

}

echo "finally, i = $i\n";



?>



If $max is set at 1 (only the first run will occur), it works as I expected: 

i = 0

finally, i = 0



But with $max at a higher value (say 2) it works different:

i = 0

increment, i = 1

increment, finally, i = 2



After the loop was finished $i was incremented! As I read right now, this is what should happen. But it doesn't occur in the first case. As I said, I was searching something using an iterator, and after the loop I wanted to know the next value. Because of this 'feature' (a.k.a. bug) that didn't work.

Be warned!

robert at extremedesigns dot de
06-Aug-2005 04:58


Here is a quick function I wrote for creating a web-true colour palette.     



function webTrueColors() {

        // create array for hexadecimal colors

        $colors = array("00", "33", "66", "99", "CC", "FF");

        // build table

        $c= "<table cellspacing=0 cellpaddding=0 border=1>";

        // loop through all combinations

        for ($i=0; $i<6; $i++) {

            for ($j=0; $j<6; $j++) {

                $c.= "<tr>";

                for ($k=0; $k<6; $k++) {

                    $thisColor = $colors[$i].$colors[$j].$colors[$k];

                    // display color and hex value

                    $c.= "<td bgcolor=\"#".$thisColor."\">".$thisColor."</td>";

                }

                $c.= "</tr>";

            }

        }

        // close table

        $c.= "</table>";

        // output to screen

        echo $c;

    }

JustinB at harvest dot org
04-Aug-2005 04:23


For those who are having issues with needing to evaluate multiple items in expression two, please note that it cannot be chained like expressions one and three can.  Although many have stated this fact, most have not stated that there is still a way to do this:



<?php

for($i = 0, $x = $nums['x_val'], $n = 15; ($i < 23 && $number != 24); $i++, $x + 5;) {

    // Do Something with All Those Fun Numbers

}

?>

dre
21-Jul-2005 11:01


In reply to M. Berndt's code sample: recursion is not "always" faster, especially not when you consider how big the stack's getting.



With FAKTOR = 50:

for(): 0.0222418308258

recursion: 0.175624132156



With FAKTOR = 1000:

for(): 0.228713035583

recursion: 0.459243059158

pmv at nipl dot net
04-Jul-2005 12:55


Actually, it seems calling echo inside a for expression is not allowed, at least as of PHP 5.0.4 (built: May 10 2005 09:56:44). Executing the said code results in the following error:



Parse error: parse error, unexpected T_ECHO in /usr/local/iwcdns/webadmin/foo.php on line 2



However, using the print() function seems to work. From this, it seems sensible to think that PHP does not treat echo as a expression. The following will work as expected:



<?php

for ($i = 'a'; $i != "aa"; $i++, print $i);

?>



(Note the use of the character notation 'a', instead of just a. Using the latter works, but is wrong, as explained in section "Why is $foo[bar] wrong?" of the array section of the manual (manual/en/language.types.array.php).)



Also, bpgordon's further shortening is not exactly equivalent to mison's examples. In a for loop, the step expression (i.e. the third one) is executed _after_ the body of the loop. Therefore, a more accurate equivalent would be:



<?php

for ($i = 'a'; $i != "aa"; print $i, $i++);

?>



And further shortening this:



<?php

for ($i = 'a'; $i != "aa"; print $i++);

?>

bpgordon at gmail dot com
12-Jun-2005 04:31


Mison's example can be shortened even further:



<?php

for ($i = a; $i!="aa"; $i++, echo $i)

?>

mison
27-May-2005 02:31


In a previous note:



Following moniarde at yahoo dot com 's idea 

<?php

for ($i = a; $i!="aa" ; $i++) {

   echo $i;

}

?>



This can be trimmed slightly further to:



<?php

for ($i = a; $i!="aa" ; $i++) echo $i;

?>

berndt at www dot michael - berndt de
12-Apr-2005 09:53


for(){} versus recursion (always faster)

http://www.michael-berndt.de/ie/tux/for_versus_recursion.htm

15-Jan-2005 04:57


Following moniarde at yahoo dot com 's idea 

<?php

for ($i = a; $i!="aa" ; $i++) {

   echo $i;

}



?>



will be good.

fb_mathman at hotmail dot com
01-Nov-2004 11:20


It is intersting how to use a structure that can substitue "for".

In the next lines, for instance, I create a function that get the smallest free_id for a table. It has good applications for the phpBB forum.



<?php



function assign_free_id($table, $id_field)

{

    global $db;



    $sql = "SELECT $id_field FROM $table WHERE 1 ORDER BY $id_field ASC";

    if( !$result = $db->sql_query($sql) )

    {

        message_die(GENERAL_ERROR, "Couldn't get id parameters for all the elements of the table", "", __LINE__, __FILE__, $sql);

    }

    

    while ($arr = $db->sql_fetchrow($result))

    {

    $row[]=$arr;

    }

    $max_elements = count($row);

    if ($max_elements==0) 

        {

        $id= 1;

        return ($id);

        }



//here I could insert a double-for structure, but I did as it follows, and I think it is more easy to process.        

      //local variable defining....

    $i=1;

    for ( $j=0; $j<$max_elements; $j++)

        {

        if ($i==intval($row[$j][0]))

            {

            // ...go to next cycle

            $i++;

            }

        else

            {

            $id=$i;

            return $id;

            }

        }



}



?>

jphansen at uga dot edu
15-Oct-2004 12:31


as harbater at teamgenesis dot com noted, don't use a comma to separate conditions in expr2. But if you do, the result will be ONLY the last condition being evaluated for the continuation of the loop. So this would iterate infinitely: for(; false, true;); and if you swapped the true and false it would break immediately. 



I wouldn't count using a comma completely out of the question, since there may be a rare circumstance where you would only want to break if the last evaluation returns false.

moniarde at yahoo dot com
31-May-2004 09:06


As previously mentioned,



<?

for ($i = a; $i <= z; $i++) {

   echo $i

}

?>



will print a -> z, then aa, ba, ca , etc -> yz.  However, doing this:



<?

for ($i = a; ; $i++) {

    echo $i;

    if ($i == "z") {

        break;

    }

}

?>

will print only a->z.  It's simple, but it works.

user at host dot com
19-Apr-2004 03:53


Also acceptable:

  for($letter = ord('a'); $letter <= ord('z'); $letter++)

   print chr($letter);

hayes029 at bama dot ua dot edu
05-Feb-2004 11:16


For the purposes of outputing the alphabet, 





<?php


for ($i="A"; $i <= "Z"; $i++) echo "$i<br>";


?>





will actually output A-Z, then begin again with AA, AB, etc. until it finishes with YZ.  Similarly, 





<?php


for ($i="A"; $i <= "ZZ"; $i++) echo "$i<br>";


?>





will output A-Z, AA, AB... ZZ, then begin again with AAA, AAB, AAC... up to ZYZ.





Thus an alternative to timrosseel's example for outputting the alphabet is:





<?php


for ($i="A"; $i != "AA"; $i++) echo "$i<br>";


?>

php at project2501 dot plus dot com
15-Sep-2003 04:45


just to clarify that you can 'chain' expr1, expr2 and expr3, for instance:



<?

for($i=0 , $j=10 ; $i<5 ; $i++ , $j+=5, $k=($i+$j))

{

    echo "i: $i<br />j: $j<br />k: $k<br /><br />";

}

?>



produces:



i: 0

j: 10

k: 



i: 1

j: 15

k: 16



i: 2

j: 20

k: 22



i: 3

j: 25

k: 28



i: 4

j: 30

k: 34



in case you need to do more than one operation per iteration.



The reason $k does not start with a value of 10, is because expr3 ($k=($i+$j)) is evaluated at the -end- of each iteration.

bishop
17-Jul-2003 04:43


If you're into 1-liners or compact code:





<?php


implode("\n", range('a','z'));


?>





Run-times (on my boxes) are comparable to the other methods using for().  This version is (arguably) more readable than other solutions.

bishop
17-Jul-2003 01:23


If you're already using the fastest algorithms you can find (on the order of O(1), O(n), or O(n log n)), and you're still worried about loop speed, unroll your loops using e.g., Duff's Device:





<?php


$n = $ITERATIONS % 8;


while ($n--) $val++;


$n = (int)($ITERATIONS / 8);


while ($n--) {


    $val++;


    $val++;


    $val++;


    $val++;


    $val++;


    $val++;


    $val++;


    $val++;


}


?>





(This is a modified form of Duff's original device, because PHP doesn't understand the original's egregious syntax.)





That's algorithmically equivalent to the common form:





<?php


for ($i = 0; $i < $ITERATIONS; $i++) {


    $val++;


}


?>





$val++ can be whatever operation you need to perform ITERATIONS number of times.





On my box, with no users, average run time across 100 samples with ITERATIONS = 10000000 (10 million) is:


Duff version:       7.9857 s


Obvious version: 27.608 s

thewhiteness at hotmail dot com
16-Dec-2002 02:10


Regarding ben's note: The reason for this is that your first example uses neither the $i++ or $i+= operators. It's like telling PHP to add nothing to it ($i+).



Another way to possibly speed a for loop would be this:



...

for (;condition;) {

// your statement/block here

}

...



This way, if you have a variable that is being updated in the body of the loop anyways, you can use it, rather than specify a number of iterations.

nzamani at cyberworldz dot de
17-Jun-2001 11:47


The point about the speed in loops is, that the middle and the last expression are executed EVERY time it loops.


So you should try to take everything that doesn't change out of the loop.


Often you use a function to check the maximum of times it should loop. Like here:





for ($i = 0; $i <= somewhat_calcMax(); $i++) {


  somewhat_doSomethingWith($i);


}





Faster would be:





$maxI = somewhat_calcMax();


for ($i = 0; $i <= $maxI; $i++) {


  somewhat_doSomethingWith($i);


}





And here a little trick:





$maxI = somewhat_calcMax();


for ($i = 0; $i <= $maxI; somewhat_doSomethingWith($i++)) ;





The $i gets changed after the copy for the function (post-increment).





I don't know if





for ($i = 0; $i <= $maxI; somewhat_doSomethingWith($i), $i++) ;





is correct...at least it's not really clear.

harbater at teamgenesis dot com
27-Mar-2001 06:55


If you want two conditions to be evaluated in expr2, you can't just separate them with commas like you can with expr1 and expr3.





For example, this does not work:


for ($i=$start, $count=0; $i<$total, $count<10; $i+=10, $count++)





But this does:


for ($i=$start, $count=0; ($i<$total && $count<10); $i+=10, $count++)