do-while
Looping through letters is possible. I'm amazed at how few people know that.
for($col = 'R'; $col != 'AD'; $col++) {
echo $col.' ';
}
returns: R S T U V W X Y Z AA AB AC
Take note that you can't use $col < 'AD'. It only works with !=
Very convenient when working with excel columns.
for
(PHP 4, PHP 5)
Los bucles for son los ciclos más complejos en PHP. Se comportan como sus contrapartes en C. La sintaxis de un bucle for es:
for (expr1; expr2; expr3)
sentencia
La primera expresión (expr1) es evaluada (ejecutada) una vez incondicionalmente al comienzo del bucle.
En el comienzo de cada iteración, la
expr2 es evaluada, Si es
TRUE, el bucle continúa y las sentencias
anidadas son ejecutadas. Si se evalúa como
FALSE, termina la ejecución del bucle.
Al final de cada iteración, la expr3 es evaluada (ejecutada).
Cada una de las expresiones puede estar vacía o contener múltiples
expresiones separadas por comas. En la expr2, todas
las expresiones separadas por una coma son evaluadas pero el resultado se toma
de la última parte.
Que la expr2 esté vacía significa que el bucle deberá
ser corrido indefinidamente (PHP implícitamente lo considera como
TRUE, como en C). Esto puede no ser tan inútil como se
pudiera pensar, ya que muchas veces se quiere terminar el bucle usando
una sentencia condicional break
en lugar de utilizar la expresión verdadera del
for.
Considere los siguientes ejemplos. Todos ellos muestran los números del 1 al 10:
<?php
/* ejemplo 1 */
for ($i = 1; $i <= 10; $i++) {
echo $i;
}
/* ejemplo 2 */
for ($i = 1; ; $i++) {
if ($i > 10) {
break;
}
echo $i;
}
/* ejemplo 3 */
$i = 1;
for (; ; ) {
if ($i > 10) {
break;
}
echo $i;
$i++;
}
/* ejemplo 4 */
for ($i = 1, $j = 0; $i <= 10; $j += $i, print $i, $i++);
?>
Por supuesto, el primer ejemplo parece ser el mejor (o quizás el cuarto), pero se puede encontrar que la posibilidad de usar expresiones vacías en los bucles for resulta útil en muchas ocasiones.
PHP también soporta la sintaxis alternativa de los dos puntos para bucles for.
for (expr1; expr2; expr3):
sentencia
...
endfor;
Es una cosa común a muchos usuarios iterar por medio de arrays como en el ejemplo de abajo.
<?php
/*
* Este es un array con algunos datos que se desean modificar
* cuando se corra por el bucle for.
*/
$people = Array(
Array('name' => 'Kalle', 'salt' => 856412),
Array('name' => 'Pierre', 'salt' => 215863)
);
for($i = 0; $i < sizeof($people); ++$i)
{
$people[$i]['salt'] = rand(000000, 999999);
}
?>
El problema radica en la segunda expresión del for. Este código puede ser lento porque tiene que calcular el tamaño del array en cada iteración. Dado que el tamaño nunca cambia, se pueden optimizar fácilmente usando una variable intermedia para almacenar el tamaño y usarla en el bucle en lugar del sizeof. El siguiente ejemplo ilustra esto:
<?php
$people = Array(
Array('name' => 'Kalle', 'salt' => 856412),
Array('name' => 'Pierre', 'salt' => 215863)
);
for($i = 0, $size = sizeof($people); $i < $size; ++$i)
{
$people[$i]['salt'] = rand(000000, 999999);
}
?>
add a note
User Contributed Notes
for
matthiaz
08-Feb-2012 02:37
kanirockz at gmail dot com
21-Mar-2010 11:49
Here is another simple example for " for loops"
<?php
$text="Welcome to PHP";
$searchchar="e";
$count="0"; //zero
for($i="0"; $i<strlen($text); $i=$i+1){
if(substr($text,$i,1)==$searchchar){
$count=$count+1;
}
}
echo $count
?>
this will be count how many "e" characters in that text (Welcome to PHP)
kanirockz at gmail dot com
21-Mar-2010 11:48
Here is another simple example for " for loops"
<?php
$text="Welcome to PHP";
$searchchar="e";
$count="0"; //zero
for($i="0"; $i<strlen($text); $i=$i+1){
if(substr($text,$i,1)==$searchchar){
$count=$count+1;
}
}
echo $count
?>
this will be count how many "e" characters in that text (Welcome to PHP)
Steven
11-Jan-2009 10:50
Alternating form rows:
<?php
$rows = 4;
echo '<table><tr>';
for($i = 0; $i < 10; $i++){
echo '<td>' . $i . '</td>';
if(($i + 1) % $rows == 0){
echo '</tr><tr>';
}
}
echo '</tr></table>';
?>
Changing $rows will change how many columns are in a row.
dkimbel13 at gmail dot com
07-Jan-2009 03:41
Just a note on looping through an array using the for() loop.
with the array...
<?php $array = array("value1","value2","value3"); ?>
then...
<?php
for(reset($array),current($array),next($array){
echo("Element ".key($array)." contains ".current($array)."<br/>";
}
?>
is the equivalent of...
<?php
for($i=0;$i<count($array);$i++){
echo("Element $i contains $array[$i]<br/>");
}
?>
I don't know if there is any advantage, just thought I would mention it.
http://badluck.tv
24-Mar-2008 05:05
Nested For Loop with the same iterator as the parent.
(Well formatted so the resulting code is clean when executed).
Useful for outputting a data array into a table, ie. images.
<?php
//Dummy data
$data = array(73,74,75,76,78,79,80,81,82,83,84,85,86,87);
//Our 'stepping' variable
$g = 0;
//Our rowcount
$rowcount = 0;
echo "<table cellspacing='0'>\r";
for ($i=0; $i<count($data); ) {
$rowcount++;
echo " <tr>\r"; //New row
$g = $i + 3; //Set our nested limit
for( ; $i<$g; $i++) { //nested for loop
if (!isset($data[$i])) { //Allow us to break on incomplete rows
break;
}
echo " <td style='border: 1px #000 solid;'>\r"; //Out put a cell
echo " <p>Row $rowcount <br/> Cell: $i <br/> Data: $data[$i]</p>\r";
echo " </td>\r";
}
echo " </tr> \r"; //End New Row
}
echo "</table>\r";?>
eduardofleury at uol dot com dot br
14-Jun-2007 06:18
<?php
//this is a different way to use the 'for'
//Essa é uma maneira diferente de usar o 'for'
for($i = $x = $z = 1; $i <= 10;$i++,$x+=2,$z=&$p){
$p = $i + $x;
print "\$i = $i , \$x = $x , \$z = $z <br />";
}
?>
lishevita at yahoo dot co (notcom) .uk
08-Sep-2006 12:33
On the combination problem again...
It seems to me like it would make more sense to go through systematically. That would take nested for loops, where each number was put through all of it's potentials sequentially.
The following would give you all of the potential combinations of a four-digit decimal combination, printed in a comma delimited format:
<?php
for($a=0;$a<10;$a++){
for($b=0;$b<10;$b++){
for($c=0;$c<10;$c++){
for($d=0;$d<10;$d++){
echo $a.$b.$c.$d.", ";
}
}
}
}
?>
Of course, if you know that the numbers you had used were in a smaller subset, you could just plunk your possible numbers into arrays $a, $b, $c, and $d and then do nested foreach loops as above.
- Elizabeth
JustinB at harvest dot org
04-Aug-2005 04:23
For those who are having issues with needing to evaluate multiple items in expression two, please note that it cannot be chained like expressions one and three can. Although many have stated this fact, most have not stated that there is still a way to do this:
<?php
for($i = 0, $x = $nums['x_val'], $n = 15; ($i < 23 && $number != 24); $i++, $x + 5;) {
// Do Something with All Those Fun Numbers
}
?>
user at host dot com
19-Apr-2004 03:53
Also acceptable:
<?php
for($letter = ord('a'); $letter <= ord('z'); $letter++)
print chr($letter);
?>
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++;
}
?>
$val++ can be whatever operation you need to perform ITERATIONS number of times.
On my box, with no users, average run time across 100 samples with ITERATIONS = 10000000 (10 million) is:
Duff version: 7.9857 s
Obvious version: 27.608 s
nzamani at cyberworldz dot de
17-Jun-2001 11:47
The point about the speed in loops is, that the middle and the last expression are executed EVERY time it loops.
So you should try to take everything that doesn't change out of the loop.
Often you use a function to check the maximum of times it should loop. Like here:
<?php
for ($i = 0; $i <= somewhat_calcMax(); $i++) {
somewhat_doSomethingWith($i);
}
?>
Faster would be:
<?php
$maxI = somewhat_calcMax();
for ($i = 0; $i <= $maxI; $i++) {
somewhat_doSomethingWith($i);
}
?>
And here a little trick:
<?php
$maxI = somewhat_calcMax();
for ($i = 0; $i <= $maxI; somewhat_doSomethingWith($i++)) ;
?>
The $i gets changed after the copy for the function (post-increment).