为什么这个 for 循环似乎没有执行，尽管条件仍然为真？

Question

在下面的SSCCE中，为什么$a的for循环不大于3，尽管条件应该让它执行到$a变为5。

最后一条语句的输出更为怪异。

我试图实现的是，我希望删除元素/变量word具有Select one值的元素，这样得到的数组为：

Array ( [0] => stdClass Object ( [word] => alpha [sentence] => A is the first letter in the word Alpha. ) [1] => stdClass Object ( [word] => beta [sentence] => B is the first letter in the word Beta. ) )

问题是，是什么把它搞砸了，我能做些什么来修复它？

<?php 
$objectOne = new stdClass;
$objectOne->word = 'alpha';
$objectOne->sentence = 'A is the first letter in the word Alpha.';
$objectTwo = new stdClass;
$objectTwo->word = 'beta';
$objectTwo->sentence = 'B is the first letter in the word Beta.';
$objectThree = new stdClass;
$objectThree->word = 'Select one';
$objectThree->sentence = '';
$items = array($objectOne, $objectTwo, $objectThree, $objectThree, $objectThree, $objectThree );

print_r($items);//check
echo '<br><br>count($items) >> '.count($items).'<br><br>';//check

for ($a=0; $a < count($items); $a++) {
    echo '<br><br>We are entering index '.$a.'<br><br>';//check
    echo '<br>'.$items[$a]->word.'<br>';//check
    if ( ($items[$a]->word)=="Select one"  ) {
        echo '<br>YES if ( ($items['.$a.']->word)=="Select one"  ) AT '.$a.' INDEX.<br>';//check
        unset($items[$a]);
        /**/array_splice($items, $a, 1);
    }
    echo '<br><br>We are leaving index '.$a.'<br><br>';//check
}

echo '<br><br>AFTER:-<br>';//check
print_r($items);//check
?>

输出：

Array ( [0] => stdClass Object ( [word] => alpha [sentence] => A is the first letter in the word Alpha. ) [1] => stdClass Object ( [word] => beta [sentence] => B is the first letter in the word Beta. ) [2] => stdClass Object ( [word] => Select one [sentence] => ) [3] => stdClass Object ( [word] => Select one [sentence] => ) [4] => stdClass Object ( [word] => Select one [sentence] => ) [5] => stdClass Object ( [word] => Select one [sentence] => ) )
count($items) >> 6

We are entering index 0

alpha

We are leaving index 0

We are entering index 1

beta

We are leaving index 1

We are entering index 2

Select one
YES if ( ($items[2]->word)=="Select one" ) AT 2 INDEX.

We are leaving index 2

We are entering index 3

Select one
YES if ( ($items[3]->word)=="Select one" ) AT 3 INDEX.

We are leaving index 3

AFTER:-
Array ( [0] => stdClass Object ( [word] => alpha [sentence] => A is the first letter in the word Alpha. ) [1] => stdClass Object ( [word] => beta [sentence] => B is the first letter in the word Beta. ) [2] => stdClass Object ( [word] => Select one [sentence] => ) ) 

Answer 1

条件并不总是真的。for循环中的条件会在每次迭代中重新计算数组的大小。每当移除某个项时，数组的长度都会发生变化。

每次检查条件时$a和count($items)的值如下：

$a | count($items) | $a < count($items)
---------------------------------------
 0 | 6             | true 
 1 | 6             | true
 2 | 6             | true
 3 | 5             | true  -- $items[2] was removed
 4 | 4             | false -- $items[3] was removed

您应该将数组的大小存储在一个变量中，然后使用它。此外，由于array_splice不保留数字键，您在尝试访问$items[4]和$items[5]时最终会收到未定义的偏移通知。该行不是必需的。

$count = count($items);
for ($a=0; $a < $count; $a++) {

更好的是，您可以使用foreach代替for，使用$item代替$items[$a]:

foreach ($items as $a=>$item) {
    echo '<br><br>We are entering index '.$a.'<br><br>';//check
    echo '<br>'.$item->word.'<br>';//check
    ...
    unset($items[$a]); //can't use $item because it is a copy and not a reference

Answer 2

在执行期间取消设置main的键时，使用temp变量进行迭代。也许，这应该起作用：-

$temp = $items;
for ($a=0; $a < count($temp); $a++) {
  echo '<br><br>We are entering index '.$a.'<br><br>';//check
  echo '<br>'.$items[$a]->word.'<br>';//check
  if ( ($items[$a]->word)=="Select one"  ) {
      echo '<br>YES if ( ($items['.$a.']->word)=="Select one"  ) AT '.$a.' INDEX.<br>';//check
      unset($items[$a]);
      /**/array_splice($items, $a, 1);
  }
  echo '<br><br>We are leaving index '.$a.'<br><br>';//check
}