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prev — 将数组的内部指针倒回一位

说明

prev(array|object &$array): mixed

将数组的内部指针倒回一位。

prev() 和 next() 的行为类似，只除了它将内部指针倒回一位而不是前移一位。

参数

array: The input array.

返回值

返回数组内部指针指向的前一个单元的值，或当没有更多单元时返回 false。

警告

此函数可能返回布尔值 false，但也可能返回等同于 false 的非布尔值。请阅读布尔类型章节以获取更多信息。应使用 === 运算符来测试此函数的返回值。

更新日志

版本	说明
8.1.0	弃用在 object 上调用此函数。要么首先使用 get_mangled_object_vars() 将 object 转换为 array，要么使用实现 Iterator 的类提供的方法，例如 ArrayIterator。
7.4.0	SPL 类的实例现在被视为没有属性的空对象，而不是调用与此函数同名的 Iterator 方法。

示例

示例 #1 prev() 及相关函数用法示例

<?php
$transport = array('foot', 'bike', 'car', 'plane');
$mode = current($transport); // $mode = 'foot';
$mode = next($transport);    // $mode = 'bike';
$mode = next($transport);    // $mode = 'car';
$mode = prev($transport);    // $mode = 'bike';
$mode = end($transport);     // $mode = 'plane';
?>

注释

警告

注意: 很难区分是遇到 boolean false 单元还是遇到了数组的开头。需要用 key() 检查 prev() 数组，是否为 null 来作区分。

参见

current() - 返回数组中的当前值
end() - 将数组的内部指针指向最后一个单元
next() - 将数组中的内部指针向前移动一位
reset() - 将数组的内部指针指向第一个单元
each() - 返回数组中当前的键／值对并将数组指针向前移动一步

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User Contributed Notes 4 notes

down

MicroVB INC ¶

9 years ago

This function searches for the closest element in an array by key value, and returns the key/value pair, or false if not found.

<?php
function nearest($array, $value, $exact=false) {
        // Initialize Variables
    $next = false;
    $prev = false;
    $return = false;
    
    if(!isset($array[$value]) && !$exact) {
        // Insert element that doesn't exist so nearest can be found
        $array[$value] = '-';
    }
    
    if($exact && isset($array[$value])) {
                // If exact match found, and searching for exact (not nearest), return result.
        $return = Array($value=>$array[$value]);
    } else {
            // Sort array so injected record is near relative numerics
        ksort($array); // Sort array on key

                // Loop through array until match is found, then set $prev and $next to the respective keys if exist
        while ( !is_null($key = key($array)) ) {
            $val = current($array);
            if($key == $value) {
                prev($array); // Get entry before this one
                $prev = key($array);
                next($array);         // Skip current entry as this was what we were looking for nearest to
                next($array); // Get entry after this one
                $next = key($array);
                break;
            }
            next($array);
        }

        if($prev && $next) {
            if(($long - $prev) > ($next - $long)) {
                                 // Previous is closer
                $return = Array($prev=>$array[$prev]);
            } else {
                                 // Next is closer
                $return = Array($next=>$array[$next]);
            }
        } elseif ($prev || $next) {
            if($prev) {
                                 // Only Previous exists
                $return = Array($prev=>$array[$prev]);
            } elseif ($next) {
                                 // Only Next exists
                $return = Array($next=>$array[$next]);
            }
        }
    }

        // Return resulting Array().   $return is false if nothing matches the closest conditions, or if exact is specified and key does not exist.        
    return $return;
}
?>

Example usage (to lookup the closest color in an array)
<?php
$mycolors= Array(
    5001046=>'Abbey',
    1774596=>'Acadia',
    8171681=>'Acapulco',
    6970651=>'Acorn',
    13238245=>'Aero Blue',
    7423635=>'Affair',
    8803850=>'Afghan Tan',
    13943976=>'Akaroa',
    16777215=>'Alabaster',
    16116179=>'Albescent White',
    10176259=>'Alert Tan',
    30371=>'Allports'
);

// Color to search for in Hex
$color = 'C0C0C0';

// Convert Hex to Long to compare with array() keys
$colorlong = base_convert($color,16,10);

// Check for an exact match
$result = nearest($mycolors, $colorlong, true);
if($result) {
    echo "An exact match was found for #" . $color . " which is called '" . $result[key($result)] . "'";
} else {
    echo "No exact match was found";
}

if(!$result) {
    // Check for closest match
    $result = nearest($mycolors, $colorlong, true);
    if($result) {
        // Match found
        echo "The closest match for #" . $color . " is #" . base_convert(key($result),10,16) . " which is called '" . $result[key($result)] . "'";
    } else {
        // No match found
        echo "No match was found for #" . $color;
    }  
}
?>

down

soapergem at gmail dot com ¶

14 years ago

Here's a slight revision to xmlich02's backwards iteration example. The problem with his/her example is that it will halt if any of the array elements are boolean false, while this version will not.

<?php

end($ar);
while ( !is_null($key = key($ar)) ) {
    $val = current($ar);
    echo "{$key} => {$val}\n";
    prev($ar);
}

?>

down

-1

Mikhail ¶

4 years ago

Function to get element in array, that goes previous your key or false if it not exeists or key doesn't isset in array.

<?php

function previousElement(array $array, $currentKey)
{
    if (!isset($array[$currentKey])) {
        return false;
    }
    end($array);
    do {
        $key = array_search(current($array), $array);
        $previousElement = prev($array);
    }
    while ($key != $currentKey);

    return $previousElement;
}

down

-10

MicroVB INC ¶

9 years ago

Some corrections to the previous function.
- When prev() and next() distance is now calculated properly returning the 'closer' one.
- When exact is false, and an item exists with the key searched for, it now returns that entry instead of next/prev closest
- At ends of the array, sometimes when moving past using next() or prev(), it caused results to die. This was corrected.
- Added field to result stating whether the match was 'exact' ('exact'=>true), or nearest ('exact'=>false)

<?php
function nearest($array, $value, $exact=false) {
    // Initialize Variables
    $next = false;
    $prev = false;
    $return = false;

    $arr_begin = false;
    $arr_end = false;
        
    $exact_match = false;
        
    
    if($exact && isset($array[$value])) {
        // If exact match found, and searching for exact (not nearest), return result.
        $return = Array($value=>$array[$value]);
        $exact_match = true;
    } elseif (!$exact && isset($array[$value])) {
        // If exact match found, and searching for nearest, return result.
        $return = Array($value=>$array[$value]);
        $exact_match = true;
    } else {

        if(!isset($array[$value]) && !$exact) {
            // Insert element that doesn't exist so nearest can be found
            $array[$value] = '-';
        }
    
        // Sort array so injected record is near relative numerics
        ksort($array); // Sort array on key

        // Set start and end keys
        $arr_begin = key($array);
        end($array);
        $arr_end = key($array);
        reset($array);
        
        // Loop through array until match is found, then set $prev and $next to the respective keys if exist
        while ( !is_null($key = key($array)) ) {
            $val = current($array);
            if($key == $value) {
                if($value <> $arr_begin) {
                    prev($array); // Get entry before this one
                    $prev = key($array);
                }
                                    
                next($array);         // Skip current entry as this was what we were looking for nearest to
                
                if($value <> $arr_end) {
                    next($array); // Get entry after this one
                    $next = key($array);
                }
                break;
            }
            next($array);
        }

        if($prev && $next) {
            if(($value - $prev) < ($next - $value)) {
                // Previous is closer
                $return = Array($prev=>$array[$prev]);
            } else {
                // Next is closer
                $return = Array($next=>$array[$next]);
            }
        } elseif ($prev || $next) {
            if($prev) {
                // Only Previous exists
                $return = Array($prev=>$array[$prev]);
            } elseif ($next) {
                // Only Next exists
                $return = Array($next=>$array[$next]);
            }
        }
    }

    // Return resulting Array().   $return is false if nothing matches the closest conditions, or if exact is specified and key does not exist.        
    if($return) {
        return $return+Array('exact'=>$exact_match);
    } else {
        return false;
    }
}
?>

Just to clarify, this function will return either an Array() containing the $key=>$value pair of the result and 'exact'=>(bool) whether the match was exact or not.

To access the $key=>$value pair, can be done as follows :
<?php

    $result = nearest( ... );  // Just here for clarification of the position for the code that follows.
     
    if($result) {
        // Output the Key
        echo key($result);

        // Output the Value
        echo $result( key($result) );

        // Output if exact match
        if($result['exact']) {
            echo 'This was an EXACT match';
        } else {
            echo 'This result was found with fuzzy logic';
        }

    } else {

        echo 'No result found';

    }

?>

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