max()

(PHP 4, PHP 5, PHP 7)

找出最大值

说明

max(array $values): mixed

max(mixed $value1,mixed $value2[,mixed$...]): mixed

如果仅有一个参数且为数组，max()返回该数组中最大的值。如果第一个参数是整数、字符串或浮点数，则至少需要两个参数而max()会返回这些值中最大的一个。可以比较无限多个值。

Note:

PHP 会将非数值的string当成0，但如果这个正是最大的数值则仍然会返回一个字符串。如果多个参数都求值为0且是最大值，max()会返回其中数值的0，如果参数中没有数值的0，则返回按字母表顺序最大的字符串。

参数

$values

包含了多个值的数组。

$value1

Anycomparablevalue.

$value2

Anycomparablevalue.

...

Anycomparablevalue.

返回值

max()返回参数中数值最大的值。 If multiple values can be considered of the same size, the one that is listed first will be returned.

Whenmax()is given multiplearrays, the longest array is returned. If all the arrays have the same length,max()will use lexicographic ordering to find the return value.

When given astringit will be cast as anintegerwhen comparing.

范例

使用max()的例子

<?php
echo max(1, 3, 5, 6, 7);  // 7
echo max(array(2, 4, 5)); // 5
// When 'hello' is cast as integer it will be 0. Both the parameters are equally
// long, so the order they are given in determines the result
echo max(0, 'hello');     // 0
echo max('hello', 0);     // hello
echo max('42', 3); // '42'
// Here 0 > -1, so 'hello' is the return value.
echo max(-1, 'hello');    // hello
// With multiple arrays of different lengths, max returns the longest
$val = max(array(2, 2, 2), array(1, 1, 1, 1)); // array(1, 1, 1, 1)
// 对多个数组，max 从左向右比较。
   // 因此在本例中：2 == 2，但 4 < 5
$val = max(array(2, 4, 8), array(2, 5, 7)); // array(2, 5, 7)
// 如果同时给出数组和非数组作为参数，则总是将数组视为
   // 最大值返回
$val = max('string', array(2, 5, 7), 42);   // array(2, 5, 7)
?>

参见

• min() 找出最小值
• count() 计算数组中的单元数目，或对象中的属性个数

The simplest way to get around the fact that max() won't give the key is array_search:
<?php
$student_grades = array ("john" => 100, "sarah" => 90, "anne" => 100);
$top_student = array_search(max($student_grades),$student_grades); // john
?>
This could also be done with array_flip, though overwriting will mean that it gets the last max value rather than the first:
<?php
$grades_index = array_flip($student_grades);
$top_student = $grades_index[max($student_grades)]; // anne
?>
To get all the max value keys:
<?php
$top_students = array_keys($student_grades,max($student_grades)); // john, anne
?>

I had several occasions that using max is a lot slower then using a if/then/else construct. Be sure to check this in your routines!
Ries

max(null, 0) = null
max(0, null) = 0

max() (and min()) on DateTime objects compares them like dates (with timezone info) and returns DateTime object.
<?php 
$dt1 = new DateTime('2014-05-07 18:53', new DateTimeZone('Europe/Kiev'));
$dt2 = new DateTime('2014-05-07 16:53', new DateTimeZone('UTC'));
echo max($dt1,$dt2)->format(DateTime::RFC3339) . PHP_EOL; // 2014-05-07T16:53:00+00:00
echo min($dt1,$dt2)->format(DateTime::RFC3339) . PHP_EOL; // 2014-05-07T18:53:00+03:00
?>
It works at least 5.3.3-7+squeeze17

Notice that whenever there is a Number in front of the String, it will be used for Comparison.
<?php
 max('7iuwmssuxue', 1); //returns 7iuwmssuxu
 max('-7suidha', -4); //returns -4
?>
But just if it is in front of the String
<?php
 max('sdihatewin7wduiw', 3); //returns 3
?>

Be aware if a array like this is used (e.g. values from a shopping cart):
<?php
Array
(
  [0] => 142,80
  [1] => 39,27
  [2] => 22,80
  [3] => 175,80
)?>
The result will be: 39,27 and not - as expected - 175,80
So, to find the max value, use integer only like:
<?php
Array
(
  [0] => 14280
  [1] => 3927
  [2] => 2280
  [3] => 17580
)?>
and you will get the correct result: 17580

Regarding boolean parameters in min() and max():
(a) If any of your parameters is boolean, max and min will cast the rest of them to boolean to do the comparison.
(b) true > false
(c) However, max and min will return the actual parameter value that wins the comparison (not the cast).
Here's some test cases to illustrate:
1. max(true,100)=true
2. max(true,0)=true
3. max(100,true)=100
4. max(false,100)=100
5. max(100,false)=100
6. min(true,100)=true
7. min(true,0)=0
8. min(100,true)=100
9. min(false,100)=false
10. min(100,false)=false
11. min(true,false)=false
12. max(true,false)=true

Sometimes you could need to get the max from an array which looks like this:
<?php
$arrTest = array(
  array( "day" => 1, "b" => 10 ),
  array( "day" => 2, "b" => 43 ),
  array( "day" => 3, "b" => 2 ),
  array( "day" => 4, "b" => -3 ),
  array( "day" => 5, "b" => 4 ),
  array( "day" => 6, "b" => -5 )
);
?>
all max functions i see around don't provide a way to get the max value of the values in (example) key "b"
<?php
function maxValueInArray($array, $keyToSearch)
{
  $currentMax = NULL;
  foreach($array as $arr)
  {
    foreach($arr as $key => $value)
    {
      if ($key == $keyToSearch && ($value >= $currentMax))
      {
        $currentMax = $value;
      }
    }
  }
  return $currentMax;
}
//                      array    key
$value = maxValueInArray($arrTest, "b");
?>
output: 43

To get the largest key in an array:
<?php
$array = array( 0 => 'first', 1=> 'second', /* ... */ 99 => 'nth' );
$max_key = max( array_keys( $array ) ); // 99
?>

Matlab users and others may feel lonely without the double argument output from min and max functions.
To have the INDEX of the highest value in an array, as well as the value itself, use the following, or a derivative:
<?php
function doublemax($mylist){
 $maxvalue=max($mylist);
 while(list($key,$value)=each($mylist)){
  if($value==$maxvalue)$maxindex=$key;
 }
 return array("m"=>$maxvalue,"i"=>$maxindex);
}
?>

In response to: keith at bifugi dot com
If you want to find the specific key(s) that match the maximum value in an array where the values may be duplicated, you can loop through and perform a simple check:
<?php
# Example Array
$a = array(
   'key1' => 100,
   'key2' => 90,
   'key3' => 100,
   'key4' => 90,
);
# First get the maximum value
$max = max($a);
# Create a new array containing all keys which have the max value
foreach($a as $key => $val) {   
   if($val === $max) $b[] = $key;
}
# If you want a string list, just do this
$b = implode(' ', $b);
?>
This produces consistent results and will scale well in terms of performance, whereas functions like array_search and array_flip can lead to degraded performance when dealing with large amounts of data.

Note that max() throws a warning if the array is empty:
<?php
$a = array();
max($a);
// Warning: max(): Array must contain at least one element
?>
So make sure your data isn't empty.

A way to bound a integer between two values is:
<?php
function bound($x, $min, $max)
{
   return min(max($x, $min), $max);
}
?>
which is the same as:
<?php
$tmp = $x;
if($tmp < $min)
{
  $tmp = $min;
}
if($tmp > $max)
{
   $tmp = $max;
}
$y = $tmp;
?>
So if you wanted to bound an integer between 1 and 12 for example:
Input:
<?php
$x = 0;
echo bound(0, 1, 12).'<br />';
$x = 1;
echo bound($x, 1, 12).'<br />';
$x = 6;
echo bound($x, 1, 12).'<br />';
$x = 12;
echo bound($x, 1, 12).'<br />';
$x = 13;
echo bound($x, 1, 12).'<br />';
?>
Output:
1
1
6
12
12

Note that max() can compare dates, so if you write something like this:
<?php
$dates = array('2009-02-15', '2009-03-15');
echo max($dates);
?>
you will get: 2009-03-15.

A little function for multi-dimensional arrays:
<?php
function amax($array){
  if(is_array($array)){
    foreach($array as $key => $value){
      $array[$key] = amax($value);
    }
    return max($array);
  }else{
    return $array;
  }
}
?>

<?php
/**
 * @param array $array
 *
 * @return int|null Returns the largest value of the array. Returns NULL if no 
 *   integers are found.
 */
function array_max_recursive(array $array) {
  $max = NULL;
  $stack = array($array);
  do {
    $current = array_pop($stack );
    foreach ($current as $value) {
      if (is_array($value)) {
        $stack[] = $value;
      } elseif (filter_var($value, FILTER_VALIDATE_INT) !== FALSE) {
        // max(NULL, 0) returns NULL, so cast it
        $max = (int) max($max, $value);
      }
    }
  } while (!empty($stack));
  return $max;
}
?>
 - This function is not actually recursive, but fulfills the requirement that it works on sub-arrays. I do this because PHP is not very good at recursion. I also did it because I enjoy doing things this way. 
 - It returns something of type int, never a string representation of an int. The exception is when you provide an array that does not contain any integers. It will then return NULL.
 - It ignores non-array, non-int values.

Be careful using max() with objects, as it returns a reference not a new object.
<?php
$min_date = date_create('2019-03-05');
$max_date = date_create('2019-03-06');
$date_by_reference = max($min_date, $max_date);
// $date_by_reference and $max_date are now the same object
echo '$max_date is ' . date_format($max_date, 'Y-m-d') . PHP_EOL; // '2019-03-06'
echo '$date_by_reference is ' . date_format($date_by_reference, 'Y-m-d') . PHP_EOL; // '2019-03-06'
date_add($date_by_reference, date_interval_create_from_date_string('1 days')); // Add 1 day to $date_by_reference, changes $max_date too
echo '$max_date is changed ' . date_format($max_date, 'Y-m-d') . PHP_EOL; // '2019-03-07'
echo '$date_by_reference is changed ' . date_format($date_by_reference, 'Y-m-d') . PHP_EOL; // '2019-03-07'
?>