我如何编写两个函数,如果它以指定的字符/字符串开头或以指定的字符串结尾,那么它们将接受字符串并返回?

例如:

$str = '|apples}';

echo startsWith($str, '|'); //Returns true
echo endsWith($str, '}'); //Returns true

当前回答

专注于startswith,如果您确定字符串不为空,那么在比较之前,在第一个字符、strlen等上添加一个测试会加快速度:

function startswith5b($haystack, $needle) {
    return ($haystack{0}==$needle{0})?strncmp($haystack, $needle, strlen($needle)) === 0:FALSE;
}

不知怎么的,速度快了(20%-30%)。添加另一个字符测试,如$haystack{1}==$needle{1},似乎并不能加快速度,甚至可能会减慢速度。

==似乎比==条件运算符(a)?b: c似乎比如果(a)b更快;否则c;


对于那些问“为什么不使用strpos?”并称其他解决方案为“不必要的工作”的人


strpos速度很快,但它不是这项工作的正确工具。

为了理解,这里有一个小模拟作为示例:

Search a12345678c inside bcdefga12345678xbbbbb.....bbbbba12345678c

电脑在里面做什么?

    With strccmp, etc...

    is a===b? NO
    return false



    With strpos

    is a===b? NO -- iterating in haysack
    is a===c? NO
    is a===d? NO
    ....
    is a===g? NO
    is a===g? NO
    is a===a? YES
    is 1===1? YES -- iterating in needle
    is 2===3? YES
    is 4===4? YES
    ....
    is 8===8? YES
    is c===x? NO: oh God,
    is a===1? NO -- iterating in haysack again
    is a===2? NO
    is a===3? NO
    is a===4? NO
    ....
    is a===x? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    ...
    ... may many times...
    ...
    is a===b? NO
    is a===a? YES -- iterating in needle again
    is 1===1? YES
    is 2===3? YES
    is 4===4? YES
    is 8===8? YES
    is c===c? YES YES YES I have found the same string! yay!
    was it at position 0? NOPE
    What you mean NO? So the string I found is useless? YEs.
    Damn.
    return false

假设strlen不迭代整个字符串(但即使在这种情况下),这一点都不方便。

其他回答

这可能奏效

function startsWith($haystack, $needle) {
     return substr($haystack, 0, strlen($needle)) == $needle;
}

资料来源:https://stackoverflow.com/a/4419658

最近,我通常使用下划线php这样的库。

require_once("vendor/autoload.php"); //use if needed
use Underscore\Types\String; 

$str = "there is a string";
echo( String::startsWith($str, 'the') ); // 1
echo( String::endsWith($str, 'ring')); // 1   

图书馆里还有很多其他方便的功能。

简而言之:

function startsWith($str, $needle){
   return substr($str, 0, strlen($needle)) === $needle;
}

function endsWith($str, $needle){
   $length = strlen($needle);
   return !$length || substr($str, - $length) === $needle;
}

您可以为此使用fnmatch函数。

// Starts with.
fnmatch('prefix*', $haystack);
// Ends with.
fnmatch('*suffix', $haystack);

专注于startswith,如果您确定字符串不为空,那么在比较之前,在第一个字符、strlen等上添加一个测试会加快速度:

function startswith5b($haystack, $needle) {
    return ($haystack{0}==$needle{0})?strncmp($haystack, $needle, strlen($needle)) === 0:FALSE;
}

不知怎么的,速度快了(20%-30%)。添加另一个字符测试,如$haystack{1}==$needle{1},似乎并不能加快速度,甚至可能会减慢速度。

==似乎比==条件运算符(a)?b: c似乎比如果(a)b更快;否则c;


对于那些问“为什么不使用strpos?”并称其他解决方案为“不必要的工作”的人


strpos速度很快,但它不是这项工作的正确工具。

为了理解,这里有一个小模拟作为示例:

Search a12345678c inside bcdefga12345678xbbbbb.....bbbbba12345678c

电脑在里面做什么?

    With strccmp, etc...

    is a===b? NO
    return false



    With strpos

    is a===b? NO -- iterating in haysack
    is a===c? NO
    is a===d? NO
    ....
    is a===g? NO
    is a===g? NO
    is a===a? YES
    is 1===1? YES -- iterating in needle
    is 2===3? YES
    is 4===4? YES
    ....
    is 8===8? YES
    is c===x? NO: oh God,
    is a===1? NO -- iterating in haysack again
    is a===2? NO
    is a===3? NO
    is a===4? NO
    ....
    is a===x? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    is a===b? NO
    ...
    ... may many times...
    ...
    is a===b? NO
    is a===a? YES -- iterating in needle again
    is 1===1? YES
    is 2===3? YES
    is 4===4? YES
    is 8===8? YES
    is c===c? YES YES YES I have found the same string! yay!
    was it at position 0? NOPE
    What you mean NO? So the string I found is useless? YEs.
    Damn.
    return false

假设strlen不迭代整个字符串(但即使在这种情况下),这一点都不方便。