位运算符
in reply to Hayley Watson:
Yes I know that's not a proper way to coding something serious, this was just a dummi example. The points I tried to mark were:
1. NULL is always less than 1. Some people that are starting to code can end up with that kind of validation and if they don't know this could loss some hours. Is just a point of something that is not documented. :)
2. Point out that if $x is null then why asking in a conditional for $x[0][0] doesn't give me a warning of some kind, like "Variable type doesn't support this kind of call".
Thats all. Hope it helps.
No more examples of code for this :P
比较运算符
比较运算符，如同它们名称所暗示的，允许对两个值进行比较。还可以参考 PHP 类型比较表看不同类型相互比较的例子。
| 例子 | 名称 | 结果 |
|---|---|---|
| $a == $b | 等于 | TRUE，如果 $a 等于 $b。 |
| $a === $b | 全等 | TRUE，如果 $a 等于 $b，并且它们的类型也相同。（PHP 4 引进） |
| $a != $b | 不等 | TRUE，如果 $a 不等于 $b。 |
| $a <> $b | 不等 | TRUE，如果 $a 不等于 $b。 |
| $a !== $b | 非全等 | TRUE，如果 $a 不等于 $b，或者它们的类型不同。（PHP 4 引进） |
| $a < $b | 小与 | TRUE，如果 $a 严格小于 $b。 |
| $a > $b | 大于 | TRUE，如果 $a 严格 $b。 |
| $a <= $b | 小于等于 | TRUE，如果 $a 小于或者等于 $b。 |
| $a >= $b | 大于等于 | TRUE，如果 $a 大于或者等于 $b。 |
如果比较一个整数和字符串，则字符串会被转换为整数。如果比较两个数字字符串，则作为整数比较。此规则也适用于 switch 语句。
<?php
var_dump(0 == "a"); // 0 == 0 -> true
var_dump("1" == "01"); // 1 == 1 -> true
switch ("a") {
case 0:
echo "0";
break;
case "a": // never reached because "a" is already matched with 0
echo "a";
break;
}
?>
对于多种类型，比较运算符根据下表比较（按顺序）。
| 运算数 1 类型 | 运算数 1 类型 | 结果 |
|---|---|---|
| null 或 string | string | 将 NULL 转换为 ""，进行数字或词汇比较 |
| bool 或 null | 任何其它类型 | 转换为 bool，FALSE < TRUE |
| object | object | 内置类可以定义自己的比较，不同类不能比较，相同类和数组同样方式比较属性（PHP 4 中），PHP 5 有其自己的说明 |
| string，resource 或 number | string，resource 或 number | 将字符串和资源转换成数字，按普通数学比较 |
| array | array | 具有较少成员的数组较小，如果运算数 1 中的键不存在于运算数 2 中则数组无法比较，否则挨个值比较（见下例） |
| array | 任何其它类型 | array 总是更大 |
| object | 任何其它类型 | object 总是更大 |
Example#1 标准数组比较代码
<?php
// 数组是用标准比较运算符这样比较的
function standard_array_compare($op1, $op2)
{
if (count($op1) < count($op2)) {
return -1; // $op1 < $op2
} elseif (count($op1) > count($op2)) {
return 1; // $op1 > $op2
}
foreach ($op1 as $key => $val) {
if (!array_key_exists($key, $op2)) {
return null; // uncomparable
} elseif ($val < $op2[$key]) {
return -1;
} elseif ($val > $op2[$key]) {
return 1;
}
}
return 0; // $op1 == $op2
}
?>
参见 strcasecmp()，strcmp()，数组运算符和类型一章。
三元运算符
另一个条件运算符是“?:”（或三元）运算符 。
Example#2 赋默认值
<?php
// Example usage for: Ternary Operator
$action = (empty($_POST['action'])) ? 'default' : $_POST['action'];
// The above is identical to this if/else statement
if (empty($_POST['action'])) {
$action = 'default';
} else {
$action = $_POST['action'];
}
?>
Note: 注意三元运算符是个语句，因此其求值不是变量，而是语句的结果。如果想通过引用返回一个变量这点就很重要。在一个通过引用返回的函数中语句 return $var == 42 ? $a : $b; 将不起作用，以后的 PHP 版本会为此发出一条警告。
add a note
User Contributed Notes比较运算符
jescribens at urbangolem dot com
05-Sep-2008 04:16
05-Sep-2008 04:16
Hayley Watson
06-Jun-2008 09:06
06-Jun-2008 09:06
@jescribens at urbangolem dot com
<?php
var_dump($x); //can be null
if($x[0][0]['value'] < 1)
?>
'If the function returns null then you get "nok" printed.'
Around here that sort of code would get you smacked.
First, it's very unstable. Once fixed so that $something and $other had values, it would still print "nok" if $x[0][0]['value'] existed but contained the value zero, or indeed anything other than a number that's at least one.
More importantly, if you want to check to see if something is null, wouldn't it be smarter (and certainly more self-explanatory) to check to see if it null instead of some random property that may or may not have anything to do with what you're wanting to check?
<?php
$x = myfunction();
if(is_null($x)) // or if($x===null)
{
echo 'nok';
}
else
{
echo 'ok';
}
jescribens at urbangolem dot com
05-Apr-2008 01:13
05-Apr-2008 01:13
Hi all:
I found this and maybe it can help you, i didn't found it documented anywhere :
Assuming you have a function that returns an array or a null value in failure and you do something like this
<?php
function myfunction()
{
if($something==$other)
{
return array(0=>array(0=>array('valor'=>$other)));
}else{
return null;
}
}
$x = myfunction();
var_dump($x); //can be null
if($x[0][0]['value'] < 1)
{
echo "nok";
}else{
echo "ok";
}
?>
If the function returns null then you get "nok" printed.
As far as I see NULL values are always less than 1, and the fact that I request a sub array ([0][0]['value']) doesn't give me any notice nor warning.
Hope this helps anybody.
Greetings
Samaliel
02-Apr-2008 09:42
02-Apr-2008 09:42
To lukzboz ~ at ~ gmx ~ dot ~ net :
It IS stated that, when compared to integers, strings are converted to a numeric value according to these rules : http://fr.php.net/manual/en/language.types.string.php
Since $txt does not begin with valid numerical data, its numeric value is 0.
If you try with $txt = '12 reasons I should type my variables', your code won't print anything.
<?php
$txt = 'yoursimpletext';
if( $txt == 0 )
{
echo $txt.' == 0';
} else {
echo $txt.' != 0';
}
// Prints 'yoursimpletext == 0'
$txt = '12 reasons I should type my variables';
if( $txt == 0 )
{
echo $txt.' == 0';
} else {
echo $txt.' != 0';
}
// Prints '12 reasons I should type my variables != 0'
var_dump( $txt );
// Prints 'string(37) "12 reasons I should type my variable"
var_dump( (int) $txt );
// Prints 'int(12)'
?>
lukzboz ~ at ~ gmx ~ dot ~ net
10-Mar-2008 06:38
10-Mar-2008 06:38
<?php
$txt = 'yoursimpletext';
if ( $txt == 0 )
{
echo "true";
}
// This sample prints true. php seems to convert the $txt string to integer
?>
Amaroq
25-Feb-2008 07:13
25-Feb-2008 07:13
Most of the time, you may be content with your conditionals evaluating to true if they are evaluating a non-false, non-zero value. You may also like it when they evaluate to false when you use the number 0.
However, there may be times where you want to make a distinction between a non-false value and a boolean true. You may also wish to make a distinction between a boolean false and a zero.
The identity operator can make this distinction for you.
<?php
$a = 'some string';
$b = 123;
$c = 0;
if($a && $b && (!$c))
{ echo "True.\n"; } else { echo "False.\n"; }
if($a == true && $b == true && $c == false)
{ echo "True.\n"; } else { echo "False.\n"; }
if($a === true || $b === true || $c === false)
{ echo "True.\n"; } else { echo "False.\n"; }
?>
The above code outputs the following:
True.
True.
False.
As you can see, in the first two cases, $a and $b are considered true, while $c is considered false. If this wasn't the case, neither of the first two conditionals would have echoed "True."
In the last case, I've cleverly used the || operator to demonstrate that both $a and $b do not evaluate to true with the identity operator, nor does $c evaluate to false.
The === operator can be used to distinguish boolean from non-boolean values.
ken at smallboxsoftware net
16-Nov-2007 12:20
16-Nov-2007 12:20
This seems a bit odd to me, but PHP can convert two strings into integers during comparison. For example: "700" == "+700" return true even though they are totally different strings.
Use === or strcmp when comparing two strings to ensure that they remain as strings during comparison.
kocurro at kocurro dot pl
16-Jul-2007 06:49
16-Jul-2007 06:49
Today I notices problem with comparision different data types.
My code is like that:
function checkAndChange( $values)
{
$core = &Core::_();
$user = &$core->getUser();
switch( $user->changePassword( $values['oldpass'], $values['newpass'], $values['newpass2']))
{
case( 'old'):
return 'Bad current password';
case( 'user'):
return 'Bad user';
case( 'new'):
return 'Can\'t change';
default:
return true;
}
return true;
}
function checkAndChange is called by callback, function changePassword returned string (indicating any error) or bool true (if no error). The problem was that if changePassword returned true it was always taking the first case - so in the example case( 'old'), if I changed order of cases giving case( 'new') as the first one then it was taken.
This code was used to track the value of $res (which was the return from changePassword
function checkAndChange( $values)
{
$core = &Core::_();
$user = &$core->getUser();
$res = $user->changePassword( $values['oldpass'], $values['newpass'], $values['newpass2']);
echo( '<pre>');
print_r( $res);
switch( $res)
{
case( 'old'):
return 'Bad current password';
case( 'user'):
return 'Bad user';
case( 'new'):
return 'Can\'t change';
default:
return true;
}
return true;
}
Even if constructions matched wrongly and even using $res === 'new'. Maybe that was a bug or something.
How I dealed with this ? I stoped mixing the types and from that time changePassword is returning an empty string for success.
Maybe this would help someone having the same problem as I had.
PHP 4.4.7, Windows XP
rofthorax at gmail dot com
04-Jul-2007 05:06
04-Jul-2007 05:06
$a = "iowjoiwef";
$b = 0;
print ($a == $b); // is 1
print ("$a" == "$b"); // is 0
Now someone mentions the point that
$y = "123";
print (int) $y; // is 123
Why would anyone want "iowjoiwef" to be 0 when casted to int? Why should int take precedence on operator casting when comparing two values, isn't php a string oriented language.. Is it strongly typed? What about operator overloading? Does it offer it? Really? Can we cover up this problem? No?
If you want people making more bugs, I suggest you stick with this form of self deception..
This is bad interpreter design.. You might as well compile PHP.. Because it can't be a valid interpreter..
hg at ecs dot soton dot ac dot uk
13-May-2007 08:12
13-May-2007 08:12
Warning, string comparison may be even stranger than you thought:
<?php
$a = "0e2";
$b = "0e3";
if ($a == $b) print '$a == $b'."\n"; // True
if ($a === $b) print '$a === $b'."\n"; // False
if ("000e002073" == "0e2857") print '"000e002073" == "0e2857"'."\n"; // True
?>
outputs
$a == $b
"000e002073" == "0e2857"
According to Table 15.4, since $a and $b are of the same type, it might have been expected that == and === gave the same result? Not necessarily.
The clue is in the text below the table:
"If you compare two numerical strings, they are compared as integers."
Since these strings are numerical strings (in fact floats, if you hadn't worked it out yet), they are compared as integers.
This is the behaviour we know and love about php, and I have no problem with it. I do have a problem that == and === give different answers for objects when "they are of the same type."
I think that Table 15.4 gives no inkling of this.
Table 15.5 might have made the situation clearer, but makes no distinction between == and ===.
Of course, to be really safe, we should have been using strcmp() in any case, but it seems to be quite common that people use == for strings, so it should be pointed out.
I am suggesting (by other mechanism) a change to the page, but in the meantime (or if it is rejected) this note may help.
By the way, this was a real problem for me.
If you are comparing a significant number of strings which happen to be hex, it is quite surprising to find that "000" == "0e0" whereas "000" != "0f0".
I think I might start using things like "0e0" for username and password on websites to see what breaks :-)
Hayley Watson
10-Apr-2007 01:38
10-Apr-2007 01:38
Note that the "ternary operator" is better described as the "conditional operator". The former name merely notes that it has three arguments without saying anything about what it does. Needless to say, if PHP picked up any more ternary operators, this will be a problem.
"Conditional Operator" is actually descriptive of the semantics, and is the name historically given to it in, e.g., C.
mjmendoza at krixware dot tk
24-Mar-2007 11:25
24-Mar-2007 11:25
I often see questions with == and === perhaps this snippet should do.
<?php
$x="1"; //a _string_ with numeric value 1
$y=1; //an _integer_ with value 1
echo "test == :".($x == $y); //equates to TRUE
echo "<br>test === :".($x === $y); //equates to FALSE
?>
floppie at quadra-tec dot net
07-Mar-2007 02:34
07-Mar-2007 02:34
I also ran a benchmark of equals versus identical on my own server. I found the difference to be surprisingly significant (average of 15% across multiple executions of the script). Just for kicks I had it compare int to float speed as well and that came up to roughly 40% faster in favor of int (in equal/identical comparisons - no idea how they perform otherwise), and had it compare ==/=== to !=/!== and came up to negligible difference (averages out to 0). Here are the results over 100k iterations - if you want to know exactly what comparisons were done let me know:
100000 iterations
total identical 36.142370939255
total equals 41.991316795349
total is 38.998074293137
total not 39.135613441467
total int 29.253834247589
total float 48.879853487015
%gain identical vs equals 13.928941272788
%gain is vs not 0.35144242350103
%gain int vs float 40.151550872875
total time 78.15989112854
thomas dot oldbury at tgohome dot com
28-Feb-2007 01:37
28-Feb-2007 01:37
Be careful when using the ternary operator!
The following will not evaluate to the expected result:
<?php
echo "a string that has a " . (true) ? 'true' : 'false' . " condition in. ";
?>
Will print true.
Instead, use this:
<?php
echo "a string that has a " . ((true) ? 'true' : 'false') . " condition in. ";
?>
This will evaluate to the expected result: "a string that has a true condition in. "
I hope this helps.
fernandoleal at dragoncs dot com
04-Feb-2007 03:19
04-Feb-2007 03:19
If you need nested ifs on I var its important to group the if so it works.
Example:
<?php
//Dont Works
//Parse error: parse error, unexpected ':'
$var='<option value="1" '.$status == "1" ? 'selected="selected"' :''.'>Value 1</option>';
//Works:
$var='<option value="1" '.($status == "1" ? 'selected="selected"' :'').'>Value 1</option>';
echo $var;
?>
stepheneliotdewey at gmail [period] com
07-Jan-2007 10:49
07-Jan-2007 10:49
Note that typecasting will NOT prevent the default behavior for converting two numeric strings to numbers when comparing them.
e.g.:
if ((string) '0123' == (string) '123')
print 'equals';
else
print 'doesn\'t equal';
Still prints 'equals'
As far as I can tell the only way to avoid this is to use the identity comparison operators (=== and !==).
Stephen
07-Jan-2007 09:12
07-Jan-2007 09:12
Regarding gamephreak AT removethis gmail DOT com,
I found those results interesting, so I decided to test them on my own dedicated server. I found that actually, the identity and equality operators are almost exactly the same speed when dealing with numbers and/or strings (I did not test arrays, objects, etc). I ran the same tests plus a couple of inequality tests as well, this time over a 100,000 iteration loop (rounded to 7 significant digits using round()). :
'foo' == 'foo': 0.0130451
'foo' === 'foo': 0.0124481
42 == 42: 0.0124421
42 === 42: 0.0124369
'foo' == 'bar': 0.0124562
'foo' === 'bar': 0.012475
42 == 43: 0.012471
42 === 43: 0.0124719
'42' == 42: 0.0124779
'42' === 42: 0.0124731
'42' == 43: 0.0124722
'42' === 43: 0.01248
42 != 43: 0.012444
42 !== 43: 0.0124211
This was just one snapshot, though. If I refreshed the page I was using to test this, sometimes the "winners" would reverse. In other words, no operator was clearly faster, and I think that system "noise" is may usually be more significant than the speed of these operators.
Here is sample code for one of the loops:
$time1 = microtime(true);
for ($i = 0; $i < 100000; $i++){
'42' == 43;
}
$time2 = microtime(true);
print "'42' == 43: " . round($time2 - $time1, 7) . '<br><br>';
The lesson here is that you should make your coding decisions based on what makes more sense from a logical perspective, and not what is faster. Personally I hate the equality operator since PHP's dynamic typing makes it unpredictable (e.g. two strings compared may both be cast as numbers...).
04-Jan-2007 11:11
Since php 5.2 the operator == for object vs object is not recursion safe, it will cause a fatal error if one of the objects contains a refernce to it self (indirect refferences also counts here).
If you are just checking if two object pointers points to the same object use === instead and aviod this issue (you might get a minimal speed bost too).
bishop
27-Oct-2006 06:49
27-Oct-2006 06:49
When you want to know if two arrays contain the same values, regardless of the values' order, you cannot use "==" or "===". In other words:
<?php
(array(1,2) == array(2,1)) === false;
?>
To answer that question, use:
<?php
function array_equal($a, $b) {
return (is_array($a) && is_array($b) && array_diff($a, $b) === array_diff($b, $a));
}
?>
A related, but more strict problem, is if you need to ensure that two arrays contain the same key=>value pairs, regardless of the order of the pairs. In that case, use:
<?php
function array_identical($a, $b) {
return (is_array($a) && is_array($b) && array_diff_assoc($a, $b) === array_diff_assoc($b, $a));
}
?>
Example:
<?php
$a = array (2, 1);
$b = array (1, 2);
// true === array_equal($a, $b);
// false === array_identical($a, $b);
$a = array ('a' => 2, 'b' => 1);
$b = array ('b' => 1, 'a' => 2);
// true === array_identical($a, $b)
// true === array_equal($a, $b)
?>
(See also the solution "rshawiii at yahoo dot com" posted)
pcdinh at phpvietnam dot net
11-Aug-2006 12:50
11-Aug-2006 12:50
You should be very careful when using == with an result set returned from a query that can be an empty array, multi-dimensional array or a boolean false value (if the query failed to execute). In PHP, an empty array is equivalent to true.
<?php
$myArray = array();
// check if there is any error with the query
if ($myArray == false)
{
echo "Yes";
}
?>
return Yes
Use === instead.
gamephreak AT removethis gmail DOT com
18-Jul-2006 07:28
18-Jul-2006 07:28
"Identical to" is faster compared to "equals to." I ran a test script that compared various combinations of strings and integers over a 10000x loop. Sample results:
'foo' v. 'foo' eq: 0.00274
'foo' v. 'foo' id: 0.001671
42 v. 42 eq: 0.00171
42 v. 42 id: 0.001436
'foo' v. 'bar' eq: 0.002589
'foo' v. 'bar' id: 0.001658
42 v. 43 eq: 0.001719
42 v. 43 id: 0.001419
'42' v. 42 eq: 0.00223
'42' v. 42 id: 0.001392
'42' v. 43 eq: 0.002281
'42' v. 43 id: 0.001389
Alex
17-May-2006 10:49
17-May-2006 10:49
I think everybody should read carefully what "jeronimo at DELETE_THIS dot transartmedia dot com" wrote. It's a great pitfall even for seasoned programmers and should be looked upon with a great attention.
For example, comparing passwords with == may result in a very large security hole.
I would add some more to it:
The workaround is to use strcmp() or ===.
Note on ===:
While the php documentation says that, basically,
($a===$b) is the same as ($a==$b && gettype($a) == gettype($b)),
this is not true.
The difference between == and === is that === never does any type conversion. So, while, according to documentation, ("+0.1" === ".1") should return true (because both are strings and == returns true), === actually returns false (which is good).
adam at caucho dot com
09-May-2006 01:49
09-May-2006 01:49
Note: according to the spec, PHP's comparison
operators are not transitive. For example,
the following are all true in PHP5:
"11" < "a" < 2 < "11"
As a result, the outcome of sorting an array
depends on the order the elements appear
in the pre-sort array. The following code will
dump out two arrays with *different* orderings:
<?php
$a = array(2, "a", "11", 2);
$b = array(2, "11", "a", 2);
sort($a);
var_dump($a);
sort($b);
var_dump($b);
?>
This is not a bug report -- given the spec on
this documentation page, what PHP does is
"correct". But that may not be what was
intended...
kroczu at interia dot pl
14-Feb-2006 09:26
14-Feb-2006 09:26
<?
//attention about this case!
$a=0;
if($a=="some string..."){ /* (...) -> true! */ }
if($a==="some string..."){ /* (...) -> false */ }
$a=1;
if($a=="some string..."){ /*(...) -> false */ }
if($a==="some string..."){ /*(...) -> false */ }
?>
rshawiii at yahoo dot com
19-Jan-2006 03:36
19-Jan-2006 03:36
You can't just compare two arrays with the === operator
like you would think to find out if they are equal or not. This is more complicated when you have multi-dimensional arrays. Here is a recursive comparison function.
/**
* Compares two arrays to see if they contain the same values. Returns TRUE or FALSE.
* usefull for determining if a record or block of data was modified (perhaps by user input)
* prior to setting a "date_last_updated" or skipping updating the db in the case of no change.
*
* @param array $a1
* @param array $a2
* @return boolean
*/
function array_compare_recursive($a1, $a2)
{
if (!(is_array($a1) and (is_array($a2)))) { return FALSE;}
if (!count($a1) == count($a2))
{
return FALSE; // arrays don't have same number of entries
}
foreach ($a1 as $key => $val)
{
if (!array_key_exists($key, $a2))
{return FALSE; // uncomparable array keys don't match
}
elseif (is_array($val) and is_array($a2[$key])) // if both entries are arrays then compare recursive
{if (!array_compare_recursive($val,$a2[$key])) return FALSE;
}
elseif (!($val === $a2[$key])) // compare entries must be of same type.
{return FALSE;
}
}
return TRUE; // $a1 === $a2
}
johnjc-phpdocs at publicinfo dot net
02-Nov-2005 02:41
02-Nov-2005 02:41
The === and !== are not fully documented in either the Comparison Operator, Booleans type sections. They are talked about a bit more in the sections on strpos() and array_search() but those sections refer you to the section on Booleans for further information.
I am putting my contribution on === and !== in the Booleans section with pointers to it from the comment areas of other sections.
http://uk.php.net/manual/en/language.types.boolean.php
sam at liddicott dot com
01-Nov-2005 11:42
01-Nov-2005 11:42
Don't be deceived into thinking that === compares objects for identity, it does not. It does a recusrive comparison which fails if there are recusrive object references involved, and will report as identical two different object instances that happen to merely have the same value.
You can see the stated problem at: http://bugs.php.net/bug.php?id=35053
My workaround is php::CompareObjects($a,$b)
# static class to hold methods to encapsulate php deficiencies in various versions
class php {
# in === on objects in php4 does a dumb recusrive check instead
function CompareObject(&$a,&$b) {
$value='Bah! Stupid===';
$key="bah".rand(0,1000);
while(isset($a->$key)) $key.=rand(0,9);
if (isset($b->$key)) return FALSE;
$a->$key=$value;
$result=($a->$key===$b->$key);
unset($a->$key);
return $result;
}
}
08-Sep-2005 04:09
The following contrasts the trinary operator associativity in PHP and Java. The first test would work as expected in Java (evaluates left-to-right, associates right-to-left, like if stmnt), the second in PHP (evaluates and associates left-to-right)
<?php
echo "\n\n######----------- trinary operator associativity\n\n";
function trinaryTest($foo){
$bar = $foo > 20
? "greater than 20"
: $foo > 10
? "greater than 10"
: $foo > 5
? "greater than 5"
: "not worthy of consideration";
echo $foo." => ".$bar."\n";
}
echo "----trinaryTest\n\n";
trinaryTest(21);
trinaryTest(11);
trinaryTest(6);
trinaryTest(4);
function trinaryTestParens($foo){
$bar = $foo > 20
? "greater than 20"
: ($foo > 10
? "greater than 10"
: ($foo > 5
? "greater than 5"
: "not worthy of consideration"));
echo $foo." => ".$bar."\n";
}
echo "----trinaryTestParens\n\n";
trinaryTestParens(21);
trinaryTestParens(11);
trinaryTest(6);
trinaryTestParens(4);
?>
Output:
######----------- trinary operator associativity
----trinaryTest
21 => greater than 5
11 => greater than 5
6 => greater than 5
4 => not worthy of consideration
----trinaryTestParens
21 => greater than 20
11 => greater than 10
6 => greater than 5
4 => not worthy of consideration
pandreas81 at NO dot gmx dot SPAM dot de
02-Jun-2005 06:40
02-Jun-2005 06:40
Please note that the example in some notes before is not 100% correct, because it misses the call of abs().
To compare a float you should use abs($myFloat - 0.3) < EPSILON. This way of comparison is very common in other languages and it's cleaner than converting the values to strings. I suggest using a constant instead of a variable for EPSILON.
mita at abv dot bg
24-Mar-2005 10:34
24-Mar-2005 10:34
Float values comparation:
if ( (string) $a == (string) $b) {
print 'a is equal to b';
} else {
print 'a is not equal to b';
}
sven dot heyll at web dot de
05-Dec-2004 05:19
05-Dec-2004 05:19
Hi folks,
to the float comparison problem...
This worked for me:
//! compare two floating point values, return true if they are equal
//! (enough) or false otherwise
function float_equal($f1, $f2)
{
return ($f1 > $f2) ? (false) : (!($f1 < $f2));
}
// compare floats
$f1 = 0.037;
$f2 = 1000387.978;
echo "$f1 and $f2 are ".(float_equal($f1,$f2)?("equal"):("not equal"))."<br />";
$f1 = 0.3;
$f2 = 0.3;
echo "$f1 and $f2 are ".(float_equal($f1,$f2)?("equal"):("not equal"))."<br />";
02-Dec-2004 07:47
Concerning floats: It is simply pointless to compare a float with the value "0.3". The number 0.3 is not exactly representable in binary. Period. So is the number 0.1, and an infinite number of others numbers. Just like 1/3 is not exactly representable in decimal. How would you code the test for your float to be one third? Maybe $myFloat == 0.33333333333 Hmm: you see: Everyone would agree that this test is not accurate.
The test $myFloat == 0.3 is making exactly the same mistake.
So the float which you think should be 0.3 is really something very close to it; if you print it in decimal, the conversion will end up with the closest decimal representation, which may well be "0.3". But "0.3" is also the "right display decimal" for hundreds of float values.
The correct way to "compare" floats is: ( $myFloat - 0.3 ) < $EPSILON where $EPSILON is something like 1e-10, depending on your application. Approximate algorithms are the only right way to treat floats, which are only approximate by themselves.
darkcryst at gmail dot com
21-Oct-2004 06:35
21-Oct-2004 06:35
be REALLY careful when comparing float values. PHP, like most languages, is vunerable to he problems of floating point precision.
Do NOT compare floats directly, and this also means any decimal value at all.
The headaches finding out if $var = 0.3 caused me are unbelievable. even though the stored variable WAS 0.3 it was still ignoring it - this was to do with the internal float not recognising it as 0.3 even though it *displayed* it as 0.3 when echo'd or printed... basically it was stored imprecisly, but not able to display it as such.
I ended up having to compare after multiplying by 10, rounding, and then dividing by 10... rediculous, but sadly the only way I could find to reliably do it. The round seemed to remove the break in the float...
jesus that took me ages to find...
hiroh2k at yahoo dot com
20-Oct-2004 02:05
20-Oct-2004 02:05
if you want to use the ?: operator, you should be careful with the precedence.
Here's an example of the priority of operators:
echo 'Hello, ' . isset($i) ? 'my friend: ' . $username . ', how are you doing?' : 'my guest, ' . $guestusername . ', please register';
This make "'Hello, ' . isset($i)" the sentence to evaluate. So, if you think to mix more sentences with the ?: operator, please use always parentheses to force the proper evaluation of the sentence.
echo 'Hello, ' . (isset($i) ? 'my friend: ' . $username . ', how are you doing?' : 'my guest, ' . $guestusername . ', please register');
for general rule, if you mix ?: with other sentences, always close it with parentheses.
jeronimo at DELETE_THIS dot transartmedia dot com
02-Jul-2004 07:01
02-Jul-2004 07:01
For converted Perl programmers: use strict comparison operators (===, !==) in place of string comparison operators (eq, ne). Don't use the simple equality operators (==, !=), because ($a == $b) will return TRUE in many situations where ($a eq $b) would return FALSE.
For instance...
"mary" == "fred" is FALSE, but
"+010" == "10.0" is TRUE (!)
In the following examples, none of the strings being compared are identical, but because PHP *can* evaluate them as numbers, it does so, and therefore finds them equal...
<?
echo ("007" == "7" ? "EQUAL" : "not equal");
// Prints: EQUAL
// Surrounding the strings with single quotes (') instead of double
// quotes (") to ensure the contents aren't evaluated, and forcing
// string types has no effect.
echo ( (string)'0001' == (string)'+1.' ? "EQUAL" : "not equal");
// Prints: EQUAL
// Including non-digit characters (like leading spaces, "e", the plus
// or minus sign, period, ...) can still result in this behavior, if
// a string happens to be valid scientific notation.
echo (' 131e-2' == '001.3100' ? "EQUAL" : "not equal");
// Prints: EQUAL
?>
If you're comparing passwords (or anything else for which "near" precision isn't good enough) this confusion could be detrimental. Stick with strict comparisons...
<?
// Same examples as above, using === instead of ==
echo ("007" === "7" ? "EQUAL" : "not equal");
// Prints: not equal
echo ( (string)'0001' === (string)'+1.' ? "EQUAL" : "not equal");
// Prints: not equal
echo (' 131e-2' === '001.3100' ? "EQUAL" : "not equal");
// Prints: not equal
?>
webmaster __AT__ digitalanime __DOT__ nl
14-Apr-2004 06:31
14-Apr-2004 06:31
WARNING!!!!
Let's say, we have this little script:
<?php
$username = 'Me';
$guestusername = 'Guest';
echo 'Hello, ' . isset($i) ? 'my friend: ' . $username . ', how are you doing?' : 'my guest, ' . $guestusername . ', please register';
?>
What you want:
If $i is set, display:
Hello, my friend: Me, how are you doing?
If not, display:
Hello, my guest, Guest, please register
BUT, you DON'T get that result!
If $i is set, you get this:
my friend: Me, how are you doing? (so, there's not "Hello, " before it)
If $i is NOT set, you get this:
my friend: Me, how are you doing?
So... That's the same!
You can solve this by using the "(" and ")" to give priority to the ternary operator:
<?php
$username = 'Me';
$guestusername = 'Guest';
echo 'Hello, ' . (isset($i) ? 'my friend: ' . $username . ', how are you doing?' : 'my guest, ' . $guestusername . ', please register');
?>
When $i is set, you get this:
Hello, my friend: Me, how are you doing? (expected)
When $i is NOT set, you get this:
Hello, my guest, Guest, please register (expected too)
So.. Please, don't be dumb and ALWAYS use the priority-signs (or.. How do you call them?), ( and ).
By using them, you won't get unneeded trouble and always know for sure your code is doing what you want: The right thing.
user@example
04-Apr-2004 06:17
04-Apr-2004 06:17
With Nested ternary Operators you have to set the logical parentheses to get the correct result.
<?php
$test=true;
$test2=true;
($test) ? "TEST1 true" : ($test2) ? "TEST2 true" : "false";
?>
This will output: TEST2 true;
correct:
<?php
$test=true;
$test2=true;
($test) ? "TEST1 true" : (($test2) ? "TEST2 true" : "false");
?>
Anyway don't nest them to much....!!
jwhiting at hampshire dot edu
10-Dec-2003 07:31
10-Dec-2003 07:31
note: the behavior below is documented in the appendix K about type comparisons, but since it is somewhat buried i thought i should raise it here for people since it threw me for a loop until i figured it out completely.
just to clarify a tricky point about the == comparison operator when dealing with strings and numbers:
('some string' == 0) returns TRUE
however, ('123' == 0) returns FALSE
also note that ((int) 'some string') returns 0
and ((int) '123') returns 123
the behavior makes senes but you must be careful when comparing strings to numbers, e.g. when you're comparing a request variable which you expect to be numeric. its easy to fall into the trap of:
if ($_GET['myvar']==0) dosomething();
as this will dosomething() even when $_GET['myvar'] is 'some string' and clearly not the value 0
i was getting lazy with my types since php vars are so flexible, so be warned to pay attention to the details...
mina86 at tlen dot pl
04-Dec-2003 07:03
04-Dec-2003 07:03
I guess quicker method would be:
<?php
$colors = array('#EEEEEE', '#FFFFFF');
while ($i++ < 10) {
?>
<tr bgColor="<?=$colors[$i%2]?>">
<td>Cell 1-<?=$i?></td>
<td>Cell 2-<?=$i?></td>
<td>Cell 3-<?=$i?></td>
<td>Cell 4-<?=$i?></td>
</tr>
<?php } ?>
And you can easily add more colors:
<?php
$colors = array('#EEEEEE', '#FFFFFF', '#DDDDDD', '#CCCCCC');
while ($i++ < 10) {
?>
<tr bgColor="<?=$colors[$i%4]?>">
<td>Cell 1-<?=$i?></td>
<td>Cell 2-<?=$i?></td>
<td>Cell 3-<?=$i?></td>
<td>Cell 4-<?=$i?></td>
</tr>
<?php } ?>
xhenxhe
13-Nov-2003 10:27
13-Nov-2003 10:27
This operator is very is usefull and will save you time and lines once you get the hang of it. Try this next time you are looping through rows in a table and want to change the bgColor:
<table border=1 width=100% align=center style="border-collapse: collapse" bgColor="lightblue">
<tr>
<td>Column 1</td>
<td>Column 2</td>
<td>Column 3</td>
<td>Column 4</td>
</tr>
<?
while ($i++ < 10)
{
$rbg = $rbg=="#EEEEEE" ? "#FFFFFF" : "#EEEEEE";
?>
<tr bgColor="<?=$rbg?>">
<td>Cell 1-<?=$i?></td>
<td>Cell 2-<?=$i?></td>
<td>Cell 3-<?=$i?></td>
<td>Cell 4-<?=$i?></td>
</tr>
<? } ?>
</table>