|
|
Глава 16. Control Structures
Any PHP script is built out of a series of statements. A statement
can be an assignment, a function call, a loop, a conditional
statement or even a statement that does nothing (an empty
statement). Statements usually end with a semicolon. In addition,
statements can be grouped into a statement-group by encapsulating a
group of statements with curly braces. A statement-group is a
statement by itself as well. The various statement types are
described in this chapter.
The if construct is one of the most important
features of many languages, PHP included. It allows for
conditional execution of code fragments. PHP features an
if structure that is similar to that of C:
As described in the section about
expressions, expression is evaluated to its
Boolean value. If expression evaluates to TRUE,
PHP will execute statement, and if it evaluates
to FALSE - it'll ignore it. More information about what values evaluate
to FALSE can be found in the 'Converting to boolean'
section.
The following example would display a is bigger
than b if $a is bigger
than $b:
Often you'd want to have more than one statement to be executed
conditionally. Of course, there's no need to wrap each statement
with an if clause. Instead, you can group
several statements into a statement group. For example, this code
would display a is bigger than b
if $a is bigger than
$b, and would then assign the value of
$a into $b:
If statements can be nested indefinitely within other
if statements, which provides you with complete
flexibility for conditional execution of the various parts of your
program.
 add a note
User Contributed Notes
Control Structures
petter[no-spam] at [no-spam]helset dot eu
03-Oct-2007 12:54
if($alfa > $beta) {
$alfa = $some;
} else {
$alfa = $thing;
}
Another method:
$alfa = ( $alfa > $beta )? $some : $thing;
wintermute
29-Aug-2007 12:45
Sinured: You can do the same thing with logical OR; if the first test is true, the second will never be executed.
<?PHP
if (empty($user_id) || in_array($user_id, $banned_list))
{
exit();
}
?>
fteng1 at gmail dot com
04-Aug-2007 06:48
Here's a lazy way of doing an comparison where multiple conditions equal the same result using arrays.
if (in_array($year,array(1948, 1960, 1972, 1984, 1996, 2008)))
echo "Rat";
elseif (in_array($year,array(1949, 1961, 1973, 1985, 1997, 2009)))
echo "Ox";
elseif (in_array($year,array(1950, 1962, 1974, 1986, 1998, 2010)))
echo "Tiger";
elseif (in_array($year,array(1951, 1963, 1975, 1987, 1999, 2011)))
echo "Rabbit";
elseif (in_array($year,array(1952, 1964, 1976, 1988, 2000, 2012)))
echo "Dragon";
elseif (in_array($year,array(1941, 1953, 1965, 1977, 1989, 2001)))
echo "Snake";
elseif (in_array($year,array(1942, 1954, 1966, 1978, 1990, 2002)))
echo "Horse";
elseif (in_array($year,array(1943, 1955, 1967, 1979, 1991, 2003)))
echo "Lamb";
elseif (in_array($year,array(1944, 1956, 1968, 1980, 1992, 2004)))
echo "Monkey";
elseif (in_array($year,array(1945, 1957, 1969, 1981, 1993, 2005)))
echo "Rooster";
elseif (in_array($year,array(1946, 1958, 1970, 1982, 1994, 2006)))
echo "Dog";
elseif (in_array($year,array(1947, 1959, 1971, 1983, 1995, 2007)))
echo "Boar";
Sinured
01-Aug-2007 11:59
As mentioned below, PHP stops evaluating expressions as soon as the result is clear. So a nice shortcut for if-statements is logical AND -- if the left expression is false, then the right expression can’t possibly change the result anymore, so it’s not executed.
<?php
if (!defined('MYAPP_DIR')) {
define('MYAPP_DIR', dirname(getcwd()));
}
!defined('MYAPP_DIR') && define('MYAPP_DIR', dirname(getcwd()));
?>
captainf
08-Mar-2007 07:53
@mongoose643
dougnoel was actually trying to provide a good example of how the if function automatically evaluates to false and goes no further if the first condition in an if expression fails can be used to optimize a script. In his example not everyone is an admin, and not all admins can delete. In his script it is an optimized failsafe to check if someone is an admin and has delete permissions while not wasting script time.
mongoose643 at gmail dot com
01-Mar-2007 06:03
@simplyduh
>Duh, both are booleans in the above case, and its obvious
>that checking one boolean is better than checking 2.
>if ($has_delete_permissions) would work better :)
I read dougnoel's post. He was providing an example of a BAD example. The last line in his post suggests that you NOT check for both - only the admin. So actually what he meant to check for is whether or not they are an admin. If so then they automatically have delete permissions - therefore it may be possible that no variable named "$has_delete_permissions" exists. The resulting statement would be as follows:
if($is_admin)
{
//Statements to delete stuff go here.
}
simplyduh
04-Nov-2006 01:33
dougnoel, you are wrong recarding optimization:
> if ($is_admin && $has_delete_permissions)
> If only an admin can have those permissions, there's no
> need to check for the permissions if the user is not an admin.
Duh, both are booleans in the above case, and its obvious that
checking one boolean is better than checking 2.
if ($has_delete_permissions) would work better :)
jupiter at nospam dot com
28-Aug-2006 01:46
// assigning a variable inside an IF conditional does assign the value,
// then if it evaluates to true, continues to the true statement group
<?php
$a = array(1, 2, ' ', true, 0, '', false, array());
foreach ($a as $b) {
if ($c[] = $b) {
echo 'true, ';
} else {
echo 'false, ';
}
}
print_r($c);
?>
// Notice $c[4] and $c[7] are assigned values, but then evaluate to false
christopher dot metz at lavafrog dot net
05-Aug-2006 04:34
in the two previous examples, both are correct.
for $foo = false, you're assigning the value while operating in the condition, so its just like:
$foo = false;
if( $foo )
print( 'a' );
else
print( 'b' );
....returns 'b';
for $foo == false, it will return true or false based on if $foo equals false or true, respectively.
28-May-2006 01:07
$foo=false will never work as it is tha assign operator.
$foo == false is required
abu_hurayrah at hidayahonline dot org
24-May-2006 01:42
Regarding lamfeust's example:
Just a note about initializing variable
<?php
if($foo=false)
print("true");
else
print("false");
?>
This will print out "false" due to the expression "$foo=false" returning the value of the assignment - boolean false, in this case.
I'm not sure what, exactly, that had to do with initialization.
dougnoel
05-May-2006 06:29
Further response to Niels:
It's not laziness, it's optimization. It saves CPUs cycles. However, it's good to know, as it allows you to optimize your code when writing. For example, when determining if someone has permissions to delete an object, you can do something like the following:
if ($is_admin && $has_delete_permissions)
If only an admin can have those permissions, there's no need to check for the permissions if the user is not an admin.
roan dot kattouw at home dot nl
09-Dec-2005 04:49
In response to Niels: The ANSI C standard also dictates this laziness .
niels dot laukens at tijd dot com
26-Dec-2004 07:49
For the people that know C: php is lazy when evaluating expressions. That is, as soon as it knows the outcome, it'll stop processing.
<?php
if ( FALSE && some_function() )
echo "something";
?>
This comes in nice in situations like this:
<?php
if ( file_exists($filename) && filemtime($filename) > time() )
do_something();
?>
|
Оператор проверки принадлежности к классу
