LOOPING
·
loops are
used to execute a set of statements repeatedly until a particular condition is
satisfied.
·
A
sequence of statements are executed until a specified condition is true.
·
This
sequence of statements to be executed is kept inside the curly braces { } known
as the Loop body.
·
After
every execution of loop body, condition is verified, and if it is found to be
true the loop body is executed again.
·
When the
condition check returns false, the loop body is not executed.
PHP supports following four loop
types.
·
for − loops through a block of code
a specified number of times.
·
foreach − loops through a block of
code for each element in an array.
·
while − loops through a block of
code if and as long as a specified condition is true.
·
do...while − loops through a block
of code once, and then repeats the loop as long as a special condition is true.
continue and break keywords
are used to control the loops execution.
1.For Loop:
A
for-loop (or simply for loop) is a control flow
statement for specifying iteration, which allows code to be executed
repeatedly. ...
For-loops are
typically used when the number of iterations is known before entering the loop.
The syntax of a for loop
in php is −
for ( inititialization ; condition; increment ) {
statement(s);
}
Flow Diagram of For Loop:
Here is the flow of control in a
'for' loop −
· The initialization step is
executed first, and only once. This step allows you to declare and initialize
any loop control variables. You are not required to put a statement here, as
long as a semicolon appears.
· Next, the condition is
evaluated. If it is true, the body of the loop is executed. If it is false, the
body of the loop does not execute and the flow of control jumps to the next
statement just after the 'for' loop.
· After the body of the 'for' loop executes, the
flow of control jumps back up to the increment statement. This
statement allows you to update any loop control variables. This statement can
be left blank, as long as a semicolon appears after the condition.
· The condition is now evaluated again. If it is
true, the loop executes and the process repeats itself (body of loop, then
increment step, and then again condition). After the condition becomes false,
the 'for' loop terminates.
Example 1:
<?php
for ($xint=0; $xint<=5;
$xint++)
{
echo "Number is : $xint <br />";
}
?>
Output:
Example 2:
.
<?php
for ($xint = 1; ; $xint++) {
if ($i >= 5) {
break;
}
echo $xint;
}
?>
Output:
Example 3:
<?php
$xint = 1;
for (; ; )
{
if ($xint >= 5)
{
break;
}
echo $xint;
$xint++;
}
?>
Output:
2.FOREACH LOOP:
Foreach and for loop
are mostly performs the same except when you're looping over an array where
each member has an index. at that point the for loop performs
better.
PHP 4
introduced "foreach" construct, it works only on arrays. The foreach
looping is the best way to access each key/value pair from an array.
Foreach loop iterates over
collections. Many of these collections are implemented as linked list.
Advantages of
foreach loop:
But there are still other
advantages using foreach loop than micro-optimisations.
·
Using
a foreach loop demonstrates to anyone using your code that you
are planning to do something to each member of a collection.
·
It also
shows you aren't modifying the original collection (and throws an exception if
you try to).
·
foreach is that
it works on any IEnumerable, where as for only
makes sense for IList, where each element actually has an index.
- At some stage it might be
easier to adapt code using foreach to run on multiple
cores.
- It increases the abstraction
level - instead of having to express the low-level details of how to
loop around a list or array (with an index or iterator), the
developer simply states that they want to loop and the language
takes care of the rest.
·
This aids
readability and clarity.
Syntax:
foreach
(array_expr as $value)
{
statement
}
array_expr
is an array. In every loop the value of the current element of the array is
assigned to $value and the internal array pointer is advanced by one and the
process continue to reach the last array element.
OR
foreach
(array_expr as $key => $value)
{
statement
}
array_expr
is an array. In every loop the current element's key is assigned to $value and
the internal array pointer is advanced by one and the process continue to reach
the last array element.
Flow
chart of Foreach loop:
Example:
<?php
$fruits_list = array("apple","banana","Mango","Orange","Grapes");
foreach($fruits _list as $array_values){
echo $array_values . "<br>";
}
?>
Output:
Foreach loop using an associative array :
Another example that loops through an associative array.
An associative array uses alphanumeric words for access keys.
Example2:
<?php
$persons = array("Mary" =>
"Female", "John" => "Male",
"Mirriam" => "Female");
foreach($persons as $key => $value){
echo "$key is
$value"."<br>";
}
?>
Output:
Here the names have been used as array keys and gender as the
values. Here the operator “=>” represents the relationship between a key and
value.Ypu can imagine that the key points => to the value.
3.WHILE LOOP:
The while
statement is simple, it executes the statement(s) repeatedly as long as the
condition is true. The condition is checked every time at the
beginning of the loop.
They are used to execute a block of code a repeatedly until the set
condition gets satisfied
When to use while loops
- While
loops are used to execute a block of code until a certain condition
becomes true.
- You
can use a while loop to read records returned from a database query.
Example: Suppose you want to calculate
gross salaries of 20 different persons or take a list of maximum and minimum
temperatures of a certain month or a year, the while loop is ideal to solve
these types of cases.
Types of while loops
- Do…
while -
executes the block of code at least once before evaluating the condition
- While… - checks the condition
first. If it evaluates to true, the block of code is executed as long as
the condition is true. If it evaluates to false, the execution of the
while loop is terminated.
Syntax of While loop:
<?php
While(condition){
Statements
to be executed;
}
?>
Here
- “while(…){…}” is the while loop
block code
- “condition” is the condition to be
evaluated by the while loop
- “statements…” is the code to be
executed if the condition gets satisfied
How it works(Data Flow)
The flow chart shown below illustrates how the while… loop works.
Example:
<?php
$i = 0;
while ($i < 5){
echo $i + 1 . "<br>";
$i++;
}
?>
Output:
4.DO…WHILE LOOP:
The difference between While… loop and Do… while loop is do… while is
executed at-least once before the condition is evaluated.
Do… while loop is executed at least once even if
the set condition evaluates to false.
Syntax:
<?php
do{
Statements
to be executed;
}
While(condition
is true)
?>
HERE,
- “do{…}
while(…)” is
the do… while loop block code
- “condition” is the condition to be
evaluated by the while loop
- “statements…” is the code that is executed
at least once by the do… while loop
How it works(Data Flow)
The flow chart shown below illustrates how the while… loop works.
Example:
<?php
$i = 9;
do{
echo "$i is"."
<br>";
}
while($i < 9);
?>
Output: 9
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