Differences and performance comparison between foreach and while loops

foreach and while both loop in php, so what is the difference between foreach and while loop? Which one will have better performance? Let me introduce to you the difference between foreach and while loop. Performance comparison.

In a while loop, Perl will read a line of input, store it in a variable and execute the loop body. Then, it goes back to find other input lines.

In the foreach loop, the entire line of input operator will be executed in the list context (because foreach needs to process the contents of the list line by line). Before the loop can start executing, it must read all the input.

When inputting large-capacity files, using foreach will occupy a lot of memory. The difference between the two will be very obvious. Therefore, the best approach is usually to use the abbreviation of while loop and let it process one line at a time.

When you want to repeatedly execute certain statements or paragraphs, C# provides 4 different loop statement options for you to use according to different current tasks:
. for statement
. foreach statement
. while statement
. do statement

1.for

The for statement is particularly useful when you know in advance how many times an contained statement should be executed. The regular syntax allows an enclosed statement (and loop expression) to be executed repeatedly when a condition is true:

for (initialization; condition; loop) enclosed statement

Please note that initialization, conditions and loops are optional. If you omit the condition, you can create an infinite loop that requires a jump statement (break or goto) to exit.

for (;;)
{
   break; // 由于某些原因
}

Another important point is that you can add multiple comma-separated statements to all three parameters of the for loop at the same time. For example, you can initialize two variables, have three conditionals, and repeat 4 variables.

2.foreach

One feature that has existed in the Visual Basic language for a long time is the collection of enumerations by using the For Each statement. C# also has a command for collecting enumerations through the foreach statement:

foreach (type identifier in expression) containing statement

The loop variable is declared by the type and identifier, and Expressions correspond to collections. The loop variable represents the collection element for which the loop is running.

3.while

When you want to execute an included statement 0 or more times, the while statement is exactly what you want:

while (condition) INCLUDED STATEMENT

The conditional statement - which is also a Boolean expression - controls the number of times the included statement is executed. You can use break and continue statements to control the execution of statements in a while statement, which operates in exactly the same way as in a for statement.

4,do

C#The last available loop statement is the do statement. It is very similar to a while statement in that the condition is verified only after the initial loop.

do
{
内含语句
}
while (条件);

The do statement guarantees that the contained statements have been executed at least once, and they continue to be executed as long as the condition evaluates to true. By using the break statement, you can force execution to exit the do block. If you want to skip this loop, use the continue statement.

Performance comparison

foreach operates on a copy of the array (by copying the array), while while operates by moving the internal index of the array. Under general logic, it is believed that while should be faster than foreach (because foreach first copies the array when it starts executing, while while directly moves the internal pointer), but the result is just the opposite.
In the loop, the array "read" operation is performed, so foreach is faster than while:

foreach ($array as $value) { 
echo $value; 
} 
while (list($key) = each($array)) { 
echo $array[$key]; 
}

In the loop, the array "write" operation is performed, then while is faster than foreach:

foreach ($array as $key => $value) { 
echo $array[$key] = $value . '...'; 
} 
while (list($key) = each($array)) { 
$array[$key] = $array[$key] . '...'; 
}

Let us first test the time it takes to traverse a one-dimensional array with 50,000 subscripts:

<?php
/*
  * @ Author: Lilov
  * @ Homepage: www.111cn.net
  * @ E-mail: zhongjiechao@gmail.com
  *
  */
$arr = array();
for($i = 0; $i < 50000; $i++){
$arr[] = $i*rand(1000,9999);
}
function GetRunTime()
{
list($usec,$sec)=exp lode(" ",microtime());
return ((float)$usec+(float)$sec);
}
######################################
$time_start = GetRunTime();
for($i = 0; $i < count($arr); $i++){
$str .= $arr[$i];
}
$time_end = GetRunTime();
$time_used = $time_end - $time_start;
echo &#39;Used time of for:&#39;.round($time_used, 7).&#39;(s)<br><br>&#39;;
unset($str, $time_start, $time_end, $time_used);
######################################
$time_start = GetRunTime();
while(list($key, $val) = each($arr)){
$str .= $val;
}
$time_end = GetRunTime();
$time_used = $time_end - $time_start;
echo &#39;Used time of while:&#39;.round($time_used, 7).&#39;(s)<br><br>&#39;;
unset($str, $key, $val, $time_start, $time_end, $time_used);
######################################
$time_start = GetRunTime();
foreach($arr as $key => $val){
$str .= $val;
}
$time_end = GetRunTime();
$time_used = $time_end - $time_start;
echo &#39;Used time of foreach:&#39;.round($time_used, 7).&#39;(s)<br><br>&#39;;
######################################
?>

Test results:

will be three times Average the test results:
corresponds to for, while, and foreach respectively
0.1311650
0.1666853
0.1237440

After repeated tests, the results show that for traversing the same array, foreach The fastest one is while. The slowest one is while. foreach is about 20% ~ 30% faster than while. Then add the array subscript to 500000 and 5000000 and the test result is the same. But from a principle point of view, foreach operates on a copy of the array (by copying the array), while while operates by moving the internal index of the array. Generally speaking, it is believed that while should be faster than foreach (because foreach first executes the The array is copied in, while while moves the internal pointer directly), but the result is just the opposite. The reason should be that foreach is an internal implementation of PHP, while while is a general loop structure.

The above is the detailed content of Differences and performance comparison between foreach and while loops. For more information, please follow other related articles on the PHP Chinese website!