PHP - Ds\Queue::peek() Function

The PHP Ds\Queue::peek() function is used to retrieve a value at the front of a queue without removing it. The front value of the queue refers to the top value. If the current queue is empty ([]), this function will throw an "UnderflowException".

Syntax

Following is the syntax of the PHP Ds\Queue::peek() function −

public Ds\Queue::peek(): mixed

Parameters

This function does not accept any parameters.

Return value

This function returns a value at the front of a queue.

Example 1

The following is the basic example of the PHP Ds\Queue::peek() function −

<?php  
   $queue = new \Ds\Queue([10, 20, 30, 40, 50]);
   echo "The queue elements are: \n";
   print_r($queue);
   echo "The peek element in queue is: ";
   print_r($queue->peek());
?>

Output

The above program produces the following output −

The queue elements are:
Ds\Queue Object
(
    [0] => 10
    [1] => 20
    [2] => 30
    [3] => 40
    [4] => 50
)
The peek element in queue is: 10

Example 2

Following is another example of the PHP Ds\Queue::peek() function. We use this function to retrieve the top element of this queue (["Tutorials", "Point", "India"]) −

<?php  
   $queue = new \Ds\Queue(["Tutorials", "Point", "India"]);
   echo "The queue elements are: \n";
   print_r($queue);
   echo "The peek element in queue is: ";
   print_r($queue->peek());
?>

Output

After executing the above program, the following output will be displayed −

The queue elements are:
Ds\Queue Object
(
    [0] => Tutorials
    [1] => Point
    [2] => India
)
The peek element in queue is: Tutorials

Example 3

If the current queue is empty ([]), this function will throw an "UnderflowException" −

<?php  
   $queue = new \Ds\Queue([]);
   echo "The queue elements are: \n";
   print_r($queue);
   echo "The peek element in queue is: ";
   print_r($queue->peek());
?>

Output

Once the above program is executing, it throws the following exception −

The queue elements are:
Ds\Queue Object
(
)
The peek element in queue is: PHP Fatal error:  Uncaught UnderflowException: 
Unexpected empty state in C:\Apache24\htdocs\index.php:6
Stack trace:
#0 C:\Apache24\htdocs\index.php(6): Ds\Queue->peek()
#1 {main}
  thrown in C:\Apache24\htdocs\index.php on line 6