The Queue in C programming

Queues

A queue is similar to a checkout line in a grocery store—the first person in line is serviced first, and other customers enter the line only at the end and wait to be serviced.

Queue nodes are removed only from the head of the queue and are inserted only at the tail(rear) of the queue . For this reason, a queue is referred to as a first-in, first-out (FIFO) data structure.

Queue in C programming — Queues in C programming

The insert and remove operations are known as enqueue and dequeue, respectively.

Function enqueue

Function enqueue receives three arguments: the address of the pointer to the head of the queue, the address of the pointer to the tail of the queue and the value to be inserted in the queue.


    // insert a node in at queue tail
    void enqueue( QueueNodePtr *headPtr, QueueNodePtr *tailPtr, 
    char value ){ 
    QueueNodePtr newPtr; // pointer to new node
    newPtr = malloc( sizeof( QueueNode ) );
    if ( newPtr != NULL ) { // is space available 
      newPtr->data = value;
      newPtr->nextPtr = NULL;
    
    // if empty, insert node at head
      if ( isEmpty( *headPtr ) ) {
         *headPtr = newPtr;
      } // end if
      else {
         ( *tailPtr )->nextPtr = newPtr;
      } // end else
      *tailPtr = newPtr;
    } // end if
    else {
      printf( "%c not inserted. No memory available.\n", value );
    } // end else
    } // end function enqueue

Function dequeue

Function dequeue receives the address of the pointer to the head of the queue and the address of the pointer to the tail of the queue as arguments and removes the first node from the queue.


    // remove node from queue head
    char dequeue( QueueNodePtr *headPtr, QueueNodePtr *tailPtr ){ 
    char value; // node value
    QueueNodePtr tempPtr; // temporary node pointer
    value = ( *headPtr )->data;      
    tempPtr = *headPtr;              
    *headPtr = ( *headPtr )->nextPtr;
    
    // if queue is empty
    if ( *headPtr == NULL ) {
      *tailPtr = NULL;
    } // end if
    free( tempPtr );
    return value;
    } // end function dequeue

Note: to run the program you need to build first a main menu using the switch (choice) statement.

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