Nodes and Linked Lists in Software

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Nodes and Linked Lists
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Display 178 Searching a Linked List
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Linked Data Structures
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Since empty lists present some minor problems that would clutter our discussion, we will at rst assume that the linked list contains at least one node Later we will come back and make sure the algorithm works for the empty list as well This search technique yields the following algorithm:
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Pseudocode for search Function
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Make the pointer variable here point to the head node (that is, first node) of the linked list
while (here is not pointing to a node containing target and here is not pointing to the last node) { Make here point to the next node in the list } if (the node pointed to by here contains target) return here; else return NULL;
To move the pointer here to the next node, we must think in terms of the named pointers we have available The next node is the one pointed to by the pointer member of the node currently pointed to by here The pointer member of the node currently pointed to by here is given by the expression
here->getLink( )
To move here to the next node, we want to change here so that it points to the node that is pointed to by the above-named pointer Hence, the following will move the pointer here to the next node in the list:
here = here->getLink( );
Putting these pieces together yields the following re nement of the algorithm pseudocode for the search function:
algorithm refinement
here = head; while (here->getData( ) != target && here->getLink( ) != NULL) here = here->getLink( ); if (here->getData( ) == target) return here; else return NULL;
Notice the Boolean expression in the while statement We test to see if here is pointing to the last node by testing to see if the member variable here->getLink( ) is equal to NULL
Nodes and Linked Lists
We still must go back and take care of the empty list If we check the previous code, we nd that there is a problem with the empty list If the list is empty, then here is equal to NULL and hence the following expressions are unde ned:
here->getData( ) here->getLink( )
empty list
When here is NULL, it is not pointing to any node, so there is no data member or link member Hence, we make a special case of the empty list The complete function definition is given in Display 179
Display 179
Function to Locate a Node in a Linked List
FUNCTION DECLARATION
IntNodePtr search(IntNodePtr head, int target); //Precondition: The pointer head points to the head of a //linked list The pointer variable in the last node is NULL //If the list is empty, then head is NULL //Returns a pointer that points to the first node that contains the //target If no node contains the target, the function returns NULL
FUNCTION DEFINITION
//Uses cstddef: IntNodePtr search(IntNodePtr head, int target) { IntNodePtr here = head;
The definitions of IntNode and IntNodePtr are given in Display 174
if (here == NULL) //if empty list { return NULL; } else { while (here->getData( ) != target && here->getLink( ) != NULL) here = here->getLink( ); if (here->getData( ) == target) return here; else return NULL; } }
Linked Data Structures
Self-Test Exercises
4 Write type definitions for the nodes and pointers in a linked list Call the node type NodeType and call the pointer type PointerType The linked lists will be lists of letters 5 A linked list is normally referred to via a pointer that points to the first node in the list, but an empty list has no first node What pointer value is normally used to represent an empty list 6 Suppose your program contains the following type definitions and pointer variable declarations:
struct Node { double data; Node *next; }; typedef Node* Pointer; Pointer p1, p2;
Suppose p1 points to a node of the above type that is in a linked list Write code that will make p1 point to the next node in this linked list (The pointer p2 is for the next exercise and has nothing to do with this exercise) 7 Suppose your program contains type definitions and pointer variable declarations as in Self-Test Exercise 6 Suppose further that p2 points to a node of the above type that is in a linked list and which is not the last node on the list Write code that will delete the node after the node pointed to by p2 After this code is executed, the linked list should be the same, except that there will be one less node in the linked list (Hint: You may want to declare another pointer variable to use) 8 Suppose your program contains the following type definitions and pointer variable declarations:
class Node { public: Node(double theData, Node* theLink) : data(theData), next(theLink){} Node* getLink( ) const { return next; } double getData( ) const { return data; } void setData(double theData) { data = theData; } void setLink(Node* pointer) { next = pointer; } private: double data; Node *next; };