EEE 243B Applied Computer Programming

EEE 243BApplied Computer Programming Linked list II Searching, Inserting and deleting

Review • Where in memory are linked lists stored? • Does each node in a linked list have a symbol name (variable name)? • What is the utility of the head pointer? Maj JGA Beaulieu & Capt MWP LeSauvage

Outline • Linked Lists (LL) • Traversing/searching in linked lists • Inserting a node • Deleting a node Maj JGA Beaulieu & Capt MWP LeSauvage

Traversing/searching through a LL • Recall from lecture 19: typedef struct STUDENT_NODE_TAG { char firstName[15]; char lastName[25]; unsigned long collegeNumber; float average; struct STUDENT_NODE_TAG *pNextStdNode; } STUDENT_NODE; //name of the type Maj JGA Beaulieu & Capt MWP LeSauvage

Traversing/searching through a LL • Declare a pointer to the type-defined structure in your declaration section (say pTraverse) STUDENT_NODE *pTraverse = NULL; • Set pTraverse to the head of the list pTraverse = pStudentList; • Check if the pointer is NULL • Process the current node • Set the pointer to the next node pTraverse = pTraverse->pNextStdNode; • Go back to step 3 until done Maj JGA Beaulieu & Capt MWP LeSauvage

Traversing/searching through a LL … int count = 0; STUDENT_NODE *pTraverse = pStudentList; while (pTraverse != NULL) { count++; //process the current node pTraverse = pTraverse->pNextStdNode; }//end while pTraverse … Maj JGA Beaulieu & Capt MWP LeSauvage

count = 0 pTraverse Allard 20001 Allen 22456 pNextStdNode pNextStdNode pStudentList Alpo 22460 Baar 20002 pNextStdNode pNextStdNode 0 pTraverse = pStudentList;

count = 1 pTraverse Allard 20001 Allen 22456 pNextStdNode pNextStdNode pStudentList Alpo 22460 Baar 20002 pNextStdNode pNextStdNode 0 pTraverse = pTraverse->pNextStdNode;

pTraverse count = 4 Allard 20001 Allen 22456 pNextStdNode pNextStdNode pStudentList Alpo 22460 Baar 20002 pNextStdNode pNextStdNode 0 while (pTraverse->pNextStdNode != NULL) {…}

Inserting a node in the LL • We already saw how to insert the first node in a LL that was empty in the previous lecture. • Before we insert any node in a LL we must first traverse it in search of our insertion point. • The only exception to the above rule is if we use an unordered list and always add the nodes at the beginning of the list. • This is a simple case that is covered by the more general ordered list insertion steps Maj JGA Beaulieu & Capt MWP LeSauvage

Inserting a node in the LL • The first steps in inserting a node are: • Declare a temporary node (pTemp) and request memory through malloc • We verify if there is enough memory available • Set all the fields pointed to by pTemp with the appropriate values • To insert at the head of the list: • Point the new block to the first node by using the head pointer: pTemp->pNextStdNode = pStudentList; • Make the head point to the new node (pTemp) pStudentList = pTemp; Maj JGA Beaulieu & Capt MWP LeSauvage

Inserting a node in the LL //First steps to create node STUDENT_NODE *pTemp = NULL; pTemp = (STUDENT_NODE *)malloc(sizeof(STUDENT_NODE)); strcpy(pTemp->firstName,"Jack"); strcpy(pTemp->lastName,"Rabbit"); pTemp->collegeNumber = 22222; pTemp->average = 99.9; //Steps to include node in structure pTemp->pNextStdNode = pStudentList; pStudentList = pTemp; Maj JGA Beaulieu & Capt MWP LeSauvage

Rabbit 22222 pTemp 0 pNextStdNode Allard 20001 0 pNextStdNode pStudentList First steps of creating the node

Rabbit 22222 pTemp pNextStdNode 1 Allard 20001 2 0 pNextStdNode pStudentList pTemp->pNextStdNode = pStudentList; pStudentList = pTemp;

Inserting a node in the LL • To insert a node anywhere else: • Repeat the steps to create the new node • To insert in the the list: • Traverse the list using pTraverse (here we assume that we want to insert after pTraverse) • Make the new node point where pTraverse points • Make pTraverse point to the new node Maj JGA Beaulieu & Capt MWP LeSauvage

Inserting a node in the LL //First steps to create node STUDENT_NODE *pTemp = NULL; pTemp = (STUDENT_NODE *)malloc(sizeof(STUDENT_NODE)); strcpy(pTemp->firstName,"Jack"); strcpy(pTemp->lastName,"Rabbit"); pTemp->collegeNumber = 22222; pTemp->average = 99.9; //Steps to include node in structure pTemp->pNextStdNode = pTraverse->pNextStdNode; pTravers->pNextStdNode = pTemp; Maj JGA Beaulieu & Capt MWP LeSauvage

Rabbit 22222 pTemp 0 pNextStdNode pTraverse All 20001 Allard 20001 pNextStdNode pNextStdNode pStudentList 0 First steps of creating the node

Rabbit 22222 pTemp pNextStdNode 1 pTraverse Allen 22456 Allard 20001 pNextStdNode pNextStdNode 2 pStudentList 0 pTemp->pNextStdNode = pTravers->pNextStdNode; pTravers->pNextStdNode = pTemp;

Deleting a node in the LL • Declare a new pointer called pPred that will follow pTraverse but one node behind • Check if the list is empty, if so print an error • All the nodes in the LL can be deleted in the same fashion except for one. If the node to be deleted is the head, you set the head pointer to the next node in the list. • Free the memory Maj JGA Beaulieu & Capt MWP LeSauvage

Deleting a node in the LL - head …//Special condition node is the head unsigned long delNumber = 0; //get this value somehow STUDENT_NODE *pTraverse = pStudentList; STUDENT_NODE *pPred = pStudentList; if (pStudentList == NULL){…} //print error and get out if (pStudentList->collegeNumber == delNumber) { pStudentList = pStudentList->pNext; free(pTraverse); } else pTraverse = pTraverse->pNextStdNode; …//pTraverse is now one node ahead of pPred Maj JGA Beaulieu & Capt MWP LeSauvage

pPred delNumber = 20001 pTraverse Allard 20001 Allen 22456 pNextStdNode pNextStdNode pStudentList Alpo 22460 Baar 20002 pNextStdNode pNextStdNode 0 pStudentList = pStudentList->pNextStdNode;

Deleting a node in the LL - general • Do a search for the node to delete by testing a condition (i.e. collegeNumber match) inside a loop • Advance both pointers (pTraverse and pPred) • Delete the node by setting the next node of pPred as follows: pPred->pNextStdNode = pTraverse->pNextStdNode; • Free the memory where pTraverse points Maj JGA Beaulieu & Capt MWP LeSauvage

Deleting a node in the LL - general …//General condition delete //This code follows the code for deleting the head while (pTraverse->collegeNumber != delNumber) { pPred = pTraverse; pTraverse = pTraverse->pNextStdNode; if (pTraverse == NULL) {…} //print error and exit fctn } pPred->pNextStdNode = pTraverse->pNextStdNode; free(pTraverse); … Maj JGA Beaulieu & Capt MWP LeSauvage

pPred delNumber = 20002 Allard 20001 Allen 22456 pNextStdNode pNextStdNode pStudentList Alpo 22460 Baar 20002 pTraverse pNextStdNode pNextStdNode 0 Advancing both pointers pTraverse ahead of pPred

pPred delNumber = 20002 Allard 20001 Allen 22456 pNextStdNode pNextStdNode pStudentList Alpo 22460 Baar 20002 pTraverse pNextStdNode pNextStdNode 0 pPred->pNextStdNode = pTraverse->pNextStdNode;

Deleting a node in the LL - general • You can also use a level of indirection to keep a hold of the predecessor to the node being deleted • The following code is equivalent to the one on slide 24 • When using this kind of indirection be careful what you free! while ((pTraverse->pNextStdNode)->collegeNumber != delNumber) { pTraverse = pTraverse->pNextStdNode; if (pTraverse == NULL) {…} //print error and exit fctn } pTemp = pTraverse->pNextStdNode; pTraverse->pNextStdNode = (pTraverse->pNextStdNode)->pNextStdNode; free(pTemp); //This line is important … Maj JGA Beaulieu & Capt MWP LeSauvage

Quiz Time • Describe the steps to traverse a LL • Describe the steps to perform a general insert operation in a LL Maj JGA Beaulieu & Capt MWP LeSauvage

EEE 243B Applied Computer Programming