Adding a node to random spot in linked list - c

Question

So I have a function that adds a node to a linked list but instead of just adding the element at the end of the linked list I am trying to get user input for the spot the user wants to add it in. I then need to add it and shift everything else over without deleting anything else. I am having a lot of trouble with this.
Below I will show my code, its a bit messy but I will explain as best I can.
void InsertGraphicElement(struct RasterGraphic *pA) {
int counter = 1;
int response = 0;
char tempString[256];
struct GraphicElement *newNode = malloc(sizeof(*newNode));
if (newNode == NULL)
return;
newNode->fileName = malloc(256 * sizeof(char));
if (newNode->fileName == NULL)
return;
newNode->pNext = NULL;
printf("Insert a GraphicElement in the RasterGraphic\nPlease enter the GraphicElement filename: ");
scanf("%s", newNode->fileName);
if (pA->GraphicElements == NULL) {
pA->GraphicElements = newNode;
printf("This is the first GraphicElement in the list\n");
} else {
struct GraphicElement *tempHead = pA->GraphicElements;
struct GraphicElement *tempTail = pA->GraphicElements;
while (tempHead->pNext != NULL) {
tempHead = tempHead->pNext;
counter++;
}
tempHead->pNext = newNode;
printf("There are %d GraphicElement(s) in the list. Please specify the position (<= %d) to insert at :", counter, counter);
scanf("%d", &response);
if (response == counter) {
return;
} else {
while (response < counter) {
tempTail = tempTail->pNext;
response++;
}
}
}
return;
}
It is incomplete Ive been messing with the code trying to figure it out but as you can see I am able to add without a problem at the end of the list. What I am having trouble with is if there are 5 elements for example in the list 1,2,3,4,,5 and I add a sixth the list would obviously look like this 1,2,3,4,5,6. What I want to do is take user input such as they want to add the sixth element to maybe spot 3 so the list would look like this 1,2,6,3,4,5. Ive tried a bunch of things a point in the right direction or some help would be greatly appreciated. Thank you,
Below are my struct definitions
struct GraphicElement {
char *fileName;
struct GraphicElement *pNext;
};
struct RasterGraphic {
//int numNodes;
struct GraphicElement *GraphicElements;
};

Answer 1

You currently have the following:
if (response == counter) {
return;
}
else {
while(response < counter){
tempTail = tempTail->pNext;
response++;
}
}
I would revise this to be something like:
if (response > counter+1 || response < 1) { // These are all invalid values
return;
}
else if (response == 1) { // We are replacing the head node
newNode->pNext = tempTail->pNext;
pA->GraphicElements = newNode;
}
else {
while(response-- > 2) { // Responses are 1-indexed
tempTail = tempTail->pNext;
}
newNode->pNext = tempTail->pNext; // IMPORTANT: Update newNode reference FIRST
tempTail->pNext = newNode; // Update tempTail to point to newNode
}
Disclaimer - Not tested in any way
I tried to comment things I thought were important here and there. The key thing to remember here though is that in order to insert something into a singly-linked list, you must update your new node to point to the rest of the linked list BEFORE you update the reference of the node previous to it, or else the last bit of your list will be lost forever and you will have a circular reference to the last node.
As a side note, it looks like your are traversing your entire linked-list twice each time you want to add a new element to it. I might recommend keeping a rolling counter of how many items are in the linked list that you update each time you add something to it to avoid the overhead of recalculating this value if efficiency is something you value.
Hope this helps!
EDIT: You'd have to remove the line that adds the new node to the end of the list higher up in the function for this to work, however. "tempHead->pNext = newNode;"