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POAdd to Linked List in sorted order, having trouble with pointers
 

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  1. POAdd to Linked List in sorted order, having trouble with pointers
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    <p>I'm trying to create a method which will add a char* to a LinkedList, ensuring that the linkedList is always sorted alphabetically. I have been given code to define a LinkedItem struct: </p> <pre><code>// Define our list item as 'struct ListItem', and also // typedef it under the same name. typedef struct ListItem { char *s; struct ListItem *p_next; } ListItem; </code></pre> <p>I have also been given a method to add an item to the beginning of a list:</p> <pre><code>// Adds a new item to the beginning of a list, returning the new // item (ie the head of the new list *) ListItem* add_item(ListItem *p_head, char *s) { // Allocate some memory for the size of our structure. ListItem *p_new_item = malloc(sizeof(ListItem)); p_new_item-&gt;p_next = p_head; // We are the new tail. p_new_item-&gt;s = s; // Set data pointer. return p_new_item; } </code></pre> <p>Now here's my code, I'll explain more after:</p> <pre><code>ListItem* addSortedItem(ListItem *p_head, char *s){ if(p_head==NULL)//if the list is empty, we add to the beginning return add_item(p_head,s); ListItem* p_new_item = malloc(sizeof(ListItem)); ListItem *p_current_item = p_head; //makes a pointer to the head of the list while (p_current_item) { // Loop while the current pointer is not NULL printf("entering while loop with current=%s\n",p_current_item-&gt;s); // now we want to look at the value contained and compare it to the value input if(aThenB(s,p_current_item-&gt;s)!=TRUE){ // if A goes after B, we want to go on to look at the next element p_current_item=p_current_item-&gt;p_next; } else if (aThenB(s,p_current_item-&gt;s)==TRUE) {printf("entered elseif\n"); p_head=add_item(p_current_item,s); return p_head; } else {printf("WHY DID WE EVER REACH THE ELSE!?"); return p_head;} } } </code></pre> <p>Now, aThenB(StringA,StringB) returns TRUE if the correct sorted order of A and B is A, then B, and false otherwise- equality is also an option, I simply haven't gotten this to work well enough to allow that :-)</p> <p>What is happening with my test data (which is <code>"sheep i"</code>, with i from 0-10) is that either I am only returning one element back, or i am randomly skipping elements, depending on the order input. I can include more code but it's a bit messy.</p> <p>I think my problem is stemming from not fully understanding the pointers and how they work- I want to ensure that p_head is always pointing to the head, whilst p_current is roving through the list. But I'm also getting seg faults when p_current reaches the last element, so I'm not sure where I'm going wrong.</p> <p>Thank you for any help on how to get my code to return properly :-)</p> <p>Edit: addSortedItem() is called in the following block in the main method:</p> <pre><code> // The empty list is represented by a pointer to NULL. ListItem *p_head = NULL; ListItem *p_head2=NULL; // Now add some items onto the beginning of the list. int i; for (i=0; i&lt;NO_ITEMS; i++) { // Allocate some memory for our string, and use snprintf to // create a formatted string (see GNU API docs) much like printf // but instead writing to memory rather than the screen. char* s = malloc(MAX_DATA_CHARS); snprintf(s, (size_t) MAX_DATA_CHARS, "sheep %d", i); p_head = addSortedItem(p_head, s); } </code></pre>
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    1. COwhat does aThenB() do?
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    2. COit compares the two parameters (stringA and stringB), one character at a time, and returns `FALSE (0) if stringB<stringA; TRUE (1) if stringA<stringB;` stringA==stringB is a separate case which I've got a function equals(StringA,StringB) for, that returns TRUE if they are equal.
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    3. COPlease include the code that calls `addSortedItem`. There are plenty of opportunities to mess things up there, because of the way the function transfers ownership of the string to the linked list.
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