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POHow to write a function within a function (list_map)
 

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  1. POHow to write a function within a function (list_map)
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    <p>Hello I recently asked some questions on linked lists in C.<br> <a href="https://stackoverflow.com/questions/2106691/c-issue-cant-figure-how-to-assign-pointer-to-beginning-of-list">The link was found here</a></p> <p>First I want to thank everyone for helping me with this. But I have one issue I cannot understand. I even asked the professor but he emailed me back with little information. Basically I am writing a linked list in C (see above link). One of the things the professor gives us in the header file is this:</p> <pre><code>void list_map( INTLIST *list, void (*f)(void *) ); /*Applies a function to each element of the list */ </code></pre> <p>So I emailed him about this and said:</p> <blockquote> <p>Another question, in the header file you did not define a sorting function, do we need to write a sorting function with the prototype and finally what is list_map</p> </blockquote> <p>And he replied with:</p> <blockquote> <p>You are asked to implement a sorting function f, which is called through list_map(list, f). Hope it clears your doubts.</p> </blockquote> <p>My only doubts are this was not fully taught. I can understand how to sort the linked list in fact here is some pseudo code:</p> <pre><code>tmp=head; while(tmp!=NULL) { tmp2=tmp-&gt;next; //pointer to next node while(tmp2!=NULL) { if (tmp2-&gt;data &lt; tmp-&gt;data) { int x = tmp2-&gt;data; tmp2-&gt;data = tmp-&gt;data; tmp2-&gt;data = x; } tmp2=tmp2-&gt;next; } tmp=tmp-&gt;next; } </code></pre> <p>I know the experts might say this is not the most efficient, and I understand that right now I am just learning and trying to get things working. I can clean up afterwords...so on to my question.</p> <p>My question is given I have the sort function (in the professor's case he calls it f). How would I call this sorting function when the signature is:</p> <pre><code>void list_map(INTLIST* list, void (*f) (void*)); </code></pre> <p>Would I just say:</p> <pre><code>list_map(myList, f()); //apply function f to the current linked list </code></pre> <p>Or do I really need to define list_map somewhere? I am not the typical student just looking for someone to do my work. I am really trying to understand this as best I can.</p> <p>Thanks to all of you.</p> <p><strong>[EDIT PORTION]</strong></p> <p>I wanted to add that one of the posters Kaleb P. said </p> <blockquote> <p>"Thus, your job is to create a sorting function that you will pass in to list_map. Note that the correct syntax for passing it in will be:"</p> </blockquote> <p>So should my code simply be this:</p> <p>in the .h file I prototype the function as:</p> <pre><code>void myCustomSort(void*); </code></pre> <p>And then in the .cpp it becomes:</p> <pre><code>void myCustomSort(void*f) { tmp=f-&gt;head; while(tmp!=NULL) { tmp2=tmp-&gt;next; //pointer to next node while(tmp2!=NULL) { if (tmp2-&gt;data &lt; tmp-&gt;data) { int x = tmp2-&gt;data; tmp2-&gt;data = tmp-&gt;data; tmp2-&gt;data = x; } tmp2=tmp2-&gt;next; } tmp=tmp-&gt;next; } } </code></pre> <p>And to call it in main I would just do:</p> <pre><code>list_map(myListPointer, &amp;myCustomSort); </code></pre> <p>But don't I need to define list_map anywhere? Because it is in the .h file do I not have to define it?</p>
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    1. COI don't think it's possible to sort a list if the function is given only a single element at a time with no other knowledge.
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    2. CObut myList is a list not a single element.
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    3. COYeah, but a map() function usually takes a sequence and another function, then applies the function on each element of the list, and you can see from the list_map() signature that that is what it does (it takes a single void* argument).
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