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POC issue - Can't figure how to assign pointer to beginning of list
 

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  1. POC issue - Can't figure how to assign pointer to beginning of list
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    <p>I have a simple assignment that the professor wants us to do. Basically to pull in some numbers from a text file and load into a linked list. I don't want to get to much into the details but I have a basic question.</p> <p>He provided us with a function like so:</p> <pre><code>INTLIST* init_intlist( int n ) { INTLIST *lst; lst = (INTLIST *)malloc(sizeof(INTLIST)); lst-&gt;datum = n; lst-&gt;next = NULL; return lst; } </code></pre> <p>This function is used to initialize the linked list with the first element. Then he asked us to define a function with this signature:</p> <pre><code>int insert_intlist( INTLIST *lst, int n ) </code></pre> <p>So I assume he just wants us to add to the linked list so I tried this:</p> <pre><code>int insert_intlist( INTLIST *lst, int n ) { INTLIST* lstTemp; lstTemp = (INTLIST *)malloc(sizeof(INTLIST)); lstTemp-&gt;datum = n; lstTemp-&gt;next = lst; lst = lstTemp; free(lstTemp); } </code></pre> <p>So what my thought process was is that it creates a temporary node, assigns the data value (Datum) and assigns the next pointer to point to where the current pointer is pointing at. Then I reassign the main pointer to this newly created temp node.</p> <p>That way we have for instance 2 nodes:</p> <p>[New Temp Node] -> [Prev Initialized Node]</p> <p>When I step through the code it looks great...</p> <p>Then back in main I have just a function to print the list:</p> <pre><code> while (lst!=NULL) { printf("The value is:%d", lst-&gt;datum); lst=lst-&gt;next; } </code></pre> <p>The problem is this only seems to print one digit (namely the first digit that I am reading in from the file, which I think is the last one in the list or at least I thought it was the last one in the list).</p> <p>But it should keep going through as I have 10 digits in the file. I know the code is very dirty and I will clean it up...here is my entire main function if anyone needs more info:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include "intlist.h" int main(int argc, char *argv[]) { char c; /* Character read from the file. */ FILE* ptr; /* Pointer to the file. FILE is a structure defined in &lt;stdio.h&gt; */ int index=0; //INTLIST* aList[10]; //will use later /* Open the file - no error checking done */ ptr = fopen("1.txt","r"); /* Read one character at a time, checking for the End of File. EOF is defined in &lt;stdio.h&gt; as -1 */ if(ptr==NULL) { printf("Error: can't open file.\n"); /* fclose(file); DON'T PASS A NULL POINTER TO fclose !! */ return 1; } //aList[index] = malloc(sizeof(INTLIST)); WE NEED THIS LATER ON.... INTLIST *lst=NULL; while ((c = fgetc(ptr)) != EOF) { if (c != ' ') { //make sure it isnt a space int i = c - '0'; //get the value from the text file if(c=='\n') { // aList[index]=lst; // index++; // aList[index] = malloc(sizeof(INTLIST)); while (lst!=NULL) { printf("The value is:%d", lst-&gt;datum); lst=lst-&gt;next; } free(lst); free(aList[index]); return 0; //new line in the file //create another linked list } if (lst==NULL) lst = init_intlist(i); else insert_intlist( lst, i); } } fclose(ptr); system("PAUSE"); return 0; } </code></pre> <p>Here is intlist.h for anyone who may need it:</p> <pre><code>#ifndef __intlist_h__ #define __intlist_h__ /* each entry in the list contains an int */ typedef struct intlist { int datum; struct intlist *next; } INTLIST; INTLIST *init_intlist( int n ); /* initializes the intlist with initial datum n */ int insert_intlist( INTLIST *lst, int n ); /* Inserts an int (n) into an intlist from the beginning*/ void list_append(INTLIST *list, void *datum); /* Inserts entry to the end of the list */ INTLIST* list_front(INTLIST *list); /*return the element at the front of the list, and remove it from the list*/ void list_map( INTLIST *list, void (*f)(void *) ); /*Applies a function to each element of the list */ void list_delete( INTLIST *list ); /* Deletes (and frees) all entries in the list */ #endif </code></pre>
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    1. USJacob
    1. USoJM86o
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      1. POC issue - Can't figure how to assign pointer to beginning of list
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      1. VTUpMod
    1. COIs the header with the prototypes given by your professor? `int insert_intlist( INTLIST *lst, int n );` with the comment that *Inserts an int (n) into an intlist from the beginning* is wrong: with the prototype, that can't be done. See my answer for details.
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      1. POC issue - Can't figure how to assign pointer to beginning of list
      1. USAlok Singhal
    2. CO@Alok: You can't do it naturally. Jerry got it, stick the new node in second, and swap payloads. I didn't read that carefully and assumed adding at the back.
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      1. POC issue - Can't figure how to assign pointer to beginning of list
      1. USdmckee
    3. CO@dmckee: aha! I like that bending of the rules - although I wonder if that's what the instructor wanted. Another possibility is that the return value is `int`, so maybe the function just returns the old head's payload, and replaces that with the new value! That will be a simple function indeed.
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      1. POC issue - Can't figure how to assign pointer to beginning of list
      1. USAlok Singhal
 

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