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problem using file pointers

Discussion in 'C' started by niso, Jan 24, 2008.

  1. niso

    niso New Member


    i have done a program to reverse the list..and here the users gives the input(i.e.,if i give ./a.out 3 my output will be like this orginal:1 2 3 and reverse:3 2 1 ......)
    but how my program should take the list as input from a file :confused: .

    (note:The name of the input file should be taken as a command line argument. This file would contain a line of white-space separated list of number. eg(19 1 4 3 18)

    can any one help me out :cryin: ......

    /**********reversing list***********
    struct link {
        int key;
        struct link *next;
    struct link * create_llist(int max)
        struct link *head;
        struct link *ptr;
        int n = 0 ;
        if(max<1)return NULL;
        ptr=(struct link *)malloc(sizeof(struct link));
        while(n++ < max-1)
            ptr->next=(struct link *)malloc(sizeof(struct link));
        return head;
    void reverse_llist(struct link *head,struct link **newHead)
        struct link **ptr=&head;
        if(*ptr == NULL || (*ptr)->next == NULL) return ;
        if((*ptr)->next->next != NULL)
            *newHead = (*ptr)->next ; 
        (*ptr)->next->next = *ptr ;
        (*ptr)->next = NULL;
    print_list(struct link *head)
        struct link *a=head;
        if(!a) return ;
            printf("  %d ", a->key);
        printf("  %d ", a->key);
    main(int argc, char*argv[])
        struct link *head;
        if(!argc) return ;
    Last edited by a moderator: Jan 24, 2008
  2. Salem

    Salem New Member

    Use fopen() to open the file for input.
    Use fscanf() to read each successive integer from the file. I would suggest using the %d format.

    > ptr=(struct link *)malloc(sizeof(struct link));
    If you had included stdlib.h (as you should have), then a cast of the result would not be necessary, at least for any ANSI/ISO C compiler.
    If, after removing the cast, you get warnings about converting void*, then that means you need to stop compiling your code with a C++ compiler, and use a C compiler. Typically, this involves renaming "prog.cpp" into "prog.c".

    Also, main should be explicitly declared as returning int, and have an explicit return 0; at the end.
  3. niso

    niso New Member


    i need output like this....

    If the input file is input.txt, and following are its contents:
    19 1 3 4 18

    After reading input from the file, call to print_list should display:
    19 → 1 → 3 → 4 → 18

    Call to reverse_llist and then calling print_list should print the following:
    18 → 4 → 3 → 1 → 19

    so where i want to modify in my program..........
    help me!!!!!!!!!!!!!!!
  4. Salem

    Salem New Member

    So when you run your test data through your program, what do you see?

    - does it crash (all the time, some of the time, specific test case)
    - does it produce bad output (all the time, some of the time, specific test case)
    - does it produce the right data in the wrong order
    - does it loop forever / not at all.
    - etc etc

    We need more than "here's my code, it doesn't work". We provide help, not a rescue service.

    Being able to debug your own code is a vital skill, as is being able to clearly communicate the exact nature of the problem.

    For example, the first test would be a list with only 1 element.
    If the test passes, fine, then move onto the next test.
    If not, then you have a really simple case to step through the code with a debugger to find the problem.

    Try it with a 2-element list.
    Try it with a 3-element list. By the time you get to this one, it should be OK for any list.

    So a report like, "it works for 1 element forwards and backwards, 2 elements forwards and then it just crashes at the start of printing 2 elements in reverse" is a meaningful description of a problem and shows us that you've done some work yourself.

    If you don't know how to use a debugger, then now is a good time to get some practice in.

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