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Pointers and arrays question.

  1. Feb 3, 2008 #1
    Hi guys,

    ***I just started c programming, ive done some reading last week and i came up with a basic program.Ive got a problem trying to assign an input as an adress to a specified value.Basically i have 3 datas in my array and it will be called by inputting a number which points to the adress.

    ***Here's the program that i wrote :


    #include <stdio.h>


    void main()
    {

    int n;

    int Array[3];
    Array[0] = 10;
    Array[1] = 20;
    Array[2] = 30;

    int *pArray;

    pArray = &Array[n];

    printf("choose a number 0-2\n");
    scanf("%d", &n);

    pArray = &Array[n];

    printf("because you choose that number ill give you : $%d\n",*pArray);
    }

    -------------------------------------------------------------------------------------

    ***Although theres an error message, the program runs just fine...


    C:\Borland\BCC55\Bin>bcc32 pointers4
    Borland C++ 5.5.1 for Win32 Copyright (c) 1993, 2000 Borland
    pointers4.cpp:
    Warning W8004 pointers4.cpp 16: 'pArray' is assigned a value that is never used
    in function main()
    Turbo Incremental Link 5.00 Copyright (c) 1997, 2000 Borland

    C:\Borland\BCC55\Bin>pointers4
    choose a number 0-2
    2
    because you choose that number ill give you : $30

    C:\Borland\BCC55\Bin>

    --------------------------------------------------------------------------------------


    ***here's my question :

    1) Can someone explain to me what's the warning all about and how to correct it? i though i have assign a value for 'pArray' according to the user's input.
    2) What if i want to have a really large amount of datas stored in my array, lets say 100 datas. Is there any simple way of doing it say :

    Array[10,20,30,40,50,60,70...............1000]?
     
  2. jcsd
  3. Feb 3, 2008 #2
    1. You're assigning to pArray and then assigning to it again without using the value you first assigned to it. Furthermore, you're using n before it's been initialised, which you should never do.
    2. There are two ways of doing this. You could either assign them each individually:
      Code (Text):

      int array[] = { 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 };
       
      …or if there are loads, you could use a for loop:
      Code (Text):

      int array[100];
      for (int i = 0; i < 100; i++)
      {
          array[i] = i = array[(i + 1) * 10];
      }
       
     
  4. Feb 3, 2008 #3
    Maybe your problem is not exactly the real problem of your program.
    The biggest problem of your program exists here:
    Code (Text):

    scanf("%d", &n);
     
    Because you haven't checked the value of the input.
    No one can guarantee the the input value is of type int and in the range of 0~2.
    That means if the input value is not 0~2 or not even a int value, there may be some
    severe errors occurrence.
     
  5. Feb 4, 2008 #4
    1) Inquisitus

    Id prefer this one cause my actual array size is going to be about 100 and the values are not predictable although it is increasing.

    int array[] = { 13, 21, 35, 41, 51, 63, 74, 81, 99...................1000 };

    2) shwx

    The program that i want to write is for an ignition system where depending on the speed of the engine, i will assign a suitable time of spark. So ill assume my input is going to be either on or off and depending on the "on time", i can calculate the speed,it will then points to an address which is the time of spark. Its really an ambitious thing to do for a beginner but i decided to do it anyways.

    ***new question***

    Eventually ill need to apply my c code to program a PIC. Im using Wiz-c as my compiler because i find that its quite easy to visualize whats happening when the code is compiled and simulated. I dont know if this is too elaborate, i hope someone can help me :

    1) What is the difference between typical c code (standard stuff such as prinf,scanf) and the ones used on a pic (movlw,movwf,trisa,trisb). I was learning C and PIC code separately and i didnt know that it can be combined(see link for further details). Is the ones on the pic is just an extension of c codes needed in order to make the PIC working?

    2) Can anyone give me some tips on how to progress from here on? Im quite confuse on how to write a code to calculate the rpm, there are some examples on the web on a DIY CDI but i just dont understand the logic..

    code.http://www.sportdevices.com/ignition/ignition.c
     
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