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Need help soon with C++ program using pointers and arrays

  1. Sep 29, 2008 #1
    Okay so I got the majority of my program complete, however when I try to run it it doesnt display the proper value of the average. I think I somehow messed up using my pointers and arrays, but I cant locate the problem and ive spent hours trying to fix it to no avail. see comments for better description of program.

    -------

    #include <iostream>

    using namespace std;

    /* Write a program to read in a series of numbers and display how many
    are greater than their average. The user will indicate that there are no more numbers.
    by entering a -999. This lab requires that you use new and delete. */

    int main ( )
    {
    // Declaration of variables
    int index = 0;
    int count = 0;
    int amount = 0;
    int sum = 0;
    int capacity = 2;
    int * number;
    number = new int[capacity];

    // Puts the numbers the user enters into an array
    // Keeps track of their sum;
    // Keeps track of the numbers entered;
    while(true)
    {
    cout <<"Enter a number: ";
    cin >> number[index];

    if (number[index] == -999)break;

    sum = sum + number[index];
    amount++;

    if (index == capacity)
    {
    int * tmp = new int[capacity];
    number = tmp;
    delete []tmp;
    }

    index++;
    }

    // Accumulator variable to count numbers larger than average
    for (index = 0; index <= amount; index++)
    {
    if (number[index] > (sum / amount)) count++;
    }

    // Output
    cout << "There are " << count
    << " numbers greater than their average." << endl;


    return 0;
    }
     
  2. jcsd
  3. Sep 29, 2008 #2

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Several problems,
    You create an array with 2 items, but you use elements greater than 0 or 1.
    You create a new array to exapand it, but the new array is the same size and you don't copy the data to the new array you copy the pointer.
    You create an array, copy the pointer and then delete the array, the memory the number pointer points at is no longer valid.

    You have most of the ideas correct,
    Allocate an array of a reasonable size 100 or 1000 entries if the user is entering them by hand.
    Keep track of how many number have been entered and if you have to allocate more space make a new array twice the size of the old one and copy the data using either memcpy() or just looping through the values.

    ps. Be careful about the result of dividing one integer by another.
     
    Last edited: Sep 29, 2008
  4. Sep 29, 2008 #3

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    At the office, if you run out of filing cabinet space, you might seek to increase your capacity. What's wrong with the following directions for doing so?

    1. Acquire new filing cabinet.
    2. Install new filing cabinet.
    3. Put old filing cabinet in the dumpster.

    (Hint: think very literally)
     
  5. Sep 29, 2008 #4
    hm.. I think I can go on this. I'm supposed to start with 2 "elements" for the array and allocate 2 more spaces for the "elements" each time they are filled up...
    and I'm supposed to use pointers or something.

    I can change them to double no problem though.
     
  6. Sep 29, 2008 #5

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Even if you only allocate 2 extra elements at a time, you still have to copy the old values to the new array before deleting the old array.
     
  7. Sep 29, 2008 #6
    do I use pointers to do this?

    EDIT: "memcpy() or just looping through the values."
    I've never used memcpy() before and am unfamiliar with it. Should I use a for loop to copy the old values? How do I go about doing this?

    if (index == capacity)
    {
    capacity = capacity + 2;
    int * tmp = new int[capacity];
    number = tmp;
    delete []tmp;
    }

    I added capacity = capacity + 2 each time this if loop runs through, that way when the whole for loop repeats, it has allocated 2 more memory.. I think. Not sure about how to copy data though.
     
  8. Sep 29, 2008 #7
    You should be able to access the values of the old array, copy them in, as they exist in the memory, and then delete it after the for loop, as you would if you were to take a pointer to an int value and store it another pointer to int, and then delete it. So yes it's possible
     
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