Making the size of an array variable

This is a discussion on Making the size of an array variable within the C++ Programming forums, part of the General Programming Boards category; Hello to everyone! I've just started studying C++ on the college, and in the program I'm working on I have ...

  1. #1
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    Question Making the size of an array variable

    Hello to everyone! I've just started studying C++ on the college, and in the program I'm working on I have this:

    Code:
    int num = 0;
    cin << num;
    int array[num];
    Before you ask, I did included any necessary libs. I get Visual Studio saying the error is in the last line: "... constant value expected..."

    What the hell is going on? I know this is simple, but I can't figure it out .

    Thanx in advance! C ya!

  2. #2
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    use new to allocate the memory. BTW: your use of cin is incorrect.
    Code:
    int num = 0;
    cin >> num;
    int * array  = new int[num];

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    Sorry about the cin kinda hurry. Now, maybe you have mistaken or something (I shouldn't use keywords in the code ). Here's the code I wanted:

    Code:
    int num = 0;
    cin >> num;
    int numbers[num];
    Ok, now this is what I've got with your help:

    Code:
    int num = 0;
    cin >> num;
    int * numbers = new int[num];
    Now just one doubt: in the Visual Studio, the window LOCALS (where I can see my variables values), the numbers int is shown an error, that the expression cannot be evaluated...

    Will I have an array of the size of NUM with that anyway? Thanx 'til now!

  4. #4
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    new will allocate an array of num number of integers. you can access variable numbers the same way you would had you hardcoded the size
    Code:
    int * numbers = new int[num];
    for(int i = 0; i < num; i++)
       numbers[i] = 0;

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    Allrighty! Thanx a lot man!!!

  6. #6
    Registered User major_small's Avatar
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    please don't tell people how to use new without telling them about delete...

    you need to delete (or free up) the memory you've allocated with new... so for example:
    Code:
    int*numbers=new int[num];
    //other code goes here
    //when you're done with the array use this next line
    delete[]num;
    if you forget to delete, you run into what's called a memory leak... it can cause big problems in some cases.
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    Ok, thanx for the note!

  8. #8
    semi-colon generator ChaosEngine's Avatar
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    now that you know how to use new and delete for arrays....


    don't.

    use a vector instead, and avoid the pain of trying to match new/delete pairs.
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    Quote Originally Posted by lfbang87
    Hello to everyone! I've just started studying C++ on the college, and in the program I'm working on I have this:

    Code:
    int num = 0;
    cin << num;
    int array[num];
    Before you ask, I did included any necessary libs. I get Visual Studio saying the error is in the last line: "... constant value expected..."

    What the hell is going on? I know this is simple, but I can't figure it out .

    Thanx in advance! C ya!
    When using a pre set value to set the size of an array - the integer that you use must be of type constant. IE:
    Code:
     const int num = 10;
    int array[num];
    I hope that explains your error.
    Oh my goodness.

  10. #10
    Registered User major_small's Avatar
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    Quote Originally Posted by ChaosEngine
    now that you know how to use new and delete for arrays....


    don't.

    use a vector instead, and avoid the pain of trying to match new/delete pairs.
    what's so hard about matching new and delete?

    vectors take care of all that for you, but I still prefer new and delete (personal preference)
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    Quote Originally Posted by major_small
    what's so hard about matching new and delete?
    What if you want to resize the array? or concatenate 2 arrays together? suddenly the problem is far bigger than just matching new[] and delete[] ...

  12. #12
    Registered User Mortissus's Avatar
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    I believe you need vector just when you want dynamic resize of a vector, i.e., use vector instead of realloc.

    Just an opinion...

  13. #13
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    Quote Originally Posted by Bench82
    What if you want to resize the array? or concatenate 2 arrays together? suddenly the problem is far bigger than just matching new[] and delete[] ...
    how so? just allocate a second (or third) array, put the data into that, and delete the original (two) array(s).
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    Quote Originally Posted by major_small
    how so? just allocate a second (or third) array, put the data into that, and delete the original (two) array(s).
    That's OK if you don't mind code fulll of new's and delete's, aswell as copying algorithms and temporary objects. But all that could easily be cut down to a line or two of STL code, and be much easier to read and debug IMO, especially when it comes to someone else trying to understand your code.

  15. #15
    Registered User major_small's Avatar
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    it's all still pretty much the same amount of code, the only thing that really changes is who writes it
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