Stupid Q regarding array

This is a discussion on Stupid Q regarding array within the C++ Programming forums, part of the General Programming Boards category; Hi,tried to define the size of an array by doing this: Code: int x=2; int y=3; int z=x*y; int someArray[z]; ...

  1. #1
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    Stupid Q regarding array

    Hi,tried to define the size of an array by doing this:
    Code:
            int x=2;
    	int y=3;
    	int z=x*y;
    	int someArray[z];
    and I keep getting these errors:
    Code:
    error C2057: expected constant expression
    
    error C2466: cannot allocate an array of constant size 0
    
    error C2133: 'someArray' : unknown size
    In the programme itself x and y would be defined by user input and the size of someArray could be anywhere from 1 to 80.All errors go when i set someArray[80].

    Im more curious as to why it doesnt work than anything else,

    thanks

  2. #2
    C++ Witch laserlight's Avatar
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    Standard C++ does not support variable length arrays. Use std::vector<int> instead.
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    Thanks Laserlight.

    Strangely enough bought accelerated C++ and was going to make a start on it this evening and one of the first subjects is the vectors

    Regards Cathalo

  4. #4
    The larch
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    That's probably because std::vector would be your default choice for storing variable amounts of stuff in.

    Note, however, that if you used the const keyword you could declare your array as well. (But only if you are dealing with real constants whose value is known to the compiler. You can't for example get input at runtime and then create an array of that size, even if you store the entered value in a const variable.)

    Code:
    int main()
    {
        const int x=2;
        const int y=3;
        const int z=x*y;
        int someArray[z];
    }
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
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    I get you Anon.So once the compiler sees that any of the variables can change and as a result the someArray could be of any size it throws up the errors.

    Normally I always knew what size the array should be so always declared haow many elements it had.

    I ended up trying the code I posted because I thought it was waste of memory and what have you setting up an array with 800 elements and only using the 1st 40 or so elements.

    And so thats what vectors are for

    Thanks for the replys and explanations

  6. #6
    DESTINY BEN10's Avatar
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    Quote Originally Posted by Cathalo View Post
    I get you Anon.So once the compiler sees that any of the variables can change and as a result the someArray could be of any size it throws up the errors.

    Normally I always knew what size the array should be so always declared haow many elements it had.

    I ended up trying the code I posted because I thought it was waste of memory and what have you setting up an array with 800 elements and only using the 1st 40 or so elements.

    And so thats what vectors are for

    Thanks for the replys and explanations
    You're right. It would be wastage of memory if only a few elements are used. That's why dynamic allocation of memory is used and in C++ as you said vectors are used for the same purpose.
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