Array of size "n"

This is a discussion on Array of size "n" within the C Programming forums, part of the General Programming Boards category; Hello, I am trying to create an array of size "n" where n is to be determined halfway through the ...

  1. #1
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    Array of size "n"

    Hello,

    I am trying to create an array of size "n" where n is to be determined halfway through the program (after various data is entered and the result calculated).

    I have defined the two variables as:

    int n;
    double x[n];

    but I get an error message each time I try and build the program.

    I'm relatively new to C so please don't use too technical language.

    Thank you in advance for your help!
    Jo

  2. #2
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    I believe only C99 supports variable arraysizes (without the use of dynamic memory). The functions you want to look up are malloc and free.

  3. #3
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    Code:
    int n;
    double *x;
    
    // Get n from user
    
    x = malloc(sizeof(double) * n);
    
    // now you can use x as an array
    
    free(x);
    If you understand what you're doing, you're not learning anything.

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    Thank you!

    Yay!

    It works. Thank you very much for your help!

    Jo

  5. #5
    Helper
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    Quote Originally Posted by itsme86
    Code:
    int n;
    double *x;
    
    // Get n from user
    
    x = malloc(sizeof(double) * n);
    
    // now you can use x as an array
    
    free(x);
    And of course, you will test the value returned by malloc() against NULL before using it.
    Emmanuel Delahaye

    "C is a sharp tool"

  6. #6
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    I thought I'd just mention calloc() and realloc(). calloc() is similar to malloc, but it clears the memory allocated. And realloc() "reallocates" memory. i.e. change the size of allocated memory.

  7. #7
    RoD
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    >I thought I'd just mention calloc() and realloc().
    It's best to avoid them unless you're sure you know what you're doing. calloc can be misleading when all bits zero is not meaningful for the type pointed to, and realloc is surprisingly tricky to use because it may or may not invalidate pointers to within the memory and mysterious bugs can arise because, depending on the arguments, realloc can function like itself, or malloc, or free. There are just too many what-if's for those two functions for my comfort.

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