Whats the best way to clear an array?

This is a discussion on Whats the best way to clear an array? within the C++ Programming forums, part of the General Programming Boards category; Does anyone know the best way to clear an array of any info? I can't figure out how to do ...

  1. #1
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    Whats the best way to clear an array?

    Does anyone know the best way to clear an array of any info? I can't figure out how to do it.

  2. #2
    Guest Sebastiani's Avatar
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    Depends on what you mean by 'clear'. If you want to simply zero out the binary representation of a data structure you can use memset. But often, this isn't adequate for more complex classes, in which case you can call the destructor, initialize it with a default-constructed object, or, in some cases, call the approprate member function of the object that serves the purpose of 'clearing' the object. Perhaps you can give an example of what kind of data structure you're dealing with (ie: the code in question)?
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  3. #3
    and the Hat of Guessing tabstop's Avatar
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    Define "clear an array of any info"? Do you mean storing marker values (like, 0, or -1, or NULL) or whatever in every element?

    At first blush, this sounds like a program-logic thing (either using a marker value, or just keeping track of "there are x values in this array", or just using a dynamic thing like a vector) more than something syntactic.

  4. #4
    Kiss the monkey. CodeMonkey's Avatar
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    What do you mean by clear? You mean remove all of its elements? That would depend on how this array is allocated.
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

  5. #5
    Kiss the monkey. CodeMonkey's Avatar
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    Wow. Triple-simultaneous response.
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

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    By clear I mean clear to the point as if no data was ever in there and not simply fill the array with zeros.

  7. #7
    Guest Sebastiani's Avatar
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    That still isn't enough information. Post some code, please.
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  8. #8
    and the Hat of Guessing tabstop's Avatar
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    There must always be data in an array. There is no way to have "no data ever in there". Even when you have an uninitialized array, there's still data there, you just can't trust it.

  9. #9
    Kiss the monkey. CodeMonkey's Avatar
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    As Sebastiani requested, post whatever (small portion of) code is compelling you to ask this question, and there will be answers abound.
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

  10. #10
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    The code is a little long so I'll break it down a bit so that it makes sense.

    Code:
    #include <iostream>
    #include <fstream>
    
    using namespace std;
    
    int g = 0;
    
    int main ()
    { 
        
    Main:
    
    
        char File [100];      
        cout <<"Enter filename and extention:\n"; 
        cin.getline (File, 100); 
    
    // does whatever the program is meant to
    
    // And here is where I need to clear "File" so that it doesnt carry on should I select 1.
    
        cout << "Would you like to continue?\nPress 1 for Yes\nPress 2 for No:\n";
        cin >> g;
        if ( g == 1) {goto Main;} else { return EXIT_SUCCESS;};

  11. #11
    Guest Sebastiani's Avatar
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    In that case, 'File' is never used after that point so you don't need to clear it, really. But if you just have some compulsion to do so, you could
    1) set the first character to 0
    2) use memset
    3) use std::fill, or otherwise cycle through the array and set each element to the default-constructed value of char (ie: 0).
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  12. #12
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    Quote Originally Posted by Hitsuin View Post
    The code is a little long so I'll break it down a bit so that it makes sense.
    It doesn't really make sense. Having an array uninitialised essentially means that any read-only access of it gives undefined behaviour.

    I take it you want the array File to be uninitialised every time through the loop.

    One way is to use a function;

    Code:
    #include <iostream>
    #include <fstream>
    
    using namespace std;
    
    void do_stuff()
    {
        char File [100];      
        cout <<"Enter filename and extention:\n"; 
        cin.getline (File, 100); 
    
         // does whatever the program is meant to
    }
    
    
    int g = 0;
    
    int main ()
    { 
        
    Main:
    
        do_stuff();
    
        cout << "Would you like to continue?\nPress 1 for Yes\nPress 2 for No:\n";
        cin >> g;
        if ( g == 1) {goto Main;} else { return EXIT_SUCCESS;};
    }
    Essentially, the array File is only guaranteed to exist within the scope of do_stuff().

    However, I suspect what you really want is for the data that was read in to be cleared, so the data is no longer physically in memory. In that case, simply overwrite the array File with some suitable pre-defined pattern (eg all zeros).
    Last edited by grumpy; 06-13-2009 at 05:37 PM.
    Right 98% of the time, and don't care about the other 3%.

  13. #13
    and the hat of sweating
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    Since it's a string, the easiest thing to do is set the first element to nul:
    Code:
    File[0] = '\0';
    If security is a concern (which it probably isn't since this string isn't used for storing passwords), you need to set all the elements to '\0' using std::fill().
    "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008

    "the internet is a scary place to be thats why i dont use it much." - billet, 03/17/2010

  14. #14
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by Hitsuin View Post
    The code is a little long so I'll break it down a bit so that it makes sense.

    Code:
    #include <iostream>
    #include <fstream>
    
    using namespace std;
    
    int g = 0;
    
    int main ()
    { 
        
    Main:
    
    
        char File [100];      
        cout <<"Enter filename and extention:\n"; 
        cin.getline (File, 100); 
    
    // does whatever the program is meant to
    
    // And here is where I need to clear "File" so that it doesnt carry on should I select 1.
    
        cout << "Would you like to continue?\nPress 1 for Yes\nPress 2 for No:\n";
        cin >> g;
        if ( g == 1) {goto Main;} else { return EXIT_SUCCESS;};
    In what way does that program (assuming the missing closing brace is added) not do exactly what you want already? The answer to this is rather important

  15. #15
    The larch
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    Could be well about missing calls to cin.ignore when mixing input with >> and getline (or failed cin generally) which makes him think array being "full" is the reason why input doesn't succeed the way he expects?...
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

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