modulo a size_t

This is a discussion on modulo a size_t within the C++ Programming forums, part of the General Programming Boards category; Hi, a curious question. Why is the value of an int % a size_t strange? Code: #include <iostream> #include <math.h> ...

  1. #1
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    modulo a size_t

    Hi, a curious question. Why is the value of an int % a size_t strange?


    Code:
    #include <iostream>
    #include <math.h>
    #include <vector>
    using namespace std;
    
    
    int main(){
    
       vector<int> somevec;
       somevec.push_back(0);
       somevec.push_back(0);
       somevec.push_back(0);
       somevec.push_back(0);
       somevec.push_back(0);
       somevec.push_back(0);
    
       int moo = -1 % 6;
       int boo = -1 % somevec.size();
       int zoo = -1 % (int)(somevec.size());
       
       
       cout << "mm : " << moo << endl;
       cout << "m2 : " << boo << endl;
       cout << "m3 : " << zoo << endl;
    
       
    }
    They should be the same.. right?

    Cheers

  2. #2
    Kiss the monkey. CodeMonkey's Avatar
    Join Date
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    Posts
    902
    I get
    Code:
    mm : -1
    m2 : 3
    m3 : -1
    For those of you who don't want to run the code. size() is unsigned, in contrast to the other types shown. That'll all I've got.
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

  3. #3
    Captain Crash brewbuck's Avatar
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    Portland, OR
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    When you operate on a signed and an unsigned, the signed argument is converted to unsigned. So -1 is converted to unsigned int, value 4294967295.

    4294967295 % 6 == 3.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  4. #4
    The larch
    Join Date
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    When you do arithmetic with mixed types, one value is cast to some other type according to some rules. If one operand is signed and the other unsigned, the signed value is cast to unsigned.

    And a hint:

    Code:
       vector<int> somevec(6);
       //somevec.push_back(0);
       //somevec.push_back(0);
       //somevec.push_back(0);
       //somevec.push_back(0);
       //somevec.push_back(0);
       //somevec.push_back(0);
    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).

  5. #5
    Registered User
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    ahh, I see. Didn't think of that. Thanks

    p.s I was thinking of putting vector.resize() but I guess I'd still miss that constructor thing.. heh

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