Problem with modus division.

This is a discussion on Problem with modus division. within the C++ Programming forums, part of the General Programming Boards category; I have made this program to take in the first nine digits of a population registration number, and calculate the ...

  1. #1
    Registered User stillwell's Avatar
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    Problem with modus division.

    I have made this program to take in the first nine digits of a population registration number, and calculate the 10th.

    The program works fine, as long as the modulus 11(method for the calculation) calculation do not return 121. In that case, the modus division returns 11....which shouldn't be possible, as it's being divided by 11, and is supposed to return 0.

    Please take a look, and see if you can locate the problem.
    Code:
    #include <vcl.h>
    #include <iostream>
    #include <stdlib.h>
    #pragma hdrstop
    using namespace std;
    
    //---------------------------------------------------------------------------
    
    #pragma argsused
    
    int main()
    {
    int Imod1, Imod2, Imod3, Imod4, Imod5,
        Imod6, Imod7, Imod8, Imod9, Imod10, moddy;
    
    char mod1[2], mod2[2], mod3[2], mod4[2], mod5[2],
         mod6[2], mod7[2], mod8[2], mod9[2], mod10[2], input[12];
    
    
    char* i = input;
    
      cout << "Indtast cprnr. der skal udregnes ";
      cin  >> input;
    
      strncpy(mod1, i, 1);
      i += 1;
      strncpy(mod2, i, 1);
      i += 1;
      strncpy(mod3, i, 1);
      i += 1;
      strncpy(mod4, i, 1);
      i += 1;
      strncpy(mod5, i, 1);
      i += 1;
      strncpy(mod6, i, 1);
      i += 2;
      strncpy(mod7, i, 1);
      i += 1;
      strncpy(mod8, i, 1);
      i += 1;
      strncpy(mod9, i, 1);
    
      Imod1 = atoi(mod1);
      Imod2 = atoi(mod2);
      Imod3 = atoi(mod3);
      Imod4 = atoi(mod4);
      Imod5 = atoi(mod5);
      Imod6 = atoi(mod6);
      Imod7 = atoi(mod7);
      Imod8 = atoi(mod8);
      Imod9 = atoi(mod9);
    
      moddy = (Imod1*4) + (Imod2*3) + (Imod3*2) + (Imod4*7) +
              (Imod5*6) + (Imod6*5) + (Imod7*4) + (Imod8*3) + (Imod9*2);//modulus 11 calculation
    
      Imod10 = moddy%11; //modus division
    
    //cout << endl << moddy << endl;
    
      Imod10 = 11 - Imod10; //could this line be the problem?
    
    //cout << Imod10;
    
      cout << "\n\nDit cprnr. er: " << Imod1 << Imod2 << Imod3 << Imod4
           << Imod5 << Imod6 << "-" << Imod7 << Imod8 << Imod9 << Imod10;
    
      cin  >> moddy;
    
            return 0;
    }
    Hehe, sorry for keep editing this. I thought I located the problem, but I didn't
    Last edited by stillwell; 10-19-2004 at 11:41 AM.

  2. #2
    Registered User
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    manipulate the components of Imod10 such that Imod10 == 121. Uncomment the cout lines below. Monitor the value of Imod10 after each statement. Congratulate yourself for debugging your own program.

    Imod10 = moddy%11; //modus division

    //cout << endl << moddy << endl;

    Imod10 = 11 - Imod10; //could this line be the problem?

    //cout << Imod10;

  3. #3
    & the hat of GPL slaying Thantos's Avatar
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    Posts
    5,681
    made a some small changes and added some more output. I think you'll see where the problem is:

    Code:
    #include <iostream>
    
    using namespace std;
    
    int main()
    {
    int Imod1, Imod2, Imod3, Imod4, Imod5,
        Imod6, Imod7, Imod8, Imod9, Imod10, moddy;
    
    char input[12];
    
    
    char* i = input;
    
      cout << "Indtast cprnr. der skal udregnes ";
      cin  >> input;
    
      Imod1 = *i - '0';
      i++;
      Imod2 = *i - '0';
      i++;
      Imod3 = *i - '0';
      i++;
      Imod4 = *i - '0';
      i++;
      Imod5 = *i - '0';
      i++;
      Imod6 = *i - '0';
      i++;
      Imod7 = *i - '0';
      i++;
      Imod8 = *i - '0';
      i++;
      Imod9 = *i - '0';
    
      moddy = (Imod1*4) + (Imod2*3) + (Imod3*2) + (Imod4*7) +
              (Imod5*6) + (Imod6*5) + (Imod7*4) + (Imod8*3) + (Imod9*2);//modulus 11 calculation
    
      cout << "moddy = " << moddy << endl;
      cout << "changing moddy to 121" << endl;
      moddy = 121;
      Imod10 = moddy%11; //modus division
    
      cout << "Imod10 = " << Imod10 << endl;
    
      Imod10 = 11 - Imod10; //could this line be the problem?
    
      cout << "Imod10 = " << Imod10 << endl;
    
    
      cout << "\n\nDit cprnr. er: " << Imod1 << Imod2 << Imod3 << Imod4
           << Imod5 << Imod6 << "-" << Imod7 << Imod8 << Imod9 << Imod10 << endl;
    
    
       return 0;
    }
    Indtast cprnr. der skal udregnes 1234567890
    moddy = 174
    changing moddy to 121
    Imod10 = 0
    Imod10 = 11


    Dit cprnr. er: 123456-78911

  4. #4
    and the hat of wrongness Salem's Avatar
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    Well none of your strncpy() calls guarantee that a \0 is placed at the end of the string
    So all your atoi() calls are roaming off through memory producing garbage results.

    > int Imod1, Imod2, Imod3, Imod4, Imod5,
    > Imod6, Imod7, Imod8, Imod9, Imod10
    Hint - use an array!
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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