Marching coordinates

This is a discussion on Marching coordinates within the C Programming forums, part of the General Programming Boards category; x[0]=0; y[0]=0; for(i=0;i<N-1;i++) { if(x[i]<1) x[i+1]=x[i]+Vx*dt; else x[i+1]=x[i]-Vx*dt; Help! I want the coordinates x[i] change from 0 over +1 to ...

  1. #1
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    Unhappy Marching coordinates

    x[0]=0;
    y[0]=0;

    for(i=0;i<N-1;i++)
    {
    if(x[i]<1) x[i+1]=x[i]+Vx*dt;
    else x[i+1]=x[i]-Vx*dt;

    Help! I want the coordinates x[i] change from 0 over +1 to -1 and back as often as i<N. The program counts from 0 up to 1 - O.K. - but then oscillates from 1 to 0.9 and so on.

  2. #2
    The superhaterodyne twomers's Avatar
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    You could do the following -

    Code:
    int main( void )
    {
        int num[] = { -1, 0, 1 }, i, max=10;
    
        for ( i=1; i<max; i++ )
        {
            printf( "&#37;d ", num[i%3] );
        }
    
        return 0;
    }
    And please consider using code tags in the future. It'll keep many of us happier

  3. #3
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    Thank you, but I forgot to mention, that the x[i] are real numbers, ranging over [-1:1]
    I want to create something like 0.0, 0.1, ...,0.9 , 1.0 , 0.9 , ... ,0.0 , -0.9 , -0.8 , ... , -1.0 , -0.9 ,... and so on as far as i<N

    Wlii i get another chance ?

    joerg
    Last edited by Salem; 04-11-2007 at 02:38 PM. Reason: snip email address

  4. #4
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    Could you please clarify the last part of the oscillation? You say:
    0.0 , -0.9 , -0.8 , ... , -1.0 , -0.9
    But there's a bit of a skip from 0.0 to -0.9, and similarly, you cannot count up from -0.9 and hope to reach -1.0.

  5. #5
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    I beg your pardon!

    It has to look like this: 0.0, 0.1 ,... ,1.0 ,0.9 ,...0.1 , 0.0 ,-0.1 ,-0.2 ,... ,-1.0 ,-0.9 ,-0.8 ,... ans so on.

    thank you for your reply anyway!

    Is there a third chance for me ?

    joerg

  6. #6
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    Here's the dependant coding of it - that is, each entry is dependant on the previous entry.
    Code:
    double val = 0.0
    double inc = 0.1
    int N = 100;
    double x[N];
    
    for (int i = 0; i < N; i++) {
      val += inc; //increase oscillation position
      if ((val == 1.0) || (val == -1.0)) inc = -inc; //reverse direction
      x[i] = val;
    }

  7. #7
    The superhaterodyne twomers's Avatar
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    Do you understand looping? If so this should be trivial.

    First things first - try to get your program to count from 0 to 1 in steps of 0.1. When you have that try to modify your code to get it to work from -1 to +1. Then worry about making it over and over and over again.

  8. #8
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    @twomers: that won't work, because then it won't oscillate, it will only modulate. Once it counts up to +1, it needs to start counting back down again.

    @joerg: Here's a more dynamic solution. It's independant of previous entries, meaning that you can just plug in any value and it will dynamically compute it without having to go back through the entire list, so it has a runtime of O(1), rather than my previous solution, which had a runtime of O(n).

    Code:
    int N = 100;
    double x[N];
    for (int i = 0; i < N; i++) {
      int v = (i + 10) &#37; 40;
      if (v <= 20) x[i] = (double)(v - 10) / 10.0;
      if (v > 20) x[i] = (double)(30 - v) / 10.0;
    }
    I was also considering a solution that used memoization, but realized that I could just modulate over the computation.

  9. #9
    The superhaterodyne twomers's Avatar
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    >> @twomers: that won't work, because then it won't oscillate, it will only modulate. Once it counts up to +1, it needs to start counting back down again.

    I understand. The OP is probably a beginner at programming, so he has to get his mind into doing things in steps and building on what he has written! I was trying to encourage him to do it himself, and hence to become a better programmer.

    edit
    >>int N = 100;
    >>double x[N];

    <nvm that>

    sigh

    Code:
    int main( void )
    {
        double i, di = -0.1;
        int num=0;
    
        for ( i=-1.0; num<100; i+=di, num++ )
        {
            printf( "&#37;f ", i );
            
            if ( i<-0.9 || i>0.9 )
                di *= -1;
        }
    
        return 0;
    }
    Enjoy.
    Last edited by twomers; 04-11-2007 at 02:55 PM.

  10. #10
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    Code:
    #define N 100
    double x[N];

  11. #11
    Captain Crash brewbuck's Avatar
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    Whenever the value reaches 1.0 or -1.0, negate the value of dt (or Vx, one of the two).

  12. #12
    Frequently Quite Prolix dwks's Avatar
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    Quote Originally Posted by IsmAvatar2 View Post
    Code:
    #define N 100
    double x[N];
    There are many threads arguing about which is better:
    1. Code:
      #define N 100
    2. Code:
      enum { N = 100 };
    3. Code:
      const int N = 100;

    Here's one, though I've changed my opinions since then: #define vs const
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


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  13. #13
    Captain Crash brewbuck's Avatar
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    Code:
    #ifndef N
    #define N 100
    #endif
    Advantages: The user can override the preprocessor definition at compile-time, without editing the source.
    Disadvantages: Literal substitution of macros can cause weird effects in certain situations. The debugger (usually) has no idea about defined values, so debugging is harder.

    Code:
    enum { N = 100; }
    Advantages: An enum is a real type, so the compiler can do type checking. The debugger can display the symbolic values, provided you've given everything an appropriate type instead of just "int."
    Disadvantages: An enumeration really shouldn't be used unless there are multiple related values to declare. Goes against intended use of enum, and might be obfuscatory.

    Code:
    const int N = 100;
    Advantages: None, really.
    Disadvantages: The variable is "real," meaning you can take its address. This means you can coerce the type to non-const, and then change the supposedly "constant" value at runtime (that is, if it doesn't crash). NOTE: big difference from what it means in C++!

    I tend to go with #define. The ability to override the constants at compile time is just too convenient most of the time.

  14. #14
    Frequently Quite Prolix dwks's Avatar
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    I generally use:
    1. Named enums for multiple related values, e.g.:
      Code:
      enum type_t {
          TYPE_NUMBER,
          TYPE_STRING,
          TYPE_LINK,
          TYPES
      };
      This is because:
      1. The numbers are incremented automatically, so adding another value is easy.
      2. You can add a variable at the end which is a count of all the preceding values.

      Note that this is the intended use of enums.
    2. Anonymous enums for constant values, e.g.:
      Code:
      enum {
          STRING_LEN = BUFSIZ
      };
      This is the controversial usage of enum. I use enum instead of #defines because the compiler checks the values rather than the preprocessor, which is rather blind. I suppose I could use constant values, but -- force of habit.
    3. #defines for strings, e.g.:
      Code:
      #define VERSION "program version 1.0.0"
      That way I can use the preprocessor string concatentation:
      Code:
      fprintf(stderr, VERSION "\n"
          "\nusage: ...\n");

    I just knew that this was going to start a debate . . .
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


    Other boards: DaniWeb, TPS
    Unofficial Wiki FAQ: cpwiki.sf.net

    My website: http://dwks.theprogrammingsite.com/
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  15. #15
    The superhaterodyne twomers's Avatar
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    Wow. A doubly listed list! You should be proud, dwks!

    >> STRING_LEN = BUFSIZ
    Is BUFSIZ a #define ... ? I would be very amused if it was.

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