Problem with ">", "<" in a for loop

This is a discussion on Problem with ">", "<" in a for loop within the C Programming forums, part of the General Programming Boards category; This is embarrassing, but i have no idea of what is happening. I'm using numerical integration to solve a ODE, ...

  1. #1
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    Problem with ">", "<" in a for loop

    This is embarrassing, but i have no idea of what is happening.

    I'm using numerical integration to solve a ODE, and I asked my program where the final point is. I devided the phase space in little cells... and because of the nature of my system it always finishes in (0,1) or in (0,-1).

    Here is my "question code":

    Code:
    if((i-DIM/2)/25.0<salidax[n][k]<(i+1-DIM/2)/25.0)
    	    if((j-DIM/2)/25.0<saliday[n][k]<(j+1-DIM/2)/25.0) {
    	      printf("&#37;f %f %f %f %f %f \t", (i-DIM/2)/25.0,salidax[n][k],(i+1-DIM/2)/25.0,(j-DIM/2)/25.0,saliday[n][k],(j+1-DIM/2)/25.0);
    }
    It shoud work. I also tried the "&&" operator insted of calling "if" twice, but the same result:

    Code:
     0.280000 -0.000000 0.320000 0.040000 -1.000000 0.080000
     0.280000 -0.000000 0.320000 0.080000 -1.000000 0.120000 	
     0.280000 -0.000000 0.320000 0.120000 -1.000000 0.160000 
     0.280000 -0.000000 0.320000 0.160000 -1.000000 0.200000 	
     0.280000 -0.000000 0.320000 0.200000 -1.000000 0.240000 
     0.280000 -0.000000 0.320000 0.240000 -1.000000 0.280000
     0.280000 -0.000000 0.320000 0.280000 -1.000000 0.320000
    &#191;Is something wrong in my "question" code?

    Thanks a lot!
    Last edited by Isolda_; 09-11-2007 at 12:27 PM.

  2. #2
    Kernel hacker
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    It looks very complex. How about pre-calculating some of those values into a set of local variables.

    --
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  3. #3
    Captain Crash brewbuck's Avatar
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    The expression of form "a < x < b" does not do what you think it does. It doesn't check that x is between a and b. First, it checks if a < x. If true, this part of the expression evaluates to 1. Then, it checks 1 < b. Or, if a >= x, then a < x == 0 and it compares 0 < b.

    You want to use: a < x && x < b.

  4. #4
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    &#191;Do you say, instead of use the limits
    Code:
    (j-DIM/2)/25.0
    , etc., asing each of this limits to a local variable and then ask?

    Code:
     
    inf_x=(i-DIMENSION/2)/25.0;
    sup_x=(i+1-DIMENSION/2)/25.0;
    inf_y=(j-DIMENSION/2)/25.0;
    sup_y=(j+1-DIMENSION/2)/25.0;
    
      if(inf_x<salidax[n][k]<sup_x)
        if(inf_y<saliday[n][k]<sup_y) {
          printf(fp,"&#37;f %f %f %f %f %f \t", inf_x,salidax[n][k],sup_x,inf_y,saliday[n][k],sup_y);}
    It doesn't work



    Code:
    0.360000 -0.000000 0.400000 0.040000 -1.000000 0.080000
    EDITION:

    O_O

    brewbuck, I did'nt see your answer. I never would imagine "a < x < b" doesn`t do what I think. Thanks a lot!

    The result is this:

    Code:
    0.000000 0.000000 0.040000 0.960000 1.000000 1.000000 	
    0.000000 0.000000 0.040000 0.960000 1.000000 1.000000 	
    0.000000 0.000000 0.040000 0.960000 1.000000 1.000000 	
    0.000000 0.000000 0.040000 0.960000 1.000000 1.000000 	
    0.000000 0.000000 0.040000 0.960000 1.000000 1.000000 	
    0.000000 0.000000 0.040000 0.960000 1.000000 1.000000 	
    0.000000 0.000000 0.040000 0.960000 1.000000 1.000000 	
    0.000000 0.000000 0.040000 0.960000 1.000000 1.000000 	
    0.000000 0.000000 0.040000 0.960000 1.000000 1.000000 	
    0.000000 0.000000 0.040000 0.960000 1.000000 1.000000...
    The problem now is i'm getting a lot of copies of the same thing; like the "i" and "j" don't evolve!, but when I put a
    Code:
    printf
    just after the declaration of local variables, it does evolve. I'm confuse.
    Last edited by Isolda_; 09-11-2007 at 12:57 PM.

  5. #5
    Registered User ssharish2005's Avatar
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    Code:
    if(inf_x<salidax[n][k]<sup_x)
    What he means is that you cant do that. This statment dosn't provide you with the right result. Instead use this

    Code:
    if( inf_x < salidax[n][k] && salidax[n][k] < sup_x )
    And you need to give some white spaces in your code, to make it read better. It is very difficult to read your code.

    ssharish

  6. #6
    Fear the Reaper...
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    It doesn't work
    Of course it doesn't. And for the very same reason the brewbuck mentionned before.

    Expressions of the type a < x < b don't do what you think they do, as they basically come down to (a < x) < b. Now, the expression (a < x), if true, returns 1, if not, it returns 0. So you're comparing the result of a<x to b. Now since its quite unlikely that x will always be 1 or 0, you're not gonna get the results you expect.

    What you want to do is :

    Code:
    (x > a && x < b)
    Then you'll be going somewhere.
    Teacher: "You connect with Internet Explorer, but what is your browser? You know, Yahoo, Webcrawler...?" It's great to see the educational system moving in the right direction

  7. #7
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    Yes, I corrected it in my edition of the post. Thanks!

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