Thread: 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!

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

--
Mats

&#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.

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.