what is wrong in this simple code

This is a discussion on what is wrong in this simple code within the C Programming forums, part of the General Programming Boards category; Hey Pals, here I wrote one simple code to be more readable but something is wrong when i am compile ...

  1. #1
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    what is wrong in this simple code

    Hey Pals, here I wrote one simple code to be more readable but something is wrong when i am compile it, the problem is print out the values of f[i][j] is wrong coz is a lot of 0's
    Code:
    #include<stdio.h>
    #include<stdlib.h>
    #include<math.h>
    #define max 300
    int
    main()
    {
    	float x[max],y[max],f[max][max],u[max][max],v[max][max],ui,xs,ys,dx;
    	int imax,jmax,i,j,il,it;
    	il=11;
    	it=31;
    	imax=41;
    	jmax=12;
    	xs=1.24;
    	ys=1.24;
    	ui=0.0;
    	dx=-1.0/(il-it);
    	for(i=il;i<=it;i++)
    	x[i]=(i-il)*dx;
    	for(i=it+1;i<=imax;i++)
    	x[i]=x[i-1]+(x[i-1]-x[i-2])*xs;
    	for(i=il-1;i>=1;i--)
    	x[i]=x[i+1]+(x[i+1]-x[i+2])*xs;
    
    	/*initial condition*/
    	for(i=1;i<=imax;i++)
    	{
    		for(j=1;j<=jmax;j++)
    		f[i][j]=ui*x[i];
    		
    	}
    	/*0000000000000000000000000000000000000000000000000*/
    	/*=====================Velocities===========================*/
    		for(i=2;i<=imax-1;i++)
    	{
    		for(j=2;j<=jmax-1;j++)
    		
    		
    		u[i][j]=(f[i+1][j]-f[i-1][j])/(x[i+1]-x[i-1]);		
    		
    					
    	}  
    	/* PRINT*/
    		for(j=1;j<=jmax;j++)
    	{
    		for(i=1;i<=imax;i++)
    		
    		printf("%f\n",f[i][j]);
    				
    	}
    }
    iam new in c so be patient with me :-)

  2. #2
    Kernel hacker
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    Considering that f[][] is initialized to all zeros:
    Code:
        ui=0.0;
        dx=-1.0/(il-it);
        for(i=il;i<=it;i++)
    	x[i]=(i-il)*dx;
        for(i=it+1;i<=imax;i++)
    	x[i]=x[i-1]+(x[i-1]-x[i-2])*xs;
        for(i=il-1;i>=1;i--)
    	x[i]=x[i+1]+(x[i+1]-x[i+2])*xs;
    
        /*initial condition*/
        for(i=1;i<=imax;i++)
        {
    	for(j=1;j<=jmax;j++)
    	    f[i][j]=ui*x[i];
    		
        }
    and doesn't change later on, I fail to see how it can become anything but zero from this
    Code:
        for(i=2;i<=imax-1;i++)
        {
    	for(j=2;j<=jmax-1;j++)
    		
    		
    	    u[i][j]=(f[i+1][j]-f[i-1][j])/(x[i+1]-x[i-1]);		
    		
    					
        }
    Usually, zero divided by some number gives zero.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  3. #3
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    op was a big mistake of course ui=1.0; but my question now is focus in the results of u[i][j] that probably will 0's and 1's, could you advise me concerning this value u[i][j]
    thanks in advance

  4. #4
    Kernel hacker
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    Cahnging ui to 1.0 gives
    Code:
    -1.961893
    -1.532172
    -1.185623
    -0.906147
    -0.680764
    -0.499003
    -0.352422
    -0.234211
    -0.138880
    -0.062000
    0.000000
    0.050000
    0.100000
    0.150000
    0.200000
    0.250000
    0.300000
    0.350000
    0.400000
    0.450000
    0.500000
    0.550000
    0.600000
    0.650000
    0.700000
    0.750000
    0.800000
    0.850000
    0.900000
    0.950000
    1.000000
    1.062000
    1.138880
    1.234211
    1.352422
    1.499004
    1.680765
    1.906148
    2.185624
    2.532173
    2.961895
    -1.961893
    -1.532172
    -1.185623
    -0.906147
    -0.680764
    -0.499003
    -0.352422
    -0.234211
    -0.138880
    -0.062000
    0.000000
    0.050000
    0.100000
    0.150000
    0.200000
    0.250000
    0.300000
    0.350000
    0.400000
    0.450000
    ...
    I have no idea what they are supposed to be, and to figure out what goes wrong, I suppose you need to understand what the intermediate values are supposed to be, and what they really are.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  5. #5
    Registered User ssharish2005's Avatar
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    Location
    Cambridge, UK
    Posts
    1,682
    I had been working on your code for few mins. I really see that maths behind is totally wrong. You need to really understand the formula before you apply them. Most of our solution is learing to negative values and your trying to double the negative value again by multipling it. Which dost sound good for me. Have a look at the maths. I guess you might have understood the typos good here. Since you have applied it right!

    -ssharish
    Life is like riding a bicycle. To keep your balance you must keep moving - Einstein

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