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assaad
Joined: 09 Dec 2009
Posts: 37
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| problem in calculation |
 Posted: Thu May 27, 2010 2:49 am |
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Hi
I am doing some complex calculations, but I get wrong results. Could you help to solve it please.
| Code: |
int8 x[10];
float sumx;
float sumy;
float sumxy;
float sumxx;
int16 try1;
int16 try2;
int16 try3;
int16 try4;
int16 y[10];
float m;
float b;
void main() {
INT8 str[10]={};
int n;
EBDIS_BIT = 1 ; // DISABLE EXTERNAL MEMORY INTERFACE
OUTPUT_J(0);
n=9;
sumx=0;
sumxx=0;
sumy=0;
sumxy=0;
////////////////////////////////////
x[0]=20;
x[1]=40;
x[2]=60;
x[3]=80;
x[4]=100;
x[5]=120;
x[6]=140;
x[7]=160;
x[8]=180;
x[9]=200;
/////////
y[0]=2184;
y[1]=4368;
y[2]=6552;
y[3]=8736;
y[4]=10920;
y[5]=13104;
y[6]=15288;
y[7]=17472;
y[8]=19656;
y[9]=21840;
///////////////////////////////////////////////////////////
for(i=0;i<=n;++i)
{sumx+=x[i];
sumy+=y[i];
sumxy+=(float)x[i]*(float)y[i];
sumxx+=(float)x[i]*(float)x[i];
}
//m=(try1-try2)/(try3-try4);
m=(((n+1)*sumxy)-(sumx*sumy))/(((n+1)*sumxx)-(sumx*sumx));
b=(sumy-(m*sumx))/(n+1);
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sumxx gives wrong result, sumxy also wrong, m and b wrong too, any help from you and I would be thankful.
Regards |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19506
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| Posted: Thu May 27, 2010 5:01 am |
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What are you getting?.
What do you expect to get?.
How are you testing these values?.
Quite a bit of accuracy _will_ be lost at a lot of points in this arithmetic. A 'float' value in CCS, is only effectively about 6 digits. So (for example), your 'sumxx' total, _will_ give a result with digits lost.
So the final results will _not_ be what you would see if you performed the arithmetic on a calculator.
However the results for sumxx, should be basically right.
It'd be faster, and more accurate, to keep all the summing as integer (int32), and only use float for the final results.
Best Wishes |
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assaad
Joined: 09 Dec 2009
Posts: 37
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| Posted: Thu May 27, 2010 5:20 am |
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| Code: |
unsigned int8 x[10];
unsigned int16 sumx;
unsigned int16 sumy;
unsigned int32 sumxy;
unsigned int32 sumxx;
unsigned int32 try1;
unsigned int32 try2;
unsigned int32 try3;
unsigned int32 try4;
unsigned int16 y[10];
x[0]=20;//20;
x[1]=40;//40;
x[2]=60;//60;
x[3]=80;//80;
x[4]=100;//100;
x[5]=120;//120;
x[6]=140;//140;
x[7]=160;//160;
x[8]=180;//180;
x[9]=200;//200;
/////////
y[0]=1;
y[1]=2;
y[2]=3;
y[3]=4;
y[4]=5;
y[5]=6;
y[6]=7;
y[7]=8;
y[8]=9;
y[9]=10;
sumxx=(_mul(x[0],x[0]))+(_mul(x[1],x[1]));
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See this test example I did:
sumxx should equal to 20*20+40*40=2000, while get 3280!!
I test the result by MPLAB SIM.
When can I use direct multply (*), instead of _mul() ?
Thank you |
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