Help with a function that suppose to normalize an array

Livnat · 07-13-2008, 11:29 PM

Hello,
This function suppose to make the mean value zero and the Standard deviation 1,
Only it makes them both zero.
Somebody has an idea why?
Thanks!

Code:

void norm_b(double b[30][30] , int i, int j)
{
double sum=0,avg, avg2;
int m,n;

for (m=0 ; m<i ; m++)
       for (n=0 ; n<j ; n++)
               sum+=b[m][n];

avg=sum/(i*j);
for (m=0 ; m<i ; m++)
       for (n=0 ; n<j ; n++)
               b[m][n]=b[m][n]-avg;

for (m=0 ; m<i ; m++)
       for (n=0 ; n<j ; n++)
               sum+=pow(b[m][n],2);
avg2=sum/(i*j);

for (m=0 ; m<i ; m++)
    for (n=0 ; n<j ; n++)
        b[m][n]=b[m][n]/sqrt(avg2);

}

**Salem** · 07-13-2008, 11:44 PM

Do you know what sum is supposed to be at various stages?

Test it with a known set of data, then run the code with the debugger and examine the program state as it runs. When reality != expectation, you've found a bug.

Without a debugger, try putting printf statements at key points to print key values.

zacs7 · 07-13-2008, 11:46 PM

You do know that array elements are sequential (regardless of dimensions too), yes?

In this case, rows and columns hold no bearing. Thus it can be written as,

Code:

void norm_b(double * b, size_t n)
{
    /* ... */
    size_t i = 0;

    for(i = 0; i < n; i++)
        sum += b[i];

    /* ... */
    return;
}

Which is far easier to read. You're also leaving a huge chunk of the problem out, how do you know that the StDev and mean are 0? You don't print them or anything. Unless you used a debugger... even then...

Also, you might want to work on your indenting

Livnat · 07-14-2008, 12:40 AM

Thanks Salem for the advise. I printed the first sum and its zero no matter what array I input.
I don't understand why...

Hi zacs7 I'm bit afraid of pointers in this point so I prefer doing it the old two dimensional way...
Any ways this is the full code, I left it out because it's long...

Code:

#include <stdio.h>
#include <math.h>

void scan_b(double b[30][30]  , int , int );
void print_b(double b[30][30] , int  , int );
void norm_b(double  b[30][30], int , int );
double std_b(double b[30][30], int , int );
double avg_b(double b[30][30], int , int );



int main()
{
int k,l;
double c[30][30];
printf("please enter the number of rows and columns\n");
scanf("%d%d" , &k, &l);
scan_b(c ,k,l);
print_b (c,k,l);
norm_b( c ,  k,  l);
printf ("the Standard deviation is: %lf\n" ,std_b( c , k,  l));
printf ("the mean value is: %lf\n" ,avg_b( c ,  k,  l));
system ("PAUSE");
return 0;
}



void scan_b(double b[30][30] , int i, int j)
{
int m,n;
printf("please enter a %d*%d array\n",i,j);
for (m=0 ; m<i ; m++)
       for (n=0 ; n<j ; n++)
               scanf("%d" , &(b[m][n]));
}

void print_b(double b[30][30] , int i, int j)
{
int m,n;

for (m=0 ; m<i ; m++)
       {
           printf("\n");
           for (n=0 ; n<j ; n++)
           printf("%d" , b[m][n]);
        }
        printf("\n");
}

void norm_b(double b[30][30] , int i, int j)
{
double sum=0,avg, avg2;
int m,n;

for (m=0 ; m<i ; m++)
       for (n=0 ; n<j ; n++)
               sum+=(b[m][n]);
               printf("sum=%lf",sum);

avg=sum/(i*j);
               printf("avg=%lf",avg);
for (m=0 ; m<i ; m++)
       for (n=0 ; n<j ; n++)
               b[m][n]=b[m][n]-avg;
               for (m=0 ; m<i ; m++)
       {
           printf("\n");
           for (n=0 ; n<j ; n++)
           printf("%d" , b[m][n]);
        }
        printf("\n");

for (m=0 ; m<i ; m++)
       for (n=0 ; n<j ; n++)
               sum+=pow(b[m][n],2);
                              printf("sum2=%lf",sum);
avg2=sum/(i*j);
               printf("avg2=%lf",avg);
for (m=0 ; m<i ; m++)
    for (n=0 ; n<j ; n++)
        b[m][n]=b[m][n]/sqrt(avg2);
        for (m=0 ; m<i ; m++)
       {
           printf("\n");
           for (n=0 ; n<j ; n++)
           printf("%d" , b[m][n]);
        }
        printf("\n");

}

double avg_b(double b[30][30], int i, int j)
{
double sum=0,avg, avg2;
int m,n;

for (m=0 ; m<i ; m++)
       for (n=0 ; n<j ; n++)
               sum+=b[m][n];

avg=sum/(i*j);
return avg;
}

double std_b(double b[30][30] , int i, int j)
{
double sum2,sum,std,avg;
int m,n;
for (m=0 ; m<i ; m++)
       for (n=0 ; n<j ; n++)
               sum+=b[m][n];
avg=sum/(i*j);
for (m=0 ; m<i ; m++)
       for (n=0 ; n<j ; n++)
               sum2=pow((b[m][n]-avg),2);

std=sqrt(sum2/(i*j));
return std;

}

vart · 07-14-2008, 12:46 AM

you should reread the format specifiers for printf and scanf

%lf used for doubles in scanf, in printf %f should be used

%d used for integers - using it for doubles can cause different errors including crashes

Livnat · 07-14-2008, 12:57 AM

Thanks that helped!
The Standard deviation is still not 1... I'll continue working on it later

**Salem** · 07-14-2008, 10:51 AM

If you're using gcc, then use
gcc -W -Wall -ansi -pedantic -O2 prog.c

Fix anything it complains about, including printf/scanf format errors.

07-13-2008, 11:44 PM	#2
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	Do you know what sum is supposed to be at various stages? Test it with a known set of data, then run the code with the debugger and examine the program state as it runs. When reality != expectation, you've found a bug. Without a debugger, try putting printf statements at key points to print key values. __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
Salem is offline

07-13-2008, 11:46 PM	#3
zacs7 Woof, woof! Join Date: Mar 2007 Location: Australia Posts: 3,139	You do know that array elements are sequential (regardless of dimensions too), yes? In this case, rows and columns hold no bearing. Thus it can be written as, Code: void norm_b(double * b, size_t n) { /* ... / size_t i = 0; for(i = 0; i < n; i++) sum += b[i]; / ... / return; } Which is far easier to read. You're also leaving a huge chunk of the problem out, how do you know* that the StDev and mean are 0? You don't print them or anything. Unless you used a debugger... even then... Also, you might want to work on your indenting __________________ "I.T. gets the chicky-babes" - M. Kelly bakefile \| vim
zacs7 is offline

07-14-2008, 12:46 AM	#5
vart CSharpener Join Date: Oct 2006 Posts: 5,242	you should reread the format specifiers for printf and scanf %lf used for doubles in scanf, in printf %f should be used %d used for integers - using it for doubles can cause different errors including crashes __________________ If I have eight hours for cutting wood, I spend six sharpening my axe.
vart is offline

07-14-2008, 12:57 AM	#6
Livnat Registered User Join Date: May 2008 Posts: 29	Thanks that helped! The Standard deviation is still not 1... I'll continue working on it later
Livnat is offline

07-14-2008, 10:51 AM	#7
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	If you're using gcc, then use gcc -W -Wall -ansi -pedantic -O2 prog.c Fix anything it complains about, including printf/scanf format errors. __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
Salem is offline

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