Problem multiplying arrays

hellhammer · #1 29-Mar-2004, 14:47

Hello,

I have what seems to be a really simple problem, but I've been unable to solve it in two days.

I need to do the following:

Find the autocorrelation of an array, which is part of a two-dimensional array.
The array is of type "double", and is of length 512.

When I print the arrays, SegmentArray[0][256+j] and SegmentArray[0][i+j], I get the correct values.

However, when I print AutocorrArray, which should be nothing but a sum of products as described in the code below, all I get is 0. This is not the correct answer for the array in question, as has been verified by Matlab and Mathcad.

What am I doing wrong?

Thank you,

CPP / C++ / C Code:

for ( i = 0; i < 256; i++ )
{
	Sum = 0.f;
	for ( j = 0; j < 256; j++ )
	{
		Sum += SegmentArray[0][256+j]*SegmentArray[0][i+j];
	}
	AutocorrArray[0][i] = Sum/256;
}

davekw7x · #2 29-Mar-2004, 14:52

Quote:

Originally Posted by hellhammer

Hello,

I have what seems to be a really simple problem, but I've been unable to solve it in two days.

I need to do the following:

Find the autocorrelation of an array, which is part of a two-dimensional array.
The array is of type "double", and is of length 512.

When I print the arrays, SegmentArray[0][256+j] and SegmentArray[0][i+j], I get the correct values.

However, when I print AutocorrArray, which should be nothing but a sum of products as described in the code below, all I get is 0. This is not the correct answer for the array in question, as has been verified by Matlab and Mathcad.

What am I doing wrong?

Thank you,

CPP / C++ / C Code:

for ( i = 0; i < 256; i++ )
{
	Sum = 0.f;
	for ( j = 0; j < 256; j++ )
	{
		Sum += SegmentArray[0][256+j]*SegmentArray[0][i+j];
	}
	AutocorrArray[0][i] = Sum/256;
}

What is the type of variable Sum?

Post a program showing declarations.

Dave

hellhammer · #3 29-Mar-2004, 15:00

Quote:

Originally Posted by davekw7x

What is the type of variable Sum?

Post a program showing declarations.

Dave

CPP / C++ / C Code:

/* Preprocessor Statements */

#include <stdio.h>

FILE *FileDataIn, *FileDataOut, *FileSegment, *FileAutocorr;

double InputArray[1280];
//This program examines the case for one segment.  In the final program there will be 7 segments and will need double SegmentArray[7][512]
double SegmentArray[512];
// This will need to be changed to double AutocorrArray[7][256]
double AutocorrArray[256];

int main()
{
	int i = 0, j = 0;
	double Sum = 0;
		
	// Step 0: Read the .wav data values from a text file
	
	if ( ( FileDataIn = fopen("WaveData.txt","r") ) == NULL )
	{
		printf("File not found \n");
		getchar();
		exit(1);
	}
	else
	{
	  i = 0;
	  while(!feof(FileDataIn))
	  { 
      // loop through and store the numbers into the array
      fscanf(FileDataIn, "%d", &InputArray[i]);
      i++;
      }	  
	  printf("File successfully read \n");
    }
    fclose(FileDataIn);
    
        
    // Step 1: Segmentation
    
    for ( i = 0; i < 512; i++ )
    {
    	SegmentArray[i] = InputArray[i];
    }
    
    printf("Segmentation Complete\n");
   
    // Write Segment Data
 
    if ( ( FileSegment = fopen("Segment.txt", "w") ) == NULL ) 
    {
     	printf("Error: can't create file.\n");
     	exit(0);
    }
    else
    {
    	for ( i = 0; i < 512; i ++ )
    	{
    		fprintf(FileSegment, "%d\n", SegmentArray[i]);
    	}
    	printf("Segment successfully written\n");      	  
    }
    fclose(FileSegment);    
  
	

	printf("\nPrinting SegmentArray[i]\n");
	for (i = 0; i < 10; i++)
	{
		printf("%d\n", SegmentArray[256+i]);
	}
	
	printf("\nPrinting SegmentArray[i+j]\n");
	for (j = 0; j < 10; j++)
	{
		printf("%d\n", SegmentArray[j]);
	}
	
	// AUTOCORRELATION

	for ( i = 0; i < 256; i++ )
	{
		Sum = 0;
		for ( j = 0; j < 256; j++ )
		{
			Sum += SegmentArray[256+j]*SegmentArray[i+j];
		}
		AutocorrArray[i] = Sum/256;
	}
		
	if ( ( FileAutocorr = fopen("Autocorr.txt", "w") ) == NULL ) 
    {
     	printf("Error: can't create file.\n");
     	exit(0);
    }
    else
    {
    	for ( i = 0; i < 256; i ++ )
    	{
    		fprintf(FileAutocorr, "%d\n", AutocorrArray[i]);
    	}
    	printf("Autocorr successfully written\n");      	  
    }
    fclose(FileAutocorr);

	printf("\n End of Routines \n"); 
}

My problem is that the contents of AutocorrArray are ALL 0.

davekw7x · #4 29-Mar-2004, 15:11

Could you supply (attach) the WaveData.txt file that you are using?

Dave

davekw7x · #5 29-Mar-2004, 15:14

Quote:

Originally Posted by davekw7x

Could you supply (attach) the WaveData.txt file that you are using?

Dave

You have the following:

CPP / C++ / C Code:

      fscanf(FileDataIn, "%d", &InputArray[i]);

but InputArray is declared

CPP / C++ / C Code:

double InputArray[1280];

So fscanf() should use "%lf", I think.

Are you sure you have read the input correctly? Hint: try printing out the first 10 or so (no need to print the entire array).

Dave

hellhammer · #6 29-Mar-2004, 15:14

Quote:

Originally Posted by davekw7x

Could you supply (attach) the WaveData.txt file that you are using?

Dave

Here it is!

Thanks, Dave.

hellhammer · #7 29-Mar-2004, 15:17

Quote:

Originally Posted by davekw7x

You have the following:

CPP / C++ / C Code:

      fscanf(FileDataIn, "%d", &InputArray[i]);

but InputArray is delcared

CPP / C++ / C Code:

double InputArray[1280];

Are you sure you have read the input correctly? Hint: try printing out the first 10 or so (no need to print the entire array).

Dave

The input's been read correctly. I used both %f and %d, and verified (both by printing to the screen and to an output file) that the contents of InputArray are the same as "WaveData.txt".

hellhammer · #8 29-Mar-2004, 15:24

Quote:

Originally Posted by davekw7x

You have the following:

CPP / C++ / C Code:

      fscanf(FileDataIn, "%d", &InputArray[i]);

but InputArray is declared

CPP / C++ / C Code:

double InputArray[1280];

So fscanf() should use "%lf", I think.

Are you sure you have read the input correctly? Hint: try printing out the first 10 or so (no need to print the entire array).

Dave

DAVE, YOU ROCK!

I changed all the %ds to %lf, and I got it! What exactly is %lf anyway? Sorry, I'm a newbie!

Thanks a lot!

davekw7x · #9 29-Mar-2004, 15:27

Quote:

Originally Posted by hellhammer

The input's been read correctly. I used both %f and %d, and verified (both by printing to the screen and to an output file) that the contents of InputArray are the same as "WaveData.txt".

You declare InputArray as double so you must (must) use %lf in scanf(), not %d, not %f.

Also in your printf() statements for SegmentArray and AutocorrArray, you must not use %d (OK to use %f or %lf in printf(), since the argument will be promoted to double, anyhow. For scanf(), must use %lf.)

Please run again, and put the following after you close WaveData.txt:

CPP / C++ / C Code:

  for (i = 0; i < 10; i++) {
    printf("<%f> ", InputArray[i]);
  }
  printf("\n");

Dave

davekw7x · #10 29-Mar-2004, 15:32

Quote:

Originally Posted by hellhammer

I changed all the %ds to %lf, and I got it! What exactly is %lf anyway? Sorry, I'm a newbie!

Thanks a lot!

You are giving scanf() the address of a variable. Scanf() will convert the appropriate number of input bytes and store in the place you tell it.

%f indicates to scanf that this is a float type variable.
%lf indicates to scanf() that this is a double type variable (lf is mnemonic for "long float", I guess).

By the way, C does all internal float math operations as double, so there's no advantage to float (as opposed to double), unless there are large arrays and storage becomes an issue.

Regards,

Dave

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