Doing calculations on a data file

Ok new problem... The assignment I'm working on right now requires us to read in a file, make calculations on the file, and write the results to a new file. The file contains like 225 sets of data but we need to make the program so that you can select a value N so that it only calculates every Nth value. So lets say you pick 5, it would calculate every 5th value and print 45 values to the screen.

I think I got the skipping values part down, I just manipulated my for loop variable in the arrays. The real question is, how can I keep a running total of my calculations. I'm calculating a displacement so you calculate first two points, get a displacement, calculate next two points, get a displacement, but that displacement has to be the total displacement so far.... Get what I'm saying?

Here's some code.

Code:

#define C_F 286.8
#define MAX_VALUES 225
#define FILENAME "pixel_load.txt"

//Start of main function
int main(void)
{
	//Declarations
	FILE *pixel_load = NULL;
	int i, j, N;
	double displacement_pixels[MAX_VALUES], displacement_inches[MAX_VALUES], load[MAX_VALUES]; 
	double num_measurements, amount_of_points;

	//Header of program and user input for number of data points
	printf("                    LOAD AND DEFLECTION \n");
	printf("------------------------------------------------------------ \n\n");
	printf("Only one out of every n data point will be recorded in the output. \n");
	printf("What value of N would you like: ");
	scanf("%i", &N);
	
	//Open file
	pixel_load = fopen(FILENAME, "r");
	
	//Check to see if file opens
	if(pixel_load == NULL)
	{
		printf("Could not open data file\n");
		return -1;
	}
	
	//Read in number of measurements
    fscanf(pixel_load, "%lf", &num_measurements);
	
	 //Calculate number of data points
	amount_of_points = num_measurements / N;
	floor(amount_of_points);
	
	//Display number of points recorded
	printf("\n\nRecording 1 out of every %i data samples.", N);
	printf(" There will be %4.0f samples in \nthe output file. \n\n", amount_of_points);
	
	//Headings for output
	printf("				DISPLACEMENT[in]				LOAD[LBS] \n");
	
	//For loop to read values 
	for( i = 0; i < num_measurements; i++)
	{
		fscanf(pixel_load, "%lf %lf", &displacement_pixels[i], &load[i] );
	}
	
	//Print zero deflection
	printf("                  0.000\n");

	//For loop to calculate  
	for( j = 0; j < amount_of_points; j++)
	{
		displacement_inches[j] = (displacement_pixels[(N * j) + 1] - displacement_pixels[N * j]) / C_F;
		printf("                 %6.3lf 					%7.3lf\n", displacement_inches[j], load[(N * j)]);
	}
	
	
	//Calculations to convert pixels to inches
	//displacement_inches[j] = (displacement_pixels[j] - displacement_pixels[j]) * C_F;
	//printf("				   %6.3lf                     %7.3lf\n", displacement_inches[j], load[j]);



    //End the program
	printf("Press ENTER to continue... ");
    getchar();
    return 0;
 }

Thanks!

Use a global variable.

Why a global? First off, you're doing all of this inside of main, so you could just as easily make this a local variable.

If you want to figure this out without having to wade through a file first, do a simple version to get the logic down:

Code:

prompt for number
read it into number
for( x = starting point; x < ending point; x += number )
{
     do whatever
}

Just use an array, throw in a handful of test variables, and use the above loop. Once you have that concept working, apply it to your file. Replace 'do whatever' it jumping number records at a time.