Thread: pointers and vectors

I cant manage to sum a few vectors. Values are gigantic and only iterates about 800 times until theres no more memory. I guess I am not handling pointers correctly, but I cant work it out. anybody see anything strange?

Code:

float *amp, *sum_amp;

SOME SKIPPED CODE

// memory allocations
	sum_amp = (float *)calloc(nx*ny, sizeof(float));
	if (!sum_amp) perror("Insufficient memory!");

	amp = (float *)calloc(nx*ny, sizeof(float));
	if (!amp) perror("Insufficient memory!");

	
	// for each line
	printf("Creating mean amplitude from files...\n"); //info
	for (ii=0;ii<kk;ii++)
	{
	
		fgets(line,sizeof(line),fp);
		// remove newline
		if ((p = strchr(line, '\n')) != NULL) *p = '\0';

		//read headerfile
		read_header(line);

		// create amplitude filename to open
		strcpy(amp_filename, path);
		strcat(amp_filename, "amp_");
		strcat(amp_filename, filename);
		strcat(amp_filename, ".bin");


		//try fopen amp_in
		if (!(amp_in = fopen(amp_filename, "rb")))
		{
			printf("\nAmplitude file not found!\n");
			exit(0);
		}
		
		// read data
		fread( (float *)amp, sizeof(float), nx*ny, amp_in);
		
		// accumulate amplitudes
		for (i=0;i<nx*ny;i++)
		{
			sum_amp[i] += amp[i];
			printf("i = %d, sum_amp = %f\n",i, sum_amp[i]); //trace
		}

		fclose(amp_in);

	}//for
	fclose(fp);

If I put the calloc allocations inside the for loop, it seems to work (numbers dont get gigantic at least) but it doesnt solve my problem since I want sum up multiple vectors, so I dont want to reset sum_amp to 0 next iteration.

OK, managed to get it to work but I am puzzled. If I put my calloc memory allocations inside the for loop instead of before the loop it works. Like this:

Code:

		amp = (float *)calloc(nx*ny, sizeof(float));
		if (!amp) perror("Insufficient memory!");
		
		if (ii==0)
		{
			sum_amp = (float *)calloc(nx*ny, sizeof(float));
			if (!sum_amp) perror("Insufficient memory!");
		}

Why is this different?

A difference is that you allocate with each iteration memory again, which isn't of course good for performance

Code:

fread( (float *)amp, sizeof(float), nx*ny, amp_in);

why the casting? amp is already float *

Thanks for your replies! I realize now that my values nx and ny are read from the header which hasnt been read before the loop. I am tired. Thanks for the casting info aswell.

As a final note, you might want to consider realloc(), since you want to change the memory you have according to nx, ny.

BUT, I understand that you want one sum_amp that adds the other amps. So what values of nx, ny are you going to use for that? You have to use the maximum values of nx, ny in order to have what you want, correct? I
f that is true than you should allocate only ONCE amp with also the maximum values. Allocating takes time, don't do it if not necessary.

Do you initialize sum_amp? If it is the sum then you should initialize it to 0, or random numbers automatically initialized will be given giving you gigantic results (it might be initialized at 2143244 for you).
You can use a loop for that or memset() is even better.