Position in file of a particular value - c

Question

I have a C binary file with 5123 values in the form of a 3-dimensional cube. I need to access the position in the cube with the highest value, which is the density. Once I have the position of the highest value, I need to create a smaller 3D cube about this position with values that are obviously smaller than 512 (the dimension of the cube). start represents the position at one corner of the smaller cube. p is the binary file obviously.
fseek(p,0,SEEK_END);
lSize = ftell(p);
rewind(p);
dim = pow(lSize/sizeof(float),1.0/3.0);
printf("File size: %lu bytes, Grid size: %d \n", lSize,(int)dim);
max = 0;
counter = 0;
index = 0;
while(fread(&density,sizeof(float),1,p),!feof(p) && !ferror(p)) {
if(density > max) max = density,index = counter;
counter += 1;
}
sub = 256;
start = index - (pow(dim,2)+dim+1)*(sub/2-1);
printf("3d coordinates of highest density: %lu,%lu,%lu, Dimension of cube: %d\n",index % 512;(index / 512) % 512;index / (512 * 512),(int)dim);
printf("The maximum density is: %e with index: %lu \n", max,index);
rewind(p);
fseek(p,start*sizeof(float),SEEK_SET);
fseek(q,start*sizeof(float),SEEK_SET);
fseek(r,start*sizeof(float),SEEK_SET);
fseek(s,start*sizeof(float),SEEK_SET);
fseek(t,start*sizeof(float),SEEK_SET);
u = fopen("results_dens.dat", "w");
if (u == NULL) { printf("Unable to open output results file!"); exit(1); }
for (ibox=0;ibox<nbox;ibox++){
for (k=0;k<nz[ibox];k++){
fseek(p,(start+k*dim*dim)*sizeof(float),SEEK_SET);
fseek(q,(start+k*dim*dim)*sizeof(float),SEEK_SET);
fseek(r,(start+k*dim*dim)*sizeof(float),SEEK_SET);
fseek(s,(start+k*dim*dim)*sizeof(float),SEEK_SET);
fseek(t,(start+k*dim*dim)*sizeof(float),SEEK_SET);
for (j=0;j<ny[ibox];j++){
fseek(p,(start+j*dim+k*dim*dim)*sizeof(float),SEEK_SET);
fseek(q,(start+j*dim+k*dim*dim)*sizeof(float),SEEK_SET);
fseek(r,(start+j*dim+k*dim*dim)*sizeof(float),SEEK_SET);
fseek(s,(start+j*dim+k*dim*dim)*sizeof(float),SEEK_SET);
fseek(t,(start+j*dim+k*dim*dim)*sizeof(float),SEEK_SET);
for (i=0;i<nx[ibox];i++){
I am aware that the above code runs without any errors. However, a lot rides on the value of index above. I am unsure of how positions are defined in C. I am aware that these are memory locations but by doing some rough calculations, the value of index that I derive seems to be close to the edge of the box and not the centre.
5123 = 134217728.
The value of index is 66978048, 130816 positions from the middle position value of 67108864. But, 130816 is approximately 512*256 meaning that if the middle position of the grid is at the edge of the box, then so is index above.

Answer 1

Perhaps this will help. First, I created a test file with the following program:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int makeCube(const char *fn, int dim)
{
FILE *p;
const int center = dim/2;
p = fopen(fn,"w");
for (int i=0; i < dim; ++i)
for (int j=0; j < dim; ++j)
for (int k=0; k < dim; ++k) {
float f = dim - sqrtf(pow(i-center,2)+
pow(j-center,2)+pow(k-center,2));
fwrite(&f, sizeof(float), 1, p);
}
fclose(p);
return 0;
}
int main()
{
const int dim = 512;
makeCube("cube.bin", dim);
return 0;
}
Next I rewrote your code to have correct syntax and to print some diagnostics which seemed useful:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int subCube(FILE *p, int dim)
{
float density;
float max = 0;
long index = 0;
long counter;
for (counter=0; fread(&density,sizeof(float),1,p); ++counter) {
if(density > max) {
max = density;
index = counter;
}
}
printf("The maximum density is: %e with index: %lu \n", max,index);
int i = index/dim/dim;
int j = (index - (i*dim*dim))/dim;
int k = (index - ((i*dim)+j)*dim);
printf("This corresponds to coordinates (%d,%d,%d)\n", i,j,k);
}
int main()
{
const int dim = 512;
FILE *p = fopen("cube.bin","r");
subCube(p, dim);
fclose(p);
return 0;
}
When I run that program, I get the following output:
The maximum density is: 5.120000e+02 with index: 67240192
This corresponds to coordinates (256,256,256)
Since the test data was basically a sphere with greatest density at the middle, this is exactly as expected.
Note that I have played fast and loose with error handling (there isn't any!) but it's omitted for clarity and not because you should actually omit it in a real program.