Reading values into 3D array in C++

Question

I'm new to C++, so this should be a fairly basic question.

Assume I have bunny.voxel.ply file. This file is written out in binary, with the first 4 bytes corresponding to the (integer) sampling resolution, res, and the next 4 x res x res x res bytes corresponding to the (single precision) floating point values.

I want to read these values into 3D array voxel. My current code:

#include <iostream>
#include <stdio.h>
#include <stdlib.h>


int main() {
    FILE* fp = fopen( "bunny.voxel.ply" , "rb" );
    if (fp==NULL) {fputs ("File error",stderr); exit (1);}
    int res;
    fread( &res , 1 , sizeof(int) , fp );
    float *voxel = new float[res*res*res];
    fread(voxel , res * res * res , sizeof(float) , fp );
    fclose( fp );
    std::cout << "Hello, World!" << std::endl;
    return 0;
}

seems to only read the last value.

Any suggestions on how I can modify this read all values?

I think you have your second and third arguments mixed up in fread? — AndyG
– AndyG, Commented Jul 10, 2017 at 17:57
Why are you using FILE* and fopen when you tagged this as a C++ question? Use some std::fstream stuff. — Charles
– Charles, Commented Jul 10, 2017 at 17:58
And, this code is rather C with std::cout and new instead of C++. — Skym0sh0
– Skym0sh0, Commented Jul 10, 2017 at 17:58
I highly recommend checking the return value of fread. The fread function returns the number of items read. — Thomas Matthews
– Thomas Matthews, Commented Jul 10, 2017 at 18:01
I don't understand the value of printing "Hello World". Why don't you actually print the values you're trying to read? — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jul 10, 2017 at 19:08

milbrandt · Accepted Answer · 2017-07-10 20:00:06Z

1

The Parameters of fread are

size_t fread(void *ptr, size_t size, size_t count, FILE *stream);

Thus you probably should exchange the order of your 2nd and 3rd parameter:

int count = res*res*res;
float *voxel = new float[count];
fread(voxel, sizeof(float), count, fp);

By accident the last value was in the correct memory position so that you could identify it.

answered Jul 10, 2017 at 20:00

milbrandt

1,4952 gold badges16 silver badges20 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

geza Over a year ago

If the read is successful, then there is no difference between size, count and count, size, so the problem definitely is not about this.

Lightness Races in Orbit Over a year ago

@geza: That is not so. There are potential aliasing issues when you get it wrong

geza Over a year ago

@LightnessRacesinOrbit: why? If the read is successful, there will be size*count bytes read, no matter of the order of parameters.

Lightness Races in Orbit Over a year ago

@geza: But you'll be writing 1 byte of a float at a time, instead of all of it. You can't just mess around with bytes in memory willy nilly. Why do you think there are two arguments in the first place?

geza Over a year ago

@LightnessRacesinOrbit: maybe it has historical reasons. I'm sure it has nothing to do with aliasing.

|

Collectives™ on Stack Overflow

Reading values into 3D array in C++

1 Answer 1

6 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Your Answer

Sign up or log in

Post as a guest

Related