Problems saving float array to binary file and reading back (C++)

Question

I am trying to simply write an array of float values to a file and then read it back.
I have tried writing it directly from an array, but when reading it back I keep hitting a problem for arrays with length greater than 153. The code example writes each float value one by one for clarity.

For values with index greater than or equal to 153 they have the value 153.0, where they should be 153.0, 154.0, 155.0, ...

Why doesn't this code work for me?

  int length = 160;

  char* fileName = "testFile.dat";

  // Write data to file

  FILE* file = fopen (fileName, "w");

  for(int i = 0; i< length; i++){
    // We are just storing the indices, so value at i is equal to i
    float f = (float) i;
    fwrite(&f, sizeof(float), 1, file);
  }

  fclose(file);

  // Read data from file into results array

  file = fopen(fileName, "r");

  float* results = new float[length];

  for(int i = 0; i< length; i++){
    float f;
    fread(&f, sizeof(float), 1, file);
    results[i] = f;
  }

  fclose(file);

  // Now check data in results array

  bool fail = false;

  for(int i = 0; i< length; i++){
    if(results[i]!=(float)i){
      fail = true; // This should not be hit, but it is!
    }
  }

  delete [] results;

Thanks, Dave

There is no C++ to be seen here. This is all C. Please do not confuse those. If you post this as a C++ question, you will get comments about how you should use vector and iostream. — Björn Pollex
– Björn Pollex, Commented Jan 13, 2011 at 13:22
You must specify that you're writing to and reading from a binary file. Check this link: cplusplus.com/reference/clibrary/cstdio/fopen If your don't do so, fread will stop reading when it hits an EOF characters, which is quite possible, since you're writing some random floats. — LavaScornedOven
– LavaScornedOven, Commented Jan 13, 2011 at 13:22
@Space_COwbOy what about the delete [] results; line at the end? :-) — Pointy
– Pointy, Commented Jan 13, 2011 at 13:22
He's newing memory so it's obviously compiled with a C++ compiler — Andreas Brinck
– Andreas Brinck, Commented Jan 13, 2011 at 13:23

Andreas Brinck · Accepted Answer · 2011-01-13 13:23:35Z

4

FILE* file = fopen (fileName, "wb");
FILE* file = fopen (fileName, "rb");

answered Jan 13, 2011 at 13:23

Andreas Brinck

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