Calculator to convert binary to float value — what am I doing wrong?

Question

I have the following code, which writes 6 floats to disk in binary form and reads them back:

#include <iostream>
#include <cstdio>

int main()
{
  int numSegs = 2;
  int numVars = 3;

  float * data = new float[numSegs * numVars];
  for (int i = 0; i < numVars * numSegs; ++i) {
    data[i] = i * .23;
    std::cout << data[i] << std::endl;
  }

  FILE * handle = std::fopen("./sandbox.out", "wb");

  long elementsWritten = 
    std::fwrite(data, sizeof(float), numVars*numSegs, handle);

    if (elementsWritten != numVars*numSegs){
      std::cout << "Error" << std::endl;
    }

    fclose(handle);
    handle = fopen("./sandbox.out", "rb");

    float * read = new float[numSegs * numVars];

    fseek(handle, 0, SEEK_SET);

    fread(read, sizeof(float), numSegs*numVars, handle);

    for (int i = 0; i < numVars * numSegs; ++i) {
      std::cout << read[i] << std::endl;
    }
}

It outputs:

0
0.23
0.46
0.69
0.92
1.15
0
0.23
0.46
0.69
0.92
1.15

When I load the file in a hexer, we get:

00 00 00 00 1f 85 6b 3e  1f 85 eb 3e d7 a3 30 3f
1f 85 6b 3f 33 33 93 3f  -- -- -- -- -- -- -- --

I want to be calculate the float value from the decimal directly. For example: 1f 85 6b 3e becomes 0.23 and 1f 85 eb 3e becomes 0.46.

I've tried a few "binary to float" calculators on the web. When I put in the hexadecimal representation of the number, 0x1f856b3e, in both calculators I get back 5.650511E-20 . But I thought the value should be 0.23 since I provided bytes 5-8 to the calculator and these bytes represent the second float written to disk.

What am I doing wrong?

Answer 1

This is an endianness issue if you for example switch:

1f 85 6b 3e 

to:

3e 6b 85 1f

this will result in .23 when you convert it using one of your converters, for example I used IEEE 754 Converter and Floating Point to Hex Converter allows you to do double as well as single precision conversions.