How to read/write float values from a binary file in python, if the file was created with C

Question

I want to read/write C float values from a binary file if it was created in C?

The file was created like this:

#include <stdio.h>

int main() {
    const int NUMBEROFTESTELEMENTS = 10;
    /* Create the file */
    float x = 1.1;
    FILE *fh = fopen ("file.bin", "wb");
    if (fh != NULL) {
       for (int i = 0; i < NUMBEROFTESTELEMENTS; ++i)
       {
            x = 1.1*i;
            fwrite (&x,1, sizeof (x), fh);
            printf("%fn", x);
       }
        fclose (fh);
    }

    return 0;
}

I found a method like this:

file=open("array.bin","rb")
number=list(file.read(3))
print (number)
file.close()

But this won’t guarantee to me the read value was a C float.

Asked By: gabor aron

||

Source

Answer 1

import struct
with open("array.bin","rb") as file:
    numbers = struct.unpack('f'*10, file.read(4*10))
print (numbers)

This should do the job. numbers is a tuple of the 10 values.

Answered By: mch

Answer 2

If you care about performance, I would suggest using numpy.fromfile for float values reading:

import numpy as np

class FloatReader:
    def __init__(self, filename):
        self.f = open(filename, "rb")
    
    def read_floats(self, count : int):
        return np.fromfile(self.f, dtype=np.float32, count=count, sep='')

This approach is much faster than struct.unpack in terms of performance!

Answered By: Anatoly

How to read/write float values from a binary file in python, if the file was created with C

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