Read binary file byte by byte

Question

I have this weird assignment in my C++ class where I must write a certain number in a binary file (132.147), using the float type, and then read it, using the char type, in such a way that the final result would be the decimal value of each byte (-94, 37, 4 and 67).

fstream binFile("blah.bin", ios::binary|ios::in|ios::out);
float a = 132.147;
binFile.write((char*)&a, sizeof(float));
char b[4];
binFile.read((char*)&b, sizeof(b));
cout << (int)b[0] << ' ' << (int)b[1] << ' ' << (int)b[2] << ' ' << (int)b[3] << endl; // -51 -51 -51 -51
cout << b[0] << ' ' << b[1] << ' ' << b[2] << ' ' << b[3]; // = = = =
binFile.close();
return 0;

I understand where they got those 4 numbers from. If I write the file and read it using an hex editor, I get 4 hexadecimal numbers that, once converted to a signed binary can then be converted to their decimal form. However, I have absolutely no clue how I can programatically do this in C++. Any clue?

Thanks !

you're almost there. You closed the file, so you must re-open it to read it again. You also want to read 4 bytes, not 10 (sizeof(b)==10 because you made it 10 elements). Note that when you write a float, use sizeof(float) not sizeof(int) — Adam
– Adam, Commented Nov 22, 2013 at 4:02

Tony Delroy · Accepted Answer · 2013-11-22 04:02:44Z

1

As Adam says you must reopen the file. You might have noticed it wasn't reading if you'd checked the return value from read. You only need to read sizoef a bytes, but asking to read more is harmless. After you read b...

cout << (int)b[0] << ' ' << ...

answered Nov 22, 2013 at 4:02

Tony Delroy

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2 Comments

user3020233 Over a year ago

Thanks for your replies, Adam and Tony. I've edited the piece of code in my initial question with your suggestions. I actually ran it this time and thrown in the outputs. I still don't get it -- both in terms of coding and understanding what's actually going on. What are supposed to represent the first 4 characters I'm reading here? Thanks !

user3020233 Over a year ago

I'm so dumb. I got it now. Thanks a bunch !

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