Width as a variable when using fscanf [duplicate]

Answer 1

Unfortunately, no, there's no modifier like '*' for printf that causes scanf to get its field width or precision from a variable. The closest you can come is dynamically creating the format string:

char format[8];
sprintf(format, "%%%dX", width);
fscanf(in_file, format, &iData);

Answer 2

I think the simplest would be to use fread() like this:

fread(buffer, nbytes, sizeof(char), in_file);

Answer 3

fscanf with %X will stop at a newline automatically, right? If the fields really are newline-terminated (as in your example), then can't you just call

fscanf(in_file, "%X", &iData);

and let fscanf figure out where the end is?

Answer 4

If you really want to be able to adjust the fscanf format programmatically, you could try stack-allocating a string with enough space, and then generating the format like so: e.g.

char formatString[100];

// writes "%max_size[0-9]", substituting max_size with the proper digits
sprintf(formatString, "%%%d[0-9]", MAX_SIZE); 

fscanf(fp, formatString, buffer); // etc...

Answer 5

You might also consider using C++ streams.

#include <ifstream>
#include <iostream>

// open the file and create a file input stream
ifstream file("test.txt" , ios::in | ios::binary);

// loop through the whole file
while (ifs.good())
{
    // extract one byte as the field width
    unsigned char width;
    file.read(&width, 1);

    // extract width number of unformatted bytes
    char * bytes = new char[width];
    file.read(bytes, width);

    // process the bytes
    ...
    delete [] bytes;

    // skip EOL characters if needed
    // file.seekg(1, ios_base::cur)
}

file.close();

A simpler way if the newlines are included as you seem to indicate, would be to use getLine(). Check out http://www.cplusplus.com/reference/iostream/ifstream/ for more ways to use read(), get(), getLine() and lots of other great stream functions.