How to save a 2-Dimensional array to a file in C?

Question

I am an beginner C programmer and I am currently working on a project to implement viola jones object detection algorithm using C. I would like to know how I would be able to store data in a 2-Dimensional array to a file that can be easily ported and accessed by different program files(e.g. main.c, header_file.h etc.)

Thank you in advance.

Answer 1

There's not quite enough detail to be sure what you're looking for, but the basic structure of what you want to do is going to look something like this:

open file.csv for writing
for(iterate through one dimension of the array using i)
{
for(iterate through the other dimension of the array using j)
   { 
   fprintf(yourfilehandle,"%d,",yourvalue[i][j]);
   }
fprintf(yourfilehandle,"\n");
}
close your file

As has been suggested by others, this will leave you with a .CSV file, which is a pretty good choice, as it's easy to read in and parse, and you can open your file in Notepad or Excel and view it no problems.

This is assuming you really meant to do this with C file I/O, which is a perfectly valid way of doing things, some just feel it's a bit dated.

Note this leaves an extraneous comma at the end of the line. If that bugs you it's easy enough to do the pre and post conditions to only get commas where you want. Hint: it involves printing the comma before the entry inside the second for loop, reducing the number of entries you iterate over for the interior for loop, and printing out the first and last case of each row special, immediately before and after the inner for loop, respectively. Harder to explain that to do, probably.

Here is a reference for C-style file I/O, and here is a tutorial.

Answer 2

Without knowing anything about what type of data you're storing, I would say to store this as a matrix. You'll need to choose a delimiter to separate your elements (tab or space are common choices, aka 'tsv' and 'csv', respectively) and then something to mark the end of a row (new line is a good choice here).

So your saved file might look something like:

10 162 1 5
7 1 4 12
9 2 2 0

You can also define your format as having some metadata in the first line -- the number of rows and columns may be useful if you want to pre-allocate memory, along with other information like character encoding. Start simple and add as necessary!