added exercise/106_files.zig

.gitignore

@@ -2,3 +2,4 @@
 /zig-out/
 /answers/
 /patches/healed/
+/output/

exercises/106_files.zig

@@ -0,0 +1,93 @@
+//
+// Until now, we've only been printing our output in the console,
+// which is good enough for fighting alien and hermit bookkeeping.
+//
+// However, many other task require some interaction with the file system,
+// which is the underlying structure for organizing files on your computer.
+//
+// The File System provide a hierarchical structure for storing files
+// by organizing files into directories, which hold files and other directories,
+// thus creating a tree structure for navigating.
+// 
+// Fortunately, zig standard library provide a simple api for interacting
+// with the file system, see the detail documentation here
+//
+// https://ziglang.org/documentation/master/std/#std.fs
+//
+// In this exercise, we'll try to 
+//   - create a new directory
+//   - open a file in the directory
+//   - write to the file.
+//
+// import std as always
+const std = @import("std");
+
+pub fn main() !void {
+    // first we get the current working directory
+    const cwd: std.fs.Dir = std.fs.cwd();
+
+    // then we'll try to make a new directory /output/
+    // to put our output files.
+    cwd.makeDir("output") catch |e| switch (e) {
+        // there are chance you might want to run this
+        // program more than once and the path might already
+        // been created, so we'll have to handle this error
+        // by doing nothing
+        //
+        // we want to catch error.PathAlreadyExists and do nothing
+        ??? => {
+            ???
+        },
+        // if is any other unexpected error we just propagate it through
+        else => return e,
+    };
+
+    // then we'll try to open our freshly created directory
+    // wait a minute
+    // opening a directory might fail!
+    // what should we do here?
+    const output_dir: std.fs.Dir = cwd.openDir("output", .{});
+
+    // we try to open the file `zigling.txt`,
+    // and propagate the error up if there are any errors
+    const file: std.fs.File = try output_dir.createFile("zigling.txt", .{});
+    // it is a good habit to close a file after you are done with
+    // so that other program can read it and prevent data corruption
+    // but here we are not yet done writing to the file
+    // if only there are a keyword in zig that
+    // allow you "defer" code execute to the end of scope...
+    file.close();
+
+    // !you are not allow to switch this two lines to before file closing line!
+    const byte_written = try file.write("It's zigling time!");
+    std.debug.print("succefully wrote {} bytes.\n", .{byte_written});
+}
+// to check if you actually write to the file, you can either,
+// 1. open the file on your text editor, or
+// 2. print the content of the file in the console with command
+//    >> cat ./output/zigling.txt
+//
+// 
+// More on Creating files
+// 
+// notice in:
+// ... try output_dir.createFile("zigling.txt", .{});
+//                                              ^^^
+//                 we passed this anonymous struct to the function call
+//                 
+// this is the struct `CreateFlag` with default fields
+// {
+//      read: bool = false,
+//      truncate: bool = true,
+//      exclusive: bool = false,
+//      lock: Lock = .none,
+//      lock_nonblocking: bool = false,
+//      mode: Mode = default_mode
+// }
+// 
+// Question:
+//   - what should you do if you want to also read the file after opening it?
+//   - go to documentation of the struct `std.fs.Dir` here
+//     https://ziglang.org/documentation/master/std/#std.fs.Dir
+//       - can you find a function for opening a file? how about deleting a file?
+//       - what kind of option can you uses with those function?