copy and modify "string" in []u8 buffer with length

[zenith veldt]

I'm learning Zig, coming from C# and TypeScript, but still have dusty C knowledge from the old days.
To get started, I want to write a small program that changes a few characters in a text file. This is read in line by line, each line being read into the same 1024 character buffer. For conversion I use a second buffer, in which the text line is copied character by character, changing some characters.
I have written the following function for this:

fn convLine(in_line: []const u8, out_line: []u8) !void { for (in_line, 0..) |_, i| { out_line[i] = switch (in_line[i]) { ',' => ';', '.' => ',', else => in_line[i], }; } //outline.len = in_line.len; // doesnt work }

The problem is that out_line.len is always the buffer size. Is the 0 termination byte not copied? Do I think too C-ish?

[wispy light]

The reason you get that error is because each function parameter is essentially declared as if it was like this:

const in_line: []const u8;
const out_line: []u8;

Thus, you cannot mutate the actual values themselves.

If you wanted this function to update the caller's slice that they passed, then you'd have to take *[]u8.
However, a generally less invasive approach is to simply return a usize, the number of bytes/elements that were written.

And then the caller is expected to do const part = my_buffer[0..return_value];.

If this was C, it would let you do that line you wrote, but it still wouldn't update the callsite.

You'd be mutating a function-local copy of the slice value in C, which would be a useless assignment in this case.

[wispy light]

An alternative is to return a subslice.
return out_line[0..in_line.len]; for example.

[wispy light]

And a second alternative: Since you always will write the same number of bytes as the input's length, you could just make that an assumption that the caller can make, and thus they just do:

var out: [4096]u8 = undefined;
const line = ...;
try convLine(line, &out);
const new_line = out[0..line.len];

...or whathaveyou.

[zenith veldt]

I Don't actually get an error, when I print out out_line after invoking convLine(), there are always 1024 characters printed, the actual converted line followed by a lot of garbage.

const in_line: []const u8; const out_line: []u8;
is ok, because buffer addresses are const, just content is not in case of out_line

I think returning a subslice would be best, thank you

[wispy light]

That garbage is because you're asking it to print out the entirety of the buffer, and not just the part that was written to 😉