\NOT A BUG/: File I/O functions / Display -- Nevermind.

[monkey0506]

Trying to display multiple characters back from a file at once, I discovered what would appear to be a bug. I did this:

Code: ags

File f = File.Open("file.txt", eFileWrite);
f.WriteRawChar('x');
f.WriteRawChar('y');
f.WriteRawChar('z');
f.Close();
f = File.Open("file.txt", eFileRead);
Display("read back: %c, %c, %c", f.ReadRawChar(), f.ReadRawChar(), f.ReadRawChar());
f.Close();

The expected output of course being:

read back: x, y, z

However the actual output I got was this:

read back: z, y, x

Splitting the single Display statement into three gave me the output I expected, printing out the letters in the desired order.

If anyone could shed any light on why this behaviour takes place it would be greatly appreciated. Until then I can simply split up my output.

[Scorpiorus]

The thing is that the parameters to pass into Display is evaluated from right to left, so the last f.ReadRawChar gets called first which returns "x"; next "y", and finally "z":

Display("read back: %c, %c, %c", f.ReadRawChar(), f.ReadRawChar(), f.ReadRawChar());

[monkey0506]

I didn't know that Display's parameters were evaluated right to left. :o Is this documented somewhere? Or just one of those things that only "t3h 3L173" know about?

[Pumaman]

It's nothing special about the Display command, it's just the standard way that function calls are done. The parameters to all function calls are evaluated right-to-left, so that's the reason for the effect you are seeing.

[monkey0506]

I had no idea...well...nevermind then I guess.

[Monsieur OUXX]

The parameters to all function calls are evaluated right-to-left

Even with String.Format(...) ?
The output above would have been the same with the code below?

Code: ags

Display(String.Format("read back : %c %c %c", f.ReadRawChar(), f.ReadRawChar(), f.ReadRawChar()));

[Gilbert]

Yes I think it would be the same, as the rule should apply to arbitrary function calls (like blah(f.ReadRawChar(), f.RawReadChar()) ). So the most secure method is to use something like:

Code: ags

a=f.ReadRawChar();
b=f.ReadRawChar();
c=f.ReadRawChar();
Display(String.Format("read back : %c %c %c", a, b, c);

[Scorpiorus]

Yeah it's all about the order in which function parameters are pushed into a stack -- last parameter goes first, so it must be evaluated first as well.

Code: ags

function test( int, int, int )
{

}

...

test( Display("1"), Display("2"), Display("3") );

...and the output will be: 3, 2, 1

This is known as a "cdecl" calling convention used by default in many "C" compilers, which besides that order also specifies the rules on how a stack memory is freed -- which makes it possible to support variable number of arguments passed into a function (eg: Display(string, ...) ).