Pascal - read/readln function impurities?

Question

I'm really interested - why do you need to put

readln;

line after reading some value from keyboard into variable? For example,

repeat
writeln('Make your choise');
read(CH);

if (CH = '1') then begin
writeln('1');
end;

{ ... }
until CH = 'q';

If you run the following code, and press '1' on keyboard, you get an output like

1
Make your choise
Make your choise
Make your choise

On the other hand, if you add that "readln;" line, it all works perfect:

repeat
writeln('Make your choise');
read(CH);
readln;

if (CH = '1') then begin
    Writeln('1');
end

until CH = 'q';

My only guess is that calling readln without arguments terminates the process of reading the keyboard input. But if so, why cant the read/readln functions stop reading input themselves, to avoid such a clumsiness?

Sdaz MacSkibbons Sdaz MacSkibbons · Accepted Answer · 2011-04-17T11:43:00

read reads a character, but still expects Enter to be pressed before returning, even though it only returns 1 character. You're pressing Enter after 1, and the console is stuffing the keyboard buffer with a CR (ASCII 0xd) and a LF (ASCII 0xa). (On a Linux/UNIX system, Make your choise will only appears twice, because UNIX uses only a LF as its linefeed character). You can see these by printing the values of the character received with ord(CH) (iirc).

The second program is pulling the CRLF combo out of the keyboard buffer with the subsequent readln and discarding it, so it behaves in the way you seem to want.

If this is a throwaway program, just do it via readln, and go on to solve whatever more important problem you're working on. If it's destined for production, build up an input function via a loop of some sort around readkey (which returns immediately after one character).

Thanks for the nostalgia.

Pascal - read/readln function impurities?

1 Answers