Detecting Arrow Key Presses in C / C++

BooKA. · 04-04-2004, 09:24 AM

Hi,
I am currently writing a program with a buffered input, when you hit return the last command is stored in a linked list and you can scroll up and down through your history.

A duplicate of the shell history in linux. Unfortunately I am unable to get my program to recognise a key press by the up and down arrows.

I have tried a few things, no success so far, was wondering if anyone had any suggestions?!

Thanks

Mega Man X · 04-04-2004, 10:20 AM

Hi BooKA!!!

Well, working with arrow keys is difficult. That's why some editors, as vi(m), don't use (it can use, but not recommended) arrow keys, because arrow keys use a sequence of ASCII codes and it's hardware related. At the machine I'm now, the up arrow key sends <Esc>-O-A in sequence.

The way to do it with other keys is using getchar from stdin... Back on Windows days, I was used to use the header conio.h and the function getch() to track user's keys.

To sum up, there's no ASCII code for arrow keys, because ASCII comes from teletypes ages, where there was no arrow keys

What you could do is to use a third part library like curses (function wgetch() I think) to use arrow keys.

Good luck!

Mega Man X · 04-04-2004, 10:24 AM

Oh yeah, currently I'm trying to learn how to use pygame, and there's a function called pygame.event.Event() that you can keep track of all keys. Most likely, SDL must have a similar function, since pygame is basically a SDL wrap. So I really believe that the only way of using arrow keys is with another library, as SDL, curses, pygame...

Regards!

Hko · 04-04-2004, 04:26 PM

You could also use the readline lib. It does all the history stuff for you. IIRC bash itself uses it.

krajzega · 04-04-2004, 04:57 PM

Hmm...you can use ncurses library:
#include <ncurses.h>

...
window *win;
initscr(); //remember about errors support

win=newwin(0,0,10,10);
keypad(win,1); //sets "using-keypad" to true
key=getch();

Arrow key's code is defined as:
KEY_UP, KEY_DOWN, KEY_LEFT, KEY_RIGHT

Good luck

BooKA. · 04-05-2004, 07:58 AM

I have a solution using <ncurses.h> but I can't use it as this is for a project and it would mean using ncurses for the whole thing

Is there a way not using ncurses

Quote:

#include<ncurses.h>

int main()
{
int ch;

/* Curses Initialisations */
initscr();
raw();
keypad(stdscr, TRUE);
noecho();

printw("Welcome - Press # to Exit\n");

while((ch = getch()) != '#')
{
switch(ch)
{
case KEY_UP: printw("\nUp Arrow");
break;
case KEY_DOWN: printw("\nDown Arrow");
break;
case KEY_LEFT: printw("\nLeft Arrow");
break;
case KEY_RIGHT: printw("\nRight Arrow");
break;
default:
{
printw("\nThe pressed key is ");
attron(A_BOLD);
printw("%c", ch);
attroff(A_BOLD);
}
}
}

printw("\n\nBye Now!\n");

refresh();
getch();
endwin();

return 0;
}

Thanks

korsuas · 04-07-2004, 01:18 AM

well, don't have a c compiler available right now, but I'll try to put it as correctly as I can.

you need the scancodes for VK_UP, VK_DOWNm etc.

I can tell you the they are about 72, 85, 70 smtg.

anyhowm to find them just

while (c=getch()!='\r') //enter or whatever
{
printf ("The char %c has the ASCII code %d\n", c, c);
}

Now, you will easily get the scancodes for all the keys.

One thing though, if you don't program right, you may have a duplicate for - I cannot remember right - lets say VK_UP the same as CTRL+M.

you have to:

c=getch();
if (c == 0)
{c = getch();
//you have an extended key - up, down, left, right
switch (c)
{
case 72: //code for up //probably
}
}
else
{
//code for normal chars like a, A, b, B, etc
}

Hope this help,
Regards,

Adrian Korsuas
korsuas@yahoo.com

Mega Man X · 04-07-2004, 02:51 AM

Here, I just did it in Free Borland:

Code:

#include<iostream>
#include<conio.h>
using namespace std;

int main(){
    int key;                 // A simple integer to keep track of ascii code
    system("cls");
  
    while (key != 27){  // ASCII for Esc key
	key = getch();
        cout << "The key you've pressed is:" << key << endl;
        system("cls");  // clean the screen
    }

    return 0;
}

My results are:

Up key = 72
Left key = 75
Right key = 77
Down key = 80

So, as you can see, my keys returns a different ASCII code then korsuas, so what I said before stands, arrow keys are hardware related (I'm running a swedish 105 keyboard). The only way to grab the keys correctly is with a third part library or using ordinary keys (a, b , c, numbers, etc).

Also note that A key returns a different value than a key (65 and 97 respectively), so when using key grabs on your program, make sure to use functions to Lower or Upper case to make it accurate. You can always check ASCII code here:

http://www.asciitable.com/

BooKA. · 04-07-2004, 06:10 AM

Thanks Guys,

Will take your advice and check out an alternative set of keys to use.

If I get any improvements I will let you know.

Thanks Again!