Terminal control/Positional read

From Rosetta Code
Terminal control/Positional read
You are encouraged to solve this task according to the task description, using any language you may know.

Determine the character displayed on the screen at column 3, row 6 and store that character in a variable. Note that it is permissible to utilize system or language provided methods or system provided facilities, system maintained records or available buffers or system maintained display records to achieve this task, rather than query the terminal directly, if those methods are more usual for the system type or language.


Works with: AutoHotkey_L

AutoHotkey is not built for the command line, so we need call the WinAPI directly.

For fun, this writes random characters to the command window so that it has something to retrieve.

DllCall( "AllocConsole" ) ; create a console if not launched from one
hConsole := DllCall( "GetStdHandle", int, STDOUT := -11 )
Loop 10
Loop 10
Random, asc, % asc("A"), % Asc("Z")
WriteConsole(hConsole, Chr(asc))
WriteConsole(hConsole, "`n")
MsgBox % ReadConsoleOutputCharacter(hConsole, 1, 3, 6)
; === The below simply wraps part of the WinAPI ===
WriteConsole(hConsole, text){
VarSetCapacity(out, 16)
If DllCall( "WriteConsole", UPtr, hConsole, Str, text, UInt, StrLen(text)
, UPtrP, out, uint, 0 )
return out
return 0
ReadConsoleOutputCharacter(hConsole, length, x, y){
VarSetCapacity(out, length * (1 << !!A_IsUnicode))
VarSetCapacity(n, 16)
if DllCall( "ReadConsoleOutputCharacter"
, UPtr, hConsole
, Str, out
, UInt, length
, UInt, x | (y << 16)
, UPtrP, n )
&& VarSetCapacity(out, -1)
return out
return 0


Applesoft BASIC[edit]

 10  DEF  FN C(H) =  SCRN( H - 1,(V - 1) * 2) +  SCRN( H - 1,(V - 1) * 2 + 1) * 16
20 LET V = 6:C$ = CHR$ ( FN C(3))

Locomotive Basic[edit]

10 LOCATE 3,6
20 a$=COPYCHR$(#0)

Amstrad CPC screen memory only stores pixels but no character information (as opposed to e.g. the C64), so the firmware routine (TXT_UNWRITE) called by BASIC works by trying to find a match between screen pixels and the shape of a currently defined character. If the character table or screen pixels in the area of the character are changed between writing and reading, COPYCHR$ will therefore fail.


The top left corner is (1, 1).

c$ = CHR$(SCREEN(6, 3))

ZX Spectrum Basic[edit]

 10 REM The top left corner is at position 0,0
20 REM So we subtract one from the coordinates
30 LET c$ = SCREEN$(5,2)


      PRINT TAB(2,5) "Here"
char$ = GET$(2,5)
PRINT ''"Character at column 3 row 6 was " char$


With the Windows console, call GetConsoleScreenBufferInfo() to find the top-left corner of the display screen. Then add (3, 6) to the top-left corner and call ReadConsoleOutputCharacterW() to read character. This program reckons that the top-left corner is (0, 0).

Library: Win32
#include <windows.h>
#include <wchar.h>
COORD pos;
HANDLE conout;
long len;
wchar_t c;
/* Create a handle to the console screen. */
0, NULL);
return 1;
/* Where is the display window? */
if (GetConsoleScreenBufferInfo(conout, &info) == 0)
return 1;
/* c = character at position. */
pos.X = info.srWindow.Left + 3; /* Column */
pos.Y = info.srWindow.Top + 6; /* Row */
if (ReadConsoleOutputCharacterW(conout, &c, 1, pos, &len) == 0 ||
len <= 0)
return 1;
wprintf(L"Character at (3, 6) had been '%lc'\n", c);
return 0;


This is based on the C entry and works on Windows 10:

// Kotlin Native version 0.3
import kotlinx.cinterop.*
import win32.*
fun main(args: Array<String>) {
for (i in 0 until (80 * 25)) print("A") // fill 80 x 25 console with 'A's
memScoped {
val conOut = GetStdHandle(-11)
val info = alloc<CONSOLE_SCREEN_BUFFER_INFO>()
val pos = alloc<COORD>()
GetConsoleScreenBufferInfo(conOut, info.ptr)
pos.X = (info.srWindow.Left + 3).toShort() // column number 3 of display window
pos.Y = (info.srWindow.Top + 6).toShort() // row number 6 of display window
val c = alloc<wchar_tVar>()
val len = alloc<IntVar>()
ReadConsoleOutputCharacterW(conOut, c.ptr, 1, pos.readValue(), len.ptr)
if (len.value == 1) {
val ch = c.value.toChar()
println("The character at column 3, row 6 is '$ch'")
else println("Something went wrong!")
The character at column 3, row 6 is 'A'


-- demo\rosetta\Positional_read.exw
-- ================================
position(6,1) -- line 6 column 1 (1-based)
integer {ch,attr} = get_screen_char(6,3)
{} = wait_key()




Works in a CMD box on Windows:

#lang racket
(require ffi/unsafe ffi/unsafe/define)
(define-ffi-definer defwin #f)
(defwin GetStdHandle (_fun _int -> _pointer))
(defwin ReadConsoleOutputCharacterA
(_fun _pointer _pointer _uint _uint [len : (_ptr o _uint)] -> _bool))
(define b (make-bytes 1 32))
(and (ReadConsoleOutputCharacterA (GetStdHandle -11) b 1 #x50002)
(printf "The character at 3x6 is <~a>\n" b))


The REXX doesn't have any cursor or screen management tools, but some REXX interpreters have added the functionality via different methods.

Works with: PC/REXX
Works with: Personal REXX
/*REXX program demonstrates reading a char at  specific screen location.*/
row = 6 /*point to row six. */
col = 3 /*point to column three. */
howMany = 1 /*read one character. */
stuff = scrRead(row, col, howMany) /*this'll do it. */
other = scrRead(40, 6, 1) /*same thing, but for row forty. */
/*stick a fork in it, we're done.*/


;;; Type definitions and constants
(typedef BOOL (enum BOOL FALSE TRUE))
(typedef HANDLE cptr)
(typedef WCHAR wchar)
(typedef DWORD uint32)
(typedef WORD uint16)
(typedef SHORT short)
(typedef COORD
(struct COORD
(typedef SMALL_RECT
(struct SMALL_RECT
(Left SHORT)
(Right SHORT)
(Bottom SHORT)))
(dwSize COORD)
(dwCursorPosition COORD)
(wAttributes WORD)
(srWindow SMALL_RECT)
(dwMaximumWindowSize COORD)))
;;; Various constants
(defvarl STD_INPUT_HANDLE (- #x100000000 10))
(defvarl STD_OUTPUT_HANDLE (- #x100000000 11))
(defvarl STD_ERROR_HANDLE (- #x100000000 12))
(defvarl NULL cptr-null)
(defvarl INVALID_HANDLE_VALUE (cptr-int -1))
;;; Foreign Function Bindings
(with-dyn-lib "kernel32.dll"
(deffi GetStdHandle "GetStdHandle" HANDLE (DWORD))
(deffi GetConsoleScreenBufferInfo "GetConsoleScreenBufferInfo"
(deffi ReadConsoleOutputCharacter "ReadConsoleOutputCharacterW"
BOOL (HANDLE (ptr-out (array 1 WCHAR))
DWORD COORD (ptr-out (array 1 DWORD)))))
;;; Now the character at <2, 5> -- column 3, row 6.
(let ((console-handle (GetStdHandle STD_OUTPUT_HANDLE)))
(when (equal console-handle INVALID_HANDLE_VALUE)
(error "couldn't get console handle"))
(let* ((cinfo (new CONSOLE_SCREEN_BUFFER_INFO))
(getinfo-ok (GetConsoleScreenBufferInfo console-handle cinfo))
(coord (if getinfo-ok
^#S(COORD X ,(+ 2 cinfo.srWindow.Left)
Y ,(+ 5 cinfo.srWindow.Top))
#S(COORD X 0 Y 0)))
(chars (vector 1))
(nread (vector 1))
(read-ok (ReadConsoleOutputCharacter console-handle chars
1 coord nread)))
(when (eq getinfo-ok 'FALSE)
(error "GetConsoleScreenBufferInfo failed"))
(prinl cinfo)
(when (eq read-ok 'FALSE)
(error "ReadConsoleOutputCharacter failed"))
(unless (plusp [nread 0])
(error "ReadConsoleOutputCharacter read zero characters"))
(format t "character is ~s\n" [chars 0])))


  • An ptr-out to an array of 1 DWORD is used for the number of characters out parameter. The FFI type (ptr-out DWORD) cannot work as a function argument, because integer objects are not mutable, and there isn't any concept of taking the address of a variable. A vector of 1 integer is mutable, and by making such a vector correspond with the FFI type (array 1 DWORD), the necessary semantics is achieved.
  • The quasiquote expression ^#S(COORD X ,(+ 2 cinfo.srWindow.Left) Y ,(+ 5 cinfo.srWindow.Top)) is equivalent to (new COORD X (+ 2 cinfo.srWindow.Left) Y (+ 5 cinfo.srWindow.Top)). It is done this way to demonstrate support for structure quasiquoting.


include c:\cxpl\stdlib;
int C;
[Cursor(3, 6); \move cursor to column 3, row 6 (top left = 0,0)
\Call BIOS interrupt routine to read character (& attribute) at cursor position
C:= CallInt($10, $0800, 0) & $00FF; \mask off attribute, leaving the character