Confused about mid() command

[johnno56]

Hi Marcus,

I was tinkering with string manipulation and came across a bit of a 'poser' in regards to the mid() command.

Here is a short example....

Code:

' Open a window and enable double buffering.
set window "Test", 800, 632, false
set redraw off

visible a = "ABCDEF"
visible b
visible c

'   display the letters "B" to "E" inclusive...
b = mid(a, 1, 4)
pln b
'   This works fine...

'   But, when I try to make the third letter "D"
'   an "X"...

'   mid(b, 3, 1) = "X"
'   pln b

'   I get a syntax error on line 17 during compile

'(remove the "remarks" from lines 17 and 18
' to see what I mean...)

system "pause"

Defining a string then assigning a variable to a "portion" of the string, using mid(), works fine...
But, trying to replace a character, using mid() causes the compilation to fail with a syntax error.

There is no rush on this one... Whenever you are able... Present situation understood... We will "keep an eye on the store" during your break...

J

[1micha.elok]

Perhaps like this one ... ?

Code:

'Console Mode
a = "ABCDEF"

pln "a                  = "+a
pln

'display the letters "B" to "E" inclusive...
b = mid(a,1,4)
pln "MID(a,1,4)         = "+b

'replace D with X
pln "REPLACE D with X   = "+replace(b,"D","X",2)
pln

system "pause"

[johnno56]

Cool... Worked like a charm... Thank you!

[1micha.elok]

Cool... Worked like a charm... Thank you!

The mid() function may be used as collision detection.
Let's try this one !

Code:

'=====================================================
'PACMAN in N7
'Control Key :
'- movement   WASD
'- exit       ESC
'
'Reference :
'Tutorial on QBasic Game Design by Andre van Wyk
'http://petesqbsite.com/sections/tutorials/tutorials/basicgd.htm
'
'Quick Tips on qbasic to N7
'-locate y,x                 - set caret x,y
'-print                      - write
'-mid$()                     - mid
'======================================================
#win32
set window "Pacman",80,60,false,10
set redraw off

'color definition
black  = [0,0,0]
white  = [255,255,255]
yellow = [255,255,0]

'initial Pacman's position
x = 40
y = 10

'maze
maze = []
maze[0] = "########"
maze[1] = "#    # #"
maze[2] = "#  #   #"
maze[3] = "# ## # #"
maze[4] = "#    # #"
maze[5] = "########"

do
    set color black ; cls
   
    'maze
    set color white
    for j=0 to 7
        for i = 0 to 5
            set caret j*10,i*10
            write mid(maze[i],j)
        next
    next
   
    'remember Pacman's position
    posX = x
    posY = y
       
    'control
    if keydown(KEY_W) and y>1   then y=y-10
    if keydown(KEY_S) and y<60  then y=y+10
    if keydown(KEY_A) and x>1   then x=x-10
    if keydown(KEY_D) and x<80  then x=x+10
   
    'collision detection Pacman and Maze
    if mid(maze[y/10],x/10)="#" then
        x = posX
        y = posY
    endif
   
    'Pacman
    set color yellow
    set caret x,y ; write "C"
   
    redraw
    fwait 10
until keydown(KEY_ESCAPE,true)

[johnno56]

Cool... Thanks for the demo... What? No ghosts? lol

Chunk text. Very retro...

[Marcus]

Sorry, you can't really modify strings, only construct new ones.

[johnno56]

hmm... Interesting response... I am receiving mixed messages... On one hand, "can't" (cannot) implies a "No"... But, "can't really" implies a measure of doubt and conveys a possible interpretation of... "No. But I dare you to try". I enjoy a good puzzle, but when it comes to coding, I find myself at the shallow end of the "smart" pool... lol

Not a problem... Time for new train of thought... The only problem is that I do not have a ticket... Boom Boom.... lol

Thank you for responding...

[Marcus]

hmm... Interesting response... I am receiving mixed messages... On one hand, "can't" (cannot) implies a "No"... But, "can't really" implies a measure of doubt and conveys a possible interpretation of... "No. But I dare you to try". I enjoy a good puzzle, but when it comes to coding, I find myself at the shallow end of the "smart" pool... lol

Not a problem... Time for new train of thought... The only problem is that I do not have a ticket... Boom Boom.... lol

Thank you for responding...

Haha

Strings are like numbers. You can't change a string without getting a new string. That's why mid/left/right/replace all return new strings. It doesn't really matter. There's no simple way of replacing a character at a certain index in a string with another character. If you know what character it is, you could probably use 'replace' to get a new string:

Code:

foo = "ABCDEF"
pln "foo = " + foo

foo = replace(foo, "C", "Z")
pln "foo = " + foo

system "pause"

Output:
foo = ABCDEF
foo = ABZDEF