Changing letters

[Lanzaa]

Well I don't think I'm going to get into peek and stuff for a while so i made a very user friendly version of the encoder and decoder which changes "aaaaaaaaaa" into "bcdefghijk" and asks you if you want it decoded so here

Code:

start: CLS
INPUT "Encode or decode"; f$
g = 0
CLS
INPUT "Input sentence"; a$
code: CLS
PRINT a$
FOR letter = 1 TO LEN(a$)
IF f$ = "decode" THEN number = (26 - (letter MOD 26))
IF f$ = "encode" THEN number = (letter MOD 26)
b = ASC(MID$(a$, letter, 1))
SELECT CASE b
    CASE 65 TO 90
        b = (((b - 65) + number) MOD 26) + 65
    CASE 97 TO 122
        b = (((b - 97) + number) MOD 26) + 97
END SELECT
MID$(a$, letter, 1) = CHR$(b)
NEXT letter
PRINT a$
PRINT
IF f$ = "encode" THEN INPUT "Do you want to decode? y / n "; f$
IF f$ = "y" THEN
    f$ = "decode"
    GOTO code
END IF
PRINT
INPUT "Again? y / n "; f$
IF f$ = "y" THEN GOTO start