Source code in the basic programming language

dim card$(5)
card$[1]="49927398716"
card$[2]="49927398717"
card$[3]="1234567812345678"
card$[4]="1234567812345670"

for test = 1 to 4
    odd = True
    sum = 0
    for n = length(card$[test]) to 1 step -1
        num = int(mid(card$[test],n,1))
        if odd then
            sum += num
            odd = False
        else
            num *= 2
            if num <= 9 then
                sum += num
            else
                sum += int(left(string(num),1)) + int(right(string(num),1))
            end if
            odd = True
        end if
    next
    if sum mod 10 = 0 then
        print card$[test], "True"
    else
        print card$[test], "False"
    end if
next test

100 cls
110 rem Luhn test
120 dim card$(5)
130 card$(1) = "49927398716"
140 card$(2) = "49927398717"
150 card$(3) = "1234567812345678"
160 card$(4) = "1234567812345670"
170 for test = 1 to 4
180   odd = true
190   sum = 0
200   for n = len(card$(test)) to 1 step -1
210     num = val(mid$(card$(test),n,1))
220     if odd then
230       sum = sum+num
240       odd = false
250     else
260       num = num*2
270       if num <= 9 then
280         sum = sum+num
290       else
300         sum = sum+val(left$(str$(num),1))+val(right$(str$(num),1))
310       endif
320       odd = true
330     endif
340   next
350   if sum mod 10 = 0 then
360     print card$(test),"True"
370   else
380     print card$(test),"False"
390   endif
400 next test


100 PROGRAM "CredCard.bas"
110 DO
120   PRINT :PRINT "Credit card number:":INPUT PROMPT ">":CCN$
130   IF CCN$="" THEN EXIT DO
140   IF LUHN(TRIM$(CCN$)) THEN
150     PRINT "Card number is valid."
160   ELSE
170     SET #102:INK 3:PRINT "Card number is invalid.":SET #102:INK 1
180   END IF
190 LOOP
200 DEF LUHN(CCN$)
210   LET L=LEN(CCN$):LET S=0
220   FOR I=1 TO L
230     LET N=VAL(CCN$(L-I+1))
240     IF I BAND 1 THEN
250       LET S=S+N
260     ELSE
270       LET N=N*2:LET S=S+MOD(N,10)+INT(N/10)
280     END IF
290   NEXT
300   LET LUHN=MOD(S,10)=0
310 END DEF
320 DEF TRIM$(S$)
330   LET T$=""
340   FOR I=1 TO LEN(S$)
350     IF S$(I)>CHR$(47) AND S$(I)<CHR$(58) THEN LET T$=T$&S$(I)
360   NEXT
370   LET TRIM$=T$
380 END DEF

CONST True = -1: False = NOT True

DIM card$(5)
card$(1) = "49927398716"
card$(2) = "49927398717"
card$(3) = "1234567812345678"
card$(4) = "1234567812345670"

FOR test = 1 TO 4
    odd = True
    sum = 0
    FOR n = LEN(card$(test)) TO 1 STEP -1
        num = VAL(MID$(card$(test), n, 1))
        IF odd THEN
           sum = sum + num
           odd = False
        ELSE
           num = num * 2
           IF num <= 9 THEN
              sum = sum + num
           ELSE
              sum = sum + VAL(LEFT$(STR$(num), 1)) + VAL(RIGHT$(STR$(num), 1))
           END IF
           odd = True
        END IF
    NEXT
    IF sum MOD 10 = 0 THEN
       PRINT card$(test), "True"
    ELSE
       PRINT card$(test), "False"
    END IF
NEXT test


LET true = -1
LET false = 0

DIM card$(5)
LET card$(1) = "49927398716"
LET card$(2) = "49927398717"
LET card$(3) = "1234567812345678"
LET card$(4) = "1234567812345670"

FOR test = 1 TO 4
    LET odd = true
    LET sum = 0
    FOR n = LEN(card$(test)) TO 1 STEP -1
        LET num = VAL((card$(test))[n:n+1-1])
        IF odd<>0 THEN
           LET sum = sum + num
           LET odd = false
        ELSE
           LET num = num*2
           IF num <= 9 THEN
              LET sum = sum + num
           ELSE
              LET sum = sum + VAL((STR$(num))[1:1]) + VAL((STR$(num))[LEN(STR$(num))-1+1:maxnum])
           END IF
           LET odd = true
        END IF
    NEXT n
    IF remainder(round(sum),10) = 0 THEN PRINT card$(test), "True" ELSE PRINT card$(test), "False"
NEXT test
END


Print "     49927398716", Show (Iif(FUNC(_Luhn ("49927398716")), "ok", "fail"))
Print "     49927398717", Show (Iif(FUNC(_Luhn ("49927398717")), "ok", "fail"))
Print "1234567812345678", Show (Iif(FUNC(_Luhn ("1234567812345678")), "ok", "fail"))
Print "1234567812345670", Show (Iif(FUNC(_Luhn ("1234567812345670")), "ok", "fail"))
End

_Luhn
  Param (1)
  Local (4)

  c@ = 1 : d@ = 0

  For b@ = Len(a@)-1 To 0 Step -1
    e@ = Peek(a@, b@) - Ord("0")
    d@ = d@ + Iif (c@, e@, e@+e@-9*(e@>4))
    c@ = c@ = 0
  Next

Return ((d@ % 10) = 0)


dim card$(5)
card$(1)="49927398716"
card$(2)="49927398717"
card$(3)="1234567812345678"
card$(4)="1234567812345670"

for test = 1 to 4
    odd = true
    sum = 0
    for n = len(card$(test)) to 1 step -1
        num = val(mid$(card$(test),n,1))
        if odd then
            sum = sum + num
            odd = false
        else
            num = num * 2
            if num <= 9 then
                sum = sum + num
            else
                sum = sum + val(left$(str$(num),1)) + val(right$(str$(num),1))
            fi
            odd = true
        fi
    next
    if mod(sum, 10) = 0 then
        print card$(test), chr$(9), "True"
    else
        print card$(test), chr$(9), "False"
    fi
next test