dim d(4)
dim chk(4)
print "The 24 Game"
print
print "Given four digits and using just the +, -, *, and / operators; and the"
print "possible use of brackets, (), enter an expression that equates to 24."

do
    d$=""
    for i = 1 to 4
        d(i)=int(rnd(1)*9)+1    '1..9
        chk(i)=d(i)
        d$=d$;d(i)  'valid digits, to check with Instr
    next

    print
    print "These are your four digits: ";
    for i = 1 to 4
        print d(i);left$(",",i<>4);
    next
    print

    Print "Enter expression:"
    Input "24 = ";expr$
    'check expr$ for validity

    'check right digits used
    failed = 0
    for i = 1 to len(expr$)
        c$=mid$(expr$,i,1)
        if instr("123456789", c$)<>0 then 'digit
            if instr(d$, c$)=0 then failed = 1: exit for
            if i>1 and instr("123456789", mid$(expr$,i-1,1))<>0 then failed = 2: exit for
            for j =1 to 4
                if chk(j)=val(c$) then chk(j)=0: exit for
            next
        end if
    next
    if failed=1 then
        print "Wrong digit (";c$;")"
        goto [fail]
    end if

    if failed=2 then
        print "Multiple digit numbers is disallowed."
        goto [fail]
    end if

    'check all digits used
    if chk(1)+ chk(2)+ chk(3)+ chk(4)<>0 then
        print "Not all digits used"
        goto [fail]
    end if

    'check valid operations
    failed = 0
    for i = 1 to len(expr$)
        c$=mid$(expr$,i,1)
        if instr("+-*/()"+d$, c$)=0 then failed = 1: exit for
    next
    if failed then
        print "Wrong operation (";c$;")"
        goto [fail]
    end if
    'some errors (like brackets) trapped by error handler
    Err$=""
    res=evalWithErrCheck(expr$)
    if Err$<>"" then
        print "Error in expression"
        goto [fail]
    end if
    if res = 24 then
        print "Correct!"
    else
        print "Wrong! (you got ";res ;")"
    end if
[fail]
    Input "Play again (y/n)? "; ans$
loop while ans$="y"
end

function evalWithErrCheck(expr$)
    on error goto [handler]
    evalWithErrCheck=eval(expr$)
    exit function
[handler]
end function