for-next repeats forever because of data type limit

[programandala.net]

Hi all,

I'm working on my first port from Sinclair Basic to ZX Basic. I'm excited about it. I'd love to program compiled versions of some old programs of mine, but first I need to acquaint with this wonderful and impressive tool.

I'm using ZX Basic 1.2.0 under Debian GNU/Linux.

Well, I think I found a kind of bug. Here you are the test:

Code:

' The following loop works as expected:

dim b0 as ubyte

for b0=0 TO 254
    print at 0,0;"b0=";b0;"    "
next b0

' The following loop works as expected too:

dim b1 as uinteger

for b1=0 TO 255
    print at 1,0;"b1=";b1;"    "
next b1

' But the following loop starts again forever:

dim b2 as ubyte

for b2=0 TO 255
    print at 2,0;"b2=";b2;"    "
next b2

border 4
print "This message never will be printed"
pause 0

I found the problem because I needed a loop from 0 to 255, so I used the ubyte data type for the index variable. Then I realized the loop repeated forever!

I guess the reason is NEXT increases the loop index before cheking it, so 256 becomes 0 after storing it back into the ubyte variable and the check gets wrong: the loop starts again because the index is zero! I guess the same happens with other data types, as long as their limits are reached, but I didn't try.

Do you think this is a language feature or a compiler bug?

I'm programming a project in FreeBASIC too, but I didn't try this. I'll try a similar code and I'll see what happens.