Problem updating a ptr in a switch statement

[Blip]

I try to update the buffer pointer in a switch statement like this:

buf[ptr]=(serialinput);
switch (buf[ptr++])

that doesn't work correctly -- doesn't update the ptr

I changed it to:
buf[ptr++]=(serialinput);
switch (buf[ptr-1])

And now it works, but I don't understand why/why the first doesn't.

Thanks, Tom

[Eric Sosman]

Blip wrote:

I try to update the buffer pointer in a switch statement like this:

buf[ptr]=(serialinput);
switch (buf[ptr++])

that doesn't work correctly -- doesn't update the ptr

I changed it to:
buf[ptr++]=(serialinput);
switch (buf[ptr-1])

And now it works, but I don't understand why/why the first doesn't.

Both should give the same outcome. Can you (1) explain
why you think the first form does not update ptr, and (2) post
a short, complete, compilable program that demonstrates the
unexpected behavior?

--
Eric Sosman
es*****@ieee-dot-org.invalid

[s0suk3]

On Jul 27, 10:20 am, Blip <[e-mail]:

I try to update the buffer pointer in a switch statement like this:

buf[ptr]=(serialinput);
switch (buf[ptr++])

that doesn't work correctly -- doesn't update the ptr

<snip>

What value do you want the switch statement to test? If it's the
pointer *before* it is incremented, then your code is right (well not
really, see below). If you want it to be the pointer *after*
incrementation, then you have to do a pre-increment:

buf[ptr]=(serialinput);
switch (buf[++ptr]) { ... }

Also, the names of the variables leads me to believe the code is wrong
anyway. 'ptr' is a pointer, right? You can't subscript an array (or a
pointer) with a pointer; you subscript it with an integer. If 'ptr'
points to an element in the array (or pointer) 'buf', then the code
should look like this:
*ptr = (serialinput);
switch (*++ptr) { ... }

Sebastian