Accessing string elements, casting?

GrendelT · 2012-08-06 21:40

In the following code, I've created a string using a long the string( ) call, and I've created a byte array to form the string "spin".

CON _clkmode = xtal1 + pll16x
    _xinfreq = 5_000_000
OBJ pst : "Parallax Serial Terminal"
VAR byte foo[10]
    long bar[10]
PUB main
    pst.Start(115200)

    foo[0] := "s"
    foo[1] := "p"
    foo[2] := "i"
    foo[3] := "n"
    foo[4] := 0

    bar := string("fun")
    
    pst.str(foo[1])
    pst.newline
    pst.str(bar)

Why on the pst.str(foo[1]) does it not print p? If I include the @ operator to make it @foo[1], I get pin, not just the p. How would I pull only the p from foo?

Do I need to do some casting trickery on it? Is that what the byte[ ] syntax I've seen used on others' code? Casting? If not, please explain what that means, I'm having a hard time finding it. (or at least tell me the name of it so I can look it up.)

kuroneko · 2012-08-06 21:46

Printing a single character requires a different method (after all you're not printing a string anymore). Use pst.char(foo[1]) (other objects use tx, out or putc). Also, you only need a single long to store a string address.

If you have a closer look at the used objects you'll find that the str method is usually based on the single character output method, e.g.

PUB [COLOR="blue"]str[/COLOR](addr)

  repeat strsize(addr)
    [COLOR="orange"]out[/COLOR](byte[addr++])

GrendelT · 2012-08-06 22:12

Awesome! Thanks.

How about the byte[ ] or word[ ] syntax I see used from time to time? What's that called/doing?

kuroneko · 2012-08-06 22:18

GrendelT wrote: »

How about the byte[ ] or word[ ] syntax I see used from time to time? What's that called/doing?

If you take the method above as an example, all it knows about is an address (of a string). IOW it doesn't have a name of a global/local variable to refer to (so @foo wouldn't work). Which is where byte[] & Co come in. They take an address (which has been communicated to this method) and return the relevant byte/word/long from hub memory (within alignment rules, e.g. a long is always loaded from 4n).

VAR long example, addr

  example := 42
  long[@example] := 42

  addr := @example
  long[addr] := 42

All three assignments do the same thing (loading VAR example with 42).

kuroneko · 2012-08-06 22:33

Note, there is another form which is used to access e.g. bytes within a long like

VAR long example, n

  example := $12345678
  n := extract(example, 3)

PUB extract(value, byte_index)

  return value.byte[byte_index]

While wrapping this into a method is clearly overkill, it should demonstrate the principle. The above is equivalent to byte[@value][byte_index].

Accessing string elements, casting?

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