If

g3cwi · 2012-04-23 10:57

I have been attempting to understand other people's SPIN code. One thing that I see often and puzzles me is a use of "If";

If VAR
Do something

This must be an implied test of some sort? I am happy with:

If VAR>100
Do something

What does the first example mean?

Richard

Duane Degn · 2012-04-23 11:08

I think "if var" is the same as "if var <> 0".

pedward · 2012-04-23 11:11

Like Duane said, non-zero value. If the variable is unset, the test fails, if it's set to anything, it passes.

Mike Green · 2012-04-23 11:15

The Propeller Manual defines FALSE as zero and TRUE as all one bits. The various Boolean operators (NOT, AND, OR) and the comparison operators (<, ==, >, <>, =>, =<) produce FALSE and TRUE results. The IF statement and the REPEAT UNTIL and REPEAT WHILE statements accept any non-zero value as TRUE.

g3cwi · 2012-04-23 11:35

Thanks - all clear now!

Richard

Dr_Acula · 2012-04-23 17:46

A question for the experts - is there any penalty in terms of extra code produced by the compiler with

if var

vs

if var == true

I'm thinking that if there is no penalty, I will try to use the second one as I think it does read more logically.

frank freedman · 2012-04-23 17:51

Dr_Acula wrote: »
A question for the experts - is there any penalty in terms of extra code produced by the compiler with
if var
vs
if var == true
I'm thinking that if there is no penalty, I will try to use the second one as I think it does read more logically.

Depends on spins implementation. Try two timing loops with both methods and see which one is faster using the counter or other timing method.

Let us know what you find

kuroneko · 2012-04-23 17:53

@Dr: The latter is bigger and slower. Your choice. That said, if you name the variables properly then there is nothing wrong with the first version:

if enabled
    ' do something

mpark · 2012-04-23 18:26

Dr_Acula wrote: »
A question for the experts - is there any penalty in terms of extra code produced by the compiler with
if var
vs
if var == true
I'm thinking that if there is no penalty, I will try to use the second one as I think it does read more logically.

Beyond the space and time penalties Kuroneko mentioned, note that the two versions are not equivalent.

Duane Degn · 2012-04-23 18:32

if var == true
    'do something if var equals -1

Wont this condition only be met if var equals -1 aka "TRUE"?

Dr_Acula · 2012-04-23 18:39

Thanks kuroneko. Well, if there is a cost in terms of speed and/or code space, that would explain why many people use the shorter version. So maybe the answer might be just to put a comment there eg

if var    '  == true

Addit : re Duane, do you mean that 'if var' is not the same as 'if var == true' ?

If so then I'm going to be asking the same question as our OP in post #1

Mike Green · 2012-04-23 19:07

"if var" is not the same as "if var == true" because the latter requires that the value of var be exactly -1 while the former only requires that var be non-zero. In most cases, the former is perfectly acceptable.

frank freedman · 2012-04-23 19:15

Mike Green wrote: »

"if var" is not the same as "if var == true" because the latter requires that the value of var be exactly -1 while the former only requires that var be non-zero. In most cases, the former is perfectly acceptable.

Thanks for pointing that one out. Kinda negates the main question here as we are no longer apples to apples, rather apples with significant side effects.

Dr_Acula · 2012-04-23 19:30

So is the equivalent code "if var" the same as ?

if var <> false

Duane Degn · 2012-04-23 19:35

Dr_Acula wrote: »
So is the equivalent code "if var" the same as ?
if var <> false

Yes.

Just to try to get this straight in my brain, I wrote this program.

CON
  _clkmode = xtal1 + pll16x
  _xinfreq = 5_000_000
 
 _DebugBaud = 57600
  
OBJ
  debug : "FullDuplexSerial"
 
PUB Main 
  
  Debug.Start(31, 30, 0, _DebugBaud)
  waitcnt(clkfreq * 3 + cnt)
   
  TestConditions(-1)
  TestConditions(true)
  TestConditions(0)
  TestConditions(false)
  TestConditions(1)
  TestConditions(2)
  TestConditions(-10)
  repeat
  
PRI TestConditions(var1)      
          
  Debug.str(string(13, 13, "var1 = "))
  Debug.dec(var1)
  if var1
    Debug.str(string(13, "(if var1) condition met."))
  else
    Debug.str(string(13, "(if var1) condition not met.")) 
  if var1 <> false
    Debug.str(string(13, "(if var1 <> false) condition met"))  
  else
    Debug.str(string(13, "(if var1 <> false) condition not met.")) 
  if var1 == true
    Debug.str(string(13, "(if var1 == true) condition met"))  
  else
    Debug.str(string(13, "(if var1 == true) condition not met.")) 
  if var1 == false
    Debug.str(string(13, "(if var1 == false) condition met"))
  else
    Debug.str(string(13, "(if var1 == false) condition not met.")) 
  ifnot var1 
    Debug.str(string(13, "(ifnot var1) condition met"))
  else
    Debug.str(string(13, "(ifnot var1) condition not met.")) 
  ifnot var1 <> false
    Debug.str(string(13, "(ifnot var1 <> false) condition met"))  
  else
    Debug.str(string(13, "(ifnot var1 <> false) condition not met.")) 
  ifnot var1 == true
    Debug.str(string(13, "(ifnot var1 == true) condition met"))  
  else
    Debug.str(string(13, "(ifnot var1 == true) condition not met.")) 
  ifnot var1 == false
    Debug.str(string(13, "(ifnot var1 == false) condition met"))
  else
    Debug.str(string(13, "(ifnot var1 == false) condition not met."))

This is the output.

var1 = -1
(if var1) condition met.
(if var1 <> false) condition met
(if var1 == true) condition met
(if var1 == false) condition not met.
(ifnot var1) condition not met.
(ifnot var1 <> false) condition not met.
(ifnot var1 == true) condition not met.
(ifnot var1 == false) condition met
var1 = -1
(if var1) condition met.
(if var1 <> false) condition met
(if var1 == true) condition met
(if var1 == false) condition not met.
(ifnot var1) condition not met.
(ifnot var1 <> false) condition not met.
(ifnot var1 == true) condition not met.
(ifnot var1 == false) condition met
var1 = 0
(if var1) condition not met.
(if var1 <> false) condition not met.
(if var1 == true) condition not met.
(if var1 == false) condition met
(ifnot var1) condition met
(ifnot var1 <> false) condition met
(ifnot var1 == true) condition met
(ifnot var1 == false) condition not met.
var1 = 0
(if var1) condition not met.
(if var1 <> false) condition not met.
(if var1 == true) condition not met.
(if var1 == false) condition met
(ifnot var1) condition met
(ifnot var1 <> false) condition met
(ifnot var1 == true) condition met
(ifnot var1 == false) condition not met.
var1 = 1
(if var1) condition met.
(if var1 <> false) condition met
(if var1 == true) condition not met.
(if var1 == false) condition not met.
(ifnot var1) condition not met.
(ifnot var1 <> false) condition not met.
(ifnot var1 == true) condition met
(ifnot var1 == false) condition met
var1 = 2
(if var1) condition met.
(if var1 <> false) condition met
(if var1 == true) condition not met.
(if var1 == false) condition not met.
(ifnot var1) condition not met.
(ifnot var1 <> false) condition not met.
(ifnot var1 == true) condition met
(ifnot var1 == false) condition met
var1 = -10
(if var1) condition met.
(if var1 <> false) condition met
(if var1 == true) condition not met.
(if var1 == false) condition not met.
(ifnot var1) condition not met.
(ifnot var1 <> false) condition not met.
(ifnot var1 == true) condition met
(ifnot var1 == false) condition met

You can see that "true" is treated the same as "-1" and "false" is treated the same as "0".

Dr_Acula · 2012-04-23 19:49

Ah, so the question by g3cwi is more intriguing than it first appears! You save space leaving out the condition, but the condition is not a "true", it is a "not false"

I am wondering about using this in my code - saves space by adding the ' as a comment and it is explicit about what the test condition is.

if var ' <> false

Duane Degn · 2012-04-23 20:06

I've started using "if var" conditions only recently.

Kye uses all sort of tricks with "if var" and "ifnot var". I often get myself into trouble when I try to do the same.

One other thing to be aware of is "true" is a long so bytes and words wont ever be true.

For example, this code:

VAR
  long varLong
  byte varByte
OBJ
  debug : "FullDuplexSerial"
 
PUB Main 
  
  Debug.Start(31, 30, 0, _DebugBaud)
  waitcnt(clkfreq * 3 + cnt)
 [B] varLong := true
  varByte := true 
[/B] 
  if varLong == true
    Debug.str(string(13, "(if varLong == true) condition met."))  
  else
    Debug.str(string(13, "(if varLong == true) condition not met.")) 
  if varByte == true
    Debug.str(string(13, "(if varByte == true) condition met."))  
  else
    Debug.str(string(13, "(if varByte == true) condition not met.")) 
  repeat

Generates this output:

(if varLong == true) condition met.
(if varByte == true) condition not met.

Dr_Acula · 2012-04-23 21:09

yikes, what does " "ifnot var" mean?

does it mean 'if not not equal true" or is it "if false".

It seems the opposite of Mike Green's explanation on post #12 but in what way is it opposite? I think I can understand it if one sticks to "true" and "false" but not when you add numbers. Definitely would need a long comment after such a statement. My brain hurts!

Duane Degn · 2012-04-23 21:12

Dr_Acula wrote: »

My brain hurts!

Mine too, whenever I try to make sense of "ifnot" statements.

Mike Green · 2012-04-23 21:43

"ifnot var" is the same as "if not var", just more efficient, so it means "if var == 0"

Mark_T · 2012-04-24 04:56

Mike Green wrote: »

"ifnot var" is the same as "if not var", just more efficient, so it means "if var == 0"

Many languages have an "unless" keyword, "ifnot" or "if not" is an awkward construction.

Ariba · 2012-04-24 05:44

As Kuroneko said, it depends a lot on the variable name if it is readable or not.
This form of IF is perfect for variables which are used for flags or have just two states like set or not set:

if enabled
    'do this

  ifnot enabled
    'do that

  buttonPressed := ina[0]
  if buttonPressed
    'do something

  ifnot buttonPressed
    'do another thing

  if pinHigh
    '...
  ifnot pinHigh
    '...

Andy

Graham Stabler · 2012-04-24 06:10

IF you can keep your head when all about you 
Are losing theirs and blaming it on you,
If you can trust yourself when all men doubt you,
But make allowance for their doubting too;
If you can wait and not be tired by waiting,
Or being lied about, don't deal in lies,
Or being hated, don't give way to hating,
And yet don't look too good, nor talk too wise:
If you can dream - and not make dreams your master;
If you can think - and not make thoughts your aim;
If you can meet with Triumph and Disaster
And treat those two impostors just the same;
If you can bear to hear the truth you've spoken
Twisted by knaves to make a trap for fools,
Or watch the things you gave your life to, broken,
And stoop and build 'em up with worn-out tools:

If you can make one heap of all your winnings 
And risk it on one turn of pitch-and-toss,
And lose, and start again at your beginnings
And never breathe a word about your loss;
If you can force your heart and nerve and sinew
To serve your turn long after they are gone,
And so hold on when there is nothing in you
Except the Will which says to them: 'Hold on!'

If you can talk with crowds and keep your virtue,
' Or walk with Kings - nor lose the common touch,
if neither foes nor loving friends can hurt you,
If all men count with you, but none too much;
If you can fill the unforgiving minute
With sixty seconds' worth of distance run,
Yours is the Earth and everything that's in it,
And - which is more - you'll be a Man, my son!

g3cwi · 2012-04-24 08:36

Oh dear. I have just read the whole thread and feel slightly sick. The manual seems rather superficial in some respects. I wish it had more explicit examples.

Cheers

Richard

If

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