Assignment expressions

[jacdelad]

I recently read about assignment expressions introduced into Python 3.8. In Python it looks like this:

if (n := len(a)) > 10:
print(f"List is too long ({n} elements, expected <= 10)")

which would be

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Define n, a.s
a = "I am a test string"

If (n := Len(a)) > 10
  Debug "Length: " + Str(n)
EndIf

in PureBasic (losely translated and converted into a small example, but showing the point I'm about to make).

Basically this stores something into a variable if an expression is true. I don't know how this is realized internally and also whether it increases speed or or just for convenience.

What are your thoughts?

Sidenote: Even if this is not useful, how about extending the syntax for loops with a static parameter like

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For n = 1 ToStatic Len(a)
  ;Do something
Next

...not calculating Len(a) over and over because we know it won't change and obsoleting the use of another variable as temporary replacement?

[STARGÅTE]

Basically this stores something into a variable if an expression is true.

The assignment is anyway, because you assign first len(a) to n and then compare n with 10.

In PureBasic is can look like:

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Procedure.i Set(*Integer.Integer, Value.i)
	*Integer\i = Value
	ProcedureReturn Value
EndProcedure

Define n, a.s
a = "I am a test string"

If Set(@n, Len(a)) > 10
  Debug "Length: " + Str(n)
EndIf

[jacdelad]

Afaik the walrus operator doesn't set the value to n, if the expression is not true. With your "Set" it is always set.

[mk-soft]

Macro ...

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Define n, a.s
a = "I am a test string"

Macro IfSet(Result, Value, Assert)
  If Value Assert : Result = Value : EndIf
EndMacro

IfSet(n, Len(a), > 10)
Debug n

[jacdelad]

Hm, great idea. Thanks! The only downside is that I can't combine this IfSet with other operations like "And".

[mk-soft]

Better ... ?

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Define n, a.s
a = "I am a test string"
n = 5
Macro IfSet(Result, Value, Assert)
  If Not (Value Assert) : Result = Value : Else :
EndMacro

IfSet(n, Len(a), > 10)
  Debug "List is too long ({n} elements, expected <= 10)"
EndIf

Debug n

[Rinzwind]

Won't happen. PB misses a lot of "modern" syntax conveniences (see other requests years old). Even something like inline array declaration and initialization is still not available (it is in plain old C, mind you).

Not to be sour, but realistic given existing experiences.

Initializing a structure could also be a one liner if syntax allowed or allowing procedure overloading or variable parameters or string formatting uhm format or enhanced variable scoping, etc.

Anyway, this one seems not to add too much convenience imho. ForStatic or similar is a good one though.

[Demivec]

I like the idea of being able to designate the ending value of a For/Next loop as a static value.

The other thought on assigning a value inside a conditional expression doesn't look to have any worth. It seems as a possible use for it is to avoid calculating a value twice, once in the conditional expression and once in the code executed after the condition is found to be true. In those situations the value of the expression should simply be assigned first and then evaluated in the conditional second.

Here is a rewrite of your example with this last thought in mind:

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Define n, a.s
a = "I am a test string"

n = Len(a)
If  n > 10
  Debug "Length: " + Str(n)
EndIf

I would say this is more readable and just as efficient and effective.

[jacdelad]

@mk-soft:

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IfSet(N,Len(a),>10) And b=7

Possible with Python, not possible with IfSet.

[STARGÅTE]

Afaik the walrus operator doesn't set the value to n, if the expression is not true. With your "Set" it is always set.

What is your reference?
https://docs.python.org/3.8/whatsnew/3. ... xpressions

There is new syntax := that assigns values to variables as part of a larger expression.

And this code:

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a = "Hallo"
if (n := len(a)) > 10:
    print(f"List is too long ({n} elements, expected <= 10)")
else:
    print(f"List is too short ({n} elements)")

sets n to 5:

List is too short (5 elements)

[mk-soft]

@mk-soft:

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IfSet(N,Len(a),>10) And b=7

Possible with Python, not possible with IfSet.

It's perfectly clear that this doesn't work if you know how macros work.

It's just unnecessary macros anyway.
I also think that you don't necessarily need the peculiarities of other programming languages. It also works with simple 'IF ELSE ENDIF' processing.

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Define n, a.s
a = "I am a test string"
n = 5
b = 7

If Len(a) > 10
  Debug "List is too long ({n} elements, expected <= 10)"
ElseIf b = 7
  n = Len(a)
EndIf

Debug n

I also think this is easier to understand than some other strange expressions in other languages.

[jacdelad]

@STARGATE: You're right, I got this wrong.
@mk-soft: That's why I wanted to talk about it and get the views of other, more advanced developers.

[Olli]

note that in ASM x86 / x64 there has been a "recent" op :

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cmove rax, rbx

which means here :

if a previous arithmetic op treated two similar values, then rax = rbx.

example :

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sub rsi, rdx
cmove rax, rbx

(x - d
if x = d
a = b)