[PD-dev] symbol comparison vs strcmp()

[Christof Ressi]

> how did you know?
Telepathy :-D

> i just find the code mostly more readable. 
That's probably a matter of taste. "!strcmp(...)" is a very familiar 
idiom for every C programmer and I find it perfectly readable.

> `!strcmp(flag, "-lib")` is of course fine (apart from the inverse 
> logic of strcmp), but `!strcmp(argv[i].a_w.w_symbol->s_name, ">")` is 
> not. 
Sure, but neither is "gensym(">") == argv[i].a_w.w_symbol".

> in this case i would happily swap that for a slow `(gensym(">") == 
> atom_getsymbol(argv+i))`. 

For a *single* comparison, it might be more readable for some people. 
But if you want to compare against several strings - maybe even in a 
loop -, you would naturally save the symbol in a variable, and then the 
strcmp() version becomes quite readable, IMO.

> all those specific comparisions for "-lib" are done in loop, so you 
> would probably put the gensym("-lib") outside of that loop. 

That would work, but I think it's overkill. Also, it would decrease 
readability, because you can't immediately see the actual flag string, 
but instead only see a variable. Also, the performance gain would be 
minimal at best, because the strings are very short and strcmp() 
terminates on the first non-matching character, anyway.

If you consider that "gensym()" needs to

1) hash the whole string,

2) do a (fast) table look-up and

3) iterate over the bucket list and do a strcmp() until it finds an 
exact match

you would need a couple of loop iterations to make up for those extra 
initial costs and gain any performance benefit.

---

Anyway, it seem like we're now talking more about readability than 
performance (which seemed to be your original concern).

I would say that for "ugly" lines like

`!strcmp(argv[i].a_w.w_symbol->s_name, ">")`

saving the string in a local variable would already increase readability 
significantly:

const char *s = argv[i].a_w.w_symbol->s_name;

if (!strcmp(s, ">")) ...


Christof

On 11.03.2021 22:13, IOhannes m zmölnig wrote:
> On 3/11/21 9:34 PM, Christof Ressi wrote:
>>> the main purpose of the the symbol table is, as i understand it, to 
>>> make string comparison super-fast (as there's no need to compare 
>>> each character but just a simple address). 
>> It is only fast if both sides are `t_symbol *`, but if one side 
>> includes a call to gensym(), it will be *slower* than a simple strcmp()!
>>
>> For example,
>>
>> flagsym == gensym("-lib")
>
>
> funnily enough, this was exactly the line that triggered my query.
> how did you know? (probably because you checked the background of my 
> other post)
>
>>
>> is my all means worse than
>>
>> !strcmp(flag, "-lib")
>
>
> well, no. all those specific comparisions for "-lib" are done in loop, 
> so you would probably put the gensym("-lib") outside of that loop.
>
>>
>> First of all, these are not the performance critical parts of Pd and 
>> most of the strings in question are very short, anyway.
>>
>> Secondly, for symbol comparison to make sense, you would have to 
>> create all those symbols upfront and store them somewhere. This would 
>> indeed pollute the symbol table and add many lines of extra code for 
>> no real benefit.
>
>
> i just find the code mostly more readable.
> `!strcmp(flag, "-lib")` is of course fine (apart from the inverse 
> logic of strcmp), but `!strcmp(argv[i].a_w.w_symbol->s_name, ">")` is 
> not.
> in this case i would happily swap that for a slow `(gensym(">") == 
> atom_getsymbol(argv+i))`.
>
> anyhow, most of those symbols (with the exception of the actual 
> flags), are also Pd-objects, so the symtable pollution shouldn't 
> really matter.
>
> gfsmdrt
> IOhannes
>
>
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