Another "por" vs "para" question

Another "por" vs "para" question
 

Doing another translation exercise in the "por vs. para" chapter...

I think that the English version of this sentence is awkward at best, so I will re-write it also......

English (given): She chooses the lottery numbers for ages of friends and for special dates.
English (Lou Ann's re-write): She chooses lottery numbers based on ages of friends and on special dates. Or, better: She uses friends' birthdays and special dates to choose her lottery numbers.
The book's Spanish translation: Ella escoge los numeros de la lotería por las edades de amigos y por fechas especiales.

I had a lot of trouble choosing por or para for this sentence, and ended up going with "para", only because I had absolutely no idea.....

I'm not quite sure why it's "por".... The only thing I can come up with is that it is some sort of mode of a method of decision making....

Can someone give me a good reason why this is "por" and not "para"? Thanks!!

Well, por is wrong and para is absurd. The right preposition there is según.

That said, and for the sake of learning both prepositions:

elige por ellos (ellos no eligen; lo iban a hacer, pero elige ella) ---> por ~ "en su reemplazo"

elige para ellos (ellos se benefician de eso; ella les asigna lo que elige) para ~ "destinado a"

elige por las fechas (si fuera otra fecha elegiría otra cosa) ---> por ~ "a causa"
elige para las fechas (para usar en esas fechas) ---> para ~ "destinado a"

elige por ellos y por las fechas (la combinación de calendario y personas le hace elegir eso ---> quizá ellos sean sus visitas en alguna fiesta tradicional)
elige para ellos y para las fechas (le va a dar a ellos lo que eligió para ese momento determinado)

elige por sus edades ---> están viejitos y les cuesta masticar y digerir
elige para sus edades ---> no le proyectemos esa película XXX a los niños

DRAE's definitions of "por" include that the preposition can refer to the means or mode to do something, so that's why the choice is valid. (Although not the immediate one for many people.)

"Para" would mean that the ages of friends or special dates benefit from her choosing lottery numbers or that there is a purpose for them as if they were subjects, which can't be the case.

These questions about por and para make me think I know nothing about my own language. :D

However, reading this, I think you can sometimes translate the idea of "based on" or "because" as "por", but never as "para".

I don't like England because of / based on (?) the weather.
No me gusta Inglaterra por el clima.

He was sacked based on / because of his lies.
Él fue despedido por sus mentiras.

I think this proposed use of "por" is way beyond those cause, mode and means that AdeA mentioned. It's just a "por by default" that speakers may use. One instance of that is the phrase "se llama Juan por el padre" where por suggests the notion conveyed in English by "after". When people are not very educated or the verbal kind and they are not able to use more complex structures in real time, well, they just use por -maybe the most elastic and multipurpose preposition in Spanish-: "Habla por las ramas", "apuesta por los cumpleaños", "se llama por su padre".

Alec - interestingly, I would slightly more often say "he was named John for his father" than "he was named John after his father", or just "he was named for his father". Hmmm....

Also, Malila - I have been looking at the RAE "por" entry (thank you for reminding me!) and completely understand your explanation. I also have a couple of questions about other given uses of "por":
- in entry #1: "Indica el agente en las oraciones en pasiva." - would this be something like "escrito por Cervantes" or something like that?
- in entry #5: "Se junta con los nombres de tiempo, determinándolo. Por San Juan. Por agosto." - would you put this use in a complete sentence for me? I don't really follow the purpose of this usage...

Thanks, all!!

#1: correct.

#5: It's for a date you're estimating without precision... Like "sometime around..."

-Recuerdo que tu cumpleaños es por San Juan, pero no sé el día exacto.
-Es el 23 de junio.

Tenía que entregar mi tesis en diciembre, pero no terminé y ahora la voy a entregar por febrero.

Thanks!! Um, so I thought San Juan is a place (which confused me). Apparently it's a date?

Some catholic saints are important for referencing the calendar dates in some regions. St. John's day is on June 24th. I used it because it was in your example.

Se me acaban las vacaciones por Reyes. (Jan. 6th.)
Voy a verte por Semana Santa. (Estimated date, not cause.)

Some more examples:
Me enfemé por octubre y sigo mal.
Tenemos que pagar la renta por el día 5. (One or two days before or after would be fine.)

So it's a little bit like when you say "por acá" or "por allá".... Super - totally clear to me now. Thanks! :)